From 1361dfc89553b146864fb8ff3526cf12cf3f293a Mon Sep 17 00:00:00 2001
From: Shahd Lala <shosho996@blackhole.local>
Date: Tue, 31 Mar 2026 10:31:02 +0200
Subject: [PATCH] feat: NCS-16 Core Separation via Patterns (#10)

Co-authored-by: Janis <janis-e@gmx.de>
Co-authored-by: shahdlala66 <shahd.lala66@gmail.com>
Co-authored-by: Janis <janis.e.20@gmx.de>
Reviewed-on: https://git.janis-eccarius.de/NowChess/NowChessSystems/pulls/10
Reviewed-by: Janis <janis-e@gmx.de>
Co-authored-by: Shahd Lala <shosho996@blackhole.local>
Co-committed-by: Shahd Lala <shosho996@blackhole.local>
---
 .idea/gradle.xml                              |   1 +
 .idea/scala_compiler.xml                      |   2 +-
 .../main/scala/de/nowchess/chess/Main.scala   |  12 -
 .../de/nowchess/chess/command/Command.scala   |  64 ++++
 .../chess/command/CommandInvoker.scala        |  73 +++++
 .../chess/controller/GameController.scala     |  45 ---
 .../de/nowchess/chess/engine/GameEngine.scala | 209 ++++++++++++
 .../de/nowchess/chess/observer/Observer.scala |  87 +++++
 .../command/CommandInvokerBranchTest.scala    | 216 ++++++++++++
 .../chess/command/CommandInvokerTest.scala    | 123 +++++++
 .../CommandInvokerThreadSafetyTest.scala      | 131 ++++++++
 .../nowchess/chess/command/CommandTest.scala  |  52 +++
 .../command/MoveCommandImmutabilityTest.scala |  65 ++++
 .../chess/controller/GameControllerTest.scala | 108 ------
 .../engine/GameEngineEdgeCasesTest.scala      | 214 ++++++++++++
 .../engine/GameEngineGameEndingTest.scala     |  93 ++++++
 .../GameEngineHandleFailedMoveTest.scala      | 110 +++++++
 .../engine/GameEngineInvalidMovesTest.scala   | 114 +++++++
 .../chess/engine/GameEngineTest.scala         | 310 ++++++++++++++++++
 .../engine/MoveCommandDefaultsTest.scala      | 110 +++++++
 .../de/nowchess/chess/main/MainTest.scala     |  12 -
 .../observer/ObservableThreadSafetyTest.scala | 168 ++++++++++
 modules/ui/build.gradle.kts                   |  73 +++++
 .../src/main/scala/de/nowchess/ui/Main.scala  |  15 +
 .../de/nowchess/ui/terminal/TerminalUI.scala  |  76 +++++
 .../test/scala/de/nowchess/ui/MainTest.scala  |  22 ++
 .../nowchess/ui/terminal/TerminalUITest.scala | 189 +++++++++++
 settings.gradle.kts                           |   2 +-
 28 files changed, 2517 insertions(+), 179 deletions(-)
 delete mode 100644 modules/core/src/main/scala/de/nowchess/chess/Main.scala
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/command/Command.scala
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/command/CommandInvoker.scala
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerBranchTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerThreadSafetyTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/command/CommandTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/command/MoveCommandImmutabilityTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineEdgeCasesTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineGameEndingTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineHandleFailedMoveTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineInvalidMovesTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/engine/MoveCommandDefaultsTest.scala
 delete mode 100644 modules/core/src/test/scala/de/nowchess/chess/main/MainTest.scala
 create mode 100644 modules/core/src/test/scala/de/nowchess/chess/observer/ObservableThreadSafetyTest.scala
 create mode 100644 modules/ui/build.gradle.kts
 create mode 100644 modules/ui/src/main/scala/de/nowchess/ui/Main.scala
 create mode 100644 modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala
 create mode 100644 modules/ui/src/test/scala/de/nowchess/ui/MainTest.scala
 create mode 100644 modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala

diff --git a/.idea/gradle.xml b/.idea/gradle.xml
index 4f4edba..f1d0a36 100644
--- a/.idea/gradle.xml
+++ b/.idea/gradle.xml
@@ -12,6 +12,7 @@
             <option value="$PROJECT_DIR$/modules" />
             <option value="$PROJECT_DIR$/modules/api" />
             <option value="$PROJECT_DIR$/modules/core" />
+            <option value="$PROJECT_DIR$/modules/ui" />
           </set>
         </option>
       </GradleProjectSettings>
diff --git a/.idea/scala_compiler.xml b/.idea/scala_compiler.xml
index 8db25d6..a0f8d4f 100644
--- a/.idea/scala_compiler.xml
+++ b/.idea/scala_compiler.xml
@@ -5,7 +5,7 @@
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
     </profile>
-    <profile name="Gradle 2" modules="NowChessSystems.modules.core.main,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test">
+    <profile name="Gradle 2" modules="NowChessSystems.modules.core.main,NowChessSystems.modules.core.scoverage,NowChessSystems.modules.core.test,NowChessSystems.modules.ui.main,NowChessSystems.modules.ui.scoverage,NowChessSystems.modules.ui.test">
       <option name="deprecationWarnings" value="true" />
       <option name="uncheckedWarnings" value="true" />
       <parameters>
diff --git a/modules/core/src/main/scala/de/nowchess/chess/Main.scala b/modules/core/src/main/scala/de/nowchess/chess/Main.scala
deleted file mode 100644
index 3fb72e6..0000000
--- a/modules/core/src/main/scala/de/nowchess/chess/Main.scala
+++ /dev/null
@@ -1,12 +0,0 @@
-package de.nowchess.chess
-
-import de.nowchess.api.board.Board
-import de.nowchess.api.board.Color
-import de.nowchess.chess.controller.GameController
-import de.nowchess.chess.logic.GameHistory
-
-object Main {
-  def main(args: Array[String]): Unit =
-    println("NowChess TUI — type moves in coordinate notation (e.g. e2e4). Type 'quit' to exit.")
-    GameController.gameLoop(Board.initial, GameHistory.empty, Color.White)
-}
diff --git a/modules/core/src/main/scala/de/nowchess/chess/command/Command.scala b/modules/core/src/main/scala/de/nowchess/chess/command/Command.scala
new file mode 100644
index 0000000..5bc93a3
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/command/Command.scala
@@ -0,0 +1,64 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, Board, Color, Piece}
+import de.nowchess.chess.logic.GameHistory
+
+/** Marker trait for all commands that can be executed and undone.
+ *  Commands encapsulate user actions and game state transitions.
+ */
+trait Command:
+  /** Execute the command and return true if successful, false otherwise. */
+  def execute(): Boolean
+
+  /** Undo the command and return true if successful, false otherwise. */
+  def undo(): Boolean
+
+  /** A human-readable description of this command. */
+  def description: String
+
+/** Command to move a piece from one square to another.
+ *  Stores the move result so undo can restore previous state.
+ */
+case class MoveCommand(
+  from: Square,
+  to: Square,
+  moveResult: Option[MoveResult] = None,
+  previousBoard: Option[Board] = None,
+  previousHistory: Option[GameHistory] = None,
+  previousTurn: Option[Color] = None
+) extends Command:
+
+  override def execute(): Boolean =
+    moveResult.isDefined
+
+  override def undo(): Boolean =
+    previousBoard.isDefined && previousHistory.isDefined && previousTurn.isDefined
+
+  override def description: String = s"Move from $from to $to"
+
+// Sealed hierarchy of move outcomes (for tracking state changes)
+sealed trait MoveResult
+object MoveResult:
+  case class Successful(newBoard: Board, newHistory: GameHistory, newTurn: Color, captured: Option[Piece]) extends MoveResult
+  case object InvalidFormat extends MoveResult
+  case object InvalidMove extends MoveResult
+
+/** Command to quit the game. */
+case class QuitCommand() extends Command:
+  override def execute(): Boolean = true
+  override def undo(): Boolean = false
+  override def description: String = "Quit game"
+
+/** Command to reset the board to initial position. */
+case class ResetCommand(
+  previousBoard: Option[Board] = None,
+  previousHistory: Option[GameHistory] = None,
+  previousTurn: Option[Color] = None
+) extends Command:
+
+  override def execute(): Boolean = true
+
+  override def undo(): Boolean =
+    previousBoard.isDefined && previousHistory.isDefined && previousTurn.isDefined
+
+  override def description: String = "Reset board"
diff --git a/modules/core/src/main/scala/de/nowchess/chess/command/CommandInvoker.scala b/modules/core/src/main/scala/de/nowchess/chess/command/CommandInvoker.scala
new file mode 100644
index 0000000..7913ba6
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/command/CommandInvoker.scala
@@ -0,0 +1,73 @@
+package de.nowchess.chess.command
+
+/** Manages command execution and history for undo/redo support. */
+class CommandInvoker:
+  private val executedCommands = scala.collection.mutable.ListBuffer[Command]()
+  private var currentIndex = -1
+
+  /** Execute a command and add it to history.
+   *  Discards any redo history if not at the end of the stack.
+   */
+  def execute(command: Command): Boolean = synchronized {
+    if command.execute() then
+      // Remove any commands after current index (redo stack is discarded)
+      while currentIndex < executedCommands.size - 1 do
+        executedCommands.remove(executedCommands.size - 1)
+      executedCommands += command
+      currentIndex += 1
+      true
+    else
+      false
+  }
+
+  /** Undo the last executed command if possible. */
+  def undo(): Boolean = synchronized {
+    if currentIndex >= 0 && currentIndex < executedCommands.size then
+      val command = executedCommands(currentIndex)
+      if command.undo() then
+        currentIndex -= 1
+        true
+      else
+        false
+    else
+      false
+  }
+
+  /** Redo the next command in history if available. */
+  def redo(): Boolean = synchronized {
+    if currentIndex + 1 < executedCommands.size then
+      val command = executedCommands(currentIndex + 1)
+      if command.execute() then
+        currentIndex += 1
+        true
+      else
+        false
+    else
+      false
+  }
+
+  /** Get the history of all executed commands. */
+  def history: List[Command] = synchronized {
+    executedCommands.toList
+  }
+
+  /** Get the current position in command history. */
+  def getCurrentIndex: Int = synchronized {
+    currentIndex
+  }
+
+  /** Clear all command history. */
+  def clear(): Unit = synchronized {
+    executedCommands.clear()
+    currentIndex = -1
+  }
+
+  /** Check if undo is available. */
+  def canUndo: Boolean = synchronized {
+    currentIndex >= 0
+  }
+
+  /** Check if redo is available. */
+  def canRedo: Boolean = synchronized {
+    currentIndex + 1 < executedCommands.size
+  }
diff --git a/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala b/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala
index 4717430..120b9e9 100644
--- a/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala
+++ b/modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala
@@ -1,9 +1,7 @@
 package de.nowchess.chess.controller
 
-import scala.io.StdIn
 import de.nowchess.api.board.{Board, Color, File, Piece, Rank, Square}
 import de.nowchess.chess.logic.*
-import de.nowchess.chess.view.Renderer
 
 // ---------------------------------------------------------------------------
 // Result ADT returned by the pure processMove function
@@ -66,46 +64,3 @@ object GameController:
                     case PositionStatus.InCheck => MoveResult.MovedInCheck(newBoard, newHistory, captured, turn.opposite)
                     case PositionStatus.Mated   => MoveResult.Checkmate(turn)
                     case PositionStatus.Drawn   => MoveResult.Stalemate
-
-  /** Thin I/O shell: renders the board, reads a line, delegates to processMove,
-   *  prints the outcome, and recurses until the game ends.
-   */
-  def gameLoop(board: Board, history: GameHistory, turn: Color): Unit =
-    println()
-    print(Renderer.render(board))
-    println(s"${turn.label}'s turn. Enter move: ")
-    val input = Option(StdIn.readLine()).getOrElse("quit").trim
-    processMove(board, history, turn, input) match
-      case MoveResult.Quit =>
-        println("Game over. Goodbye!")
-      case MoveResult.InvalidFormat(raw) =>
-        println(s"Invalid move format '$raw'. Use coordinate notation, e.g. e2e4.")
-        gameLoop(board, history, turn)
-      case MoveResult.NoPiece =>
-        println(s"No piece on ${Parser.parseMove(input).map(_._1).fold("?")(_.toString)}.")
-        gameLoop(board, history, turn)
-      case MoveResult.WrongColor =>
-        println(s"That is not your piece.")
-        gameLoop(board, history, turn)
-      case MoveResult.IllegalMove =>
-        println(s"Illegal move.")
-        gameLoop(board, history, turn)
-      case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
-        val prevTurn = newTurn.opposite
-        captured.foreach: cap =>
-          val toSq = Parser.parseMove(input).map(_._2).fold("?")(_.toString)
-          println(s"${prevTurn.label} captures ${cap.color.label} ${cap.pieceType.label} on $toSq")
-        gameLoop(newBoard, newHistory, newTurn)
-      case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
-        val prevTurn = newTurn.opposite
-        captured.foreach: cap =>
-          val toSq = Parser.parseMove(input).map(_._2).fold("?")(_.toString)
-          println(s"${prevTurn.label} captures ${cap.color.label} ${cap.pieceType.label} on $toSq")
-        println(s"${newTurn.label} is in check!")
-        gameLoop(newBoard, newHistory, newTurn)
-      case MoveResult.Checkmate(winner) =>
-        println(s"Checkmate! ${winner.label} wins.")
-        gameLoop(Board.initial, GameHistory.empty, Color.White)
-      case MoveResult.Stalemate =>
-        println("Stalemate! The game is a draw.")
-        gameLoop(Board.initial, GameHistory.empty, Color.White)
diff --git a/modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala b/modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala
new file mode 100644
index 0000000..59aafe4
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/engine/GameEngine.scala
@@ -0,0 +1,209 @@
+package de.nowchess.chess.engine
+
+import de.nowchess.api.board.{Board, Color, Piece, Square}
+import de.nowchess.chess.logic.{GameHistory, GameRules, PositionStatus}
+import de.nowchess.chess.controller.{GameController, Parser, MoveResult}
+import de.nowchess.chess.observer.*
+import de.nowchess.chess.command.{CommandInvoker, MoveCommand}
+
+/** Pure game engine that manages game state and notifies observers of state changes.
+ *  This class is the single source of truth for the game state.
+ *  All user interactions must go through this engine via Commands, and all state changes
+ *  are communicated to observers via GameEvent notifications.
+ */
+class GameEngine extends Observable:
+  private var currentBoard: Board = Board.initial
+  private var currentHistory: GameHistory = GameHistory.empty
+  private var currentTurn: Color = Color.White
+  private val invoker = new CommandInvoker()
+
+  // Synchronized accessors for current state
+  def board: Board = synchronized { currentBoard }
+  def history: GameHistory = synchronized { currentHistory }
+  def turn: Color = synchronized { currentTurn }
+
+  /** Check if undo is available. */
+  def canUndo: Boolean = synchronized { invoker.canUndo }
+
+  /** Check if redo is available. */
+  def canRedo: Boolean = synchronized { invoker.canRedo }
+
+  /** Get the command history for inspection (testing/debugging). */
+  def commandHistory: List[de.nowchess.chess.command.Command] = synchronized { invoker.history }
+
+  /** Process a raw move input string and update game state if valid.
+   *  Notifies all observers of the outcome via GameEvent.
+   */
+  def processUserInput(rawInput: String): Unit = synchronized {
+    val trimmed = rawInput.trim.toLowerCase
+    trimmed match
+      case "quit" | "q" =>
+        // Client should handle quit logic; we just return
+        ()
+
+      case "undo" =>
+        performUndo()
+
+      case "redo" =>
+        performRedo()
+
+      case "" =>
+        val event = InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "Please enter a valid move or command."
+        )
+        notifyObservers(event)
+
+      case moveInput =>
+        // Try to parse as a move
+        Parser.parseMove(moveInput) match
+          case None =>
+            val event = InvalidMoveEvent(
+              currentBoard,
+              currentHistory,
+              currentTurn,
+              s"Invalid move format '$moveInput'. Use coordinate notation, e.g. e2e4."
+            )
+            notifyObservers(event)
+
+          case Some((from, to)) =>
+            // Create a move command with current state snapshot
+            val cmd = MoveCommand(
+              from = from,
+              to = to,
+              previousBoard = Some(currentBoard),
+              previousHistory = Some(currentHistory),
+              previousTurn = Some(currentTurn)
+            )
+
+            // Execute the move through GameController
+            GameController.processMove(currentBoard, currentHistory, currentTurn, moveInput) match
+              case MoveResult.InvalidFormat(_) | MoveResult.NoPiece | MoveResult.WrongColor | MoveResult.IllegalMove | MoveResult.Quit =>
+                handleFailedMove(moveInput)
+
+              case MoveResult.Moved(newBoard, newHistory, captured, newTurn) =>
+                // Move succeeded - store result and execute through invoker
+                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
+                invoker.execute(updatedCmd)
+                updateGameState(newBoard, newHistory, newTurn)
+                emitMoveEvent(from.toString, to.toString, captured, newTurn)
+
+              case MoveResult.MovedInCheck(newBoard, newHistory, captured, newTurn) =>
+                // Move succeeded with check
+                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(newBoard, newHistory, newTurn, captured)))
+                invoker.execute(updatedCmd)
+                updateGameState(newBoard, newHistory, newTurn)
+                emitMoveEvent(from.toString, to.toString, captured, newTurn)
+                notifyObservers(CheckDetectedEvent(currentBoard, currentHistory, currentTurn))
+
+              case MoveResult.Checkmate(winner) =>
+                // Move resulted in checkmate
+                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
+                invoker.execute(updatedCmd)
+                currentBoard = Board.initial
+                currentHistory = GameHistory.empty
+                currentTurn = Color.White
+                notifyObservers(CheckmateEvent(currentBoard, currentHistory, currentTurn, winner))
+
+              case MoveResult.Stalemate =>
+                // Move resulted in stalemate
+                val updatedCmd = cmd.copy(moveResult = Some(de.nowchess.chess.command.MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)))
+                invoker.execute(updatedCmd)
+                currentBoard = Board.initial
+                currentHistory = GameHistory.empty
+                currentTurn = Color.White
+                notifyObservers(StalemateEvent(currentBoard, currentHistory, currentTurn))
+  }
+
+  /** Undo the last move. */
+  def undo(): Unit = synchronized {
+    performUndo()
+  }
+
+  /** Redo the last undone move. */
+  def redo(): Unit = synchronized {
+    performRedo()
+  }
+
+  /** Reset the board to initial position. */
+  def reset(): Unit = synchronized {
+    currentBoard = Board.initial
+    currentHistory = GameHistory.empty
+    currentTurn = Color.White
+    invoker.clear()
+    notifyObservers(BoardResetEvent(
+      currentBoard,
+      currentHistory,
+      currentTurn
+    ))
+  }
+
+  // ──── Private Helpers ────
+
+  private def performUndo(): Unit =
+    if invoker.canUndo then
+      val cmd = invoker.history(invoker.getCurrentIndex)
+      (cmd: @unchecked) match
+        case moveCmd: MoveCommand =>
+          moveCmd.previousBoard.foreach(currentBoard = _)
+          moveCmd.previousHistory.foreach(currentHistory = _)
+          moveCmd.previousTurn.foreach(currentTurn = _)
+          invoker.undo()
+          notifyObservers(BoardResetEvent(currentBoard, currentHistory, currentTurn))
+    else
+      notifyObservers(InvalidMoveEvent(currentBoard, currentHistory, currentTurn, "Nothing to undo."))
+
+  private def performRedo(): Unit =
+    if invoker.canRedo then
+      val cmd = invoker.history(invoker.getCurrentIndex + 1)
+      (cmd: @unchecked) match
+        case moveCmd: MoveCommand =>
+          for case de.nowchess.chess.command.MoveResult.Successful(nb, nh, nt, cap) <- moveCmd.moveResult do
+            updateGameState(nb, nh, nt)
+            invoker.redo()
+            emitMoveEvent(moveCmd.from.toString, moveCmd.to.toString, cap, nt)
+    else
+      notifyObservers(InvalidMoveEvent(currentBoard, currentHistory, currentTurn, "Nothing to redo."))
+
+  private def updateGameState(newBoard: Board, newHistory: GameHistory, newTurn: Color): Unit =
+    currentBoard = newBoard
+    currentHistory = newHistory
+    currentTurn = newTurn
+
+  private def emitMoveEvent(fromSq: String, toSq: String, captured: Option[Piece], newTurn: Color): Unit =
+    val capturedDesc = captured.map(c => s"${c.color.label} ${c.pieceType.label}")
+    notifyObservers(MoveExecutedEvent(
+      currentBoard,
+      currentHistory,
+      newTurn,
+      fromSq,
+      toSq,
+      capturedDesc
+    ))
+
+  private def handleFailedMove(moveInput: String): Unit =
+    (GameController.processMove(currentBoard, currentHistory, currentTurn, moveInput): @unchecked) match
+      case MoveResult.NoPiece =>
+        notifyObservers(InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "No piece on that square."
+        ))
+      case MoveResult.WrongColor =>
+        notifyObservers(InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "That is not your piece."
+        ))
+      case MoveResult.IllegalMove =>
+        notifyObservers(InvalidMoveEvent(
+          currentBoard,
+          currentHistory,
+          currentTurn,
+          "Illegal move."
+        ))
+
diff --git a/modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala b/modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala
new file mode 100644
index 0000000..3ed526b
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/observer/Observer.scala
@@ -0,0 +1,87 @@
+package de.nowchess.chess.observer
+
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+
+/** Base trait for all game state events.
+ *  Events are immutable snapshots of game state changes.
+ */
+sealed trait GameEvent:
+  def board: Board
+  def history: GameHistory
+  def turn: Color
+
+/** Fired when a move is successfully executed. */
+case class MoveExecutedEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  fromSquare: String,
+  toSquare: String,
+  capturedPiece: Option[String]
+) extends GameEvent
+
+/** Fired when the current player is in check. */
+case class CheckDetectedEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired when the game reaches checkmate. */
+case class CheckmateEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  winner: Color
+) extends GameEvent
+
+/** Fired when the game reaches stalemate. */
+case class StalemateEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Fired when a move is invalid. */
+case class InvalidMoveEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color,
+  reason: String
+) extends GameEvent
+
+/** Fired when the board is reset. */
+case class BoardResetEvent(
+  board: Board,
+  history: GameHistory,
+  turn: Color
+) extends GameEvent
+
+/** Observer trait: implement to receive game state updates. */
+trait Observer:
+  def onGameEvent(event: GameEvent): Unit
+
+/** Observable trait: manages observers and notifies them of events. */
+trait Observable:
+  private val observers = scala.collection.mutable.Set[Observer]()
+
+  /** Register an observer to receive game events. */
+  def subscribe(observer: Observer): Unit = synchronized {
+    observers += observer
+  }
+
+  /** Unregister an observer. */
+  def unsubscribe(observer: Observer): Unit = synchronized {
+    observers -= observer
+  }
+
+  /** Notify all observers of a game event. */
+  protected def notifyObservers(event: GameEvent): Unit = synchronized {
+    observers.foreach(_.onGameEvent(event))
+  }
+
+  /** Return current list of observers (for testing). */
+  def observerCount: Int = synchronized {
+    observers.size
+  }
diff --git a/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerBranchTest.scala b/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerBranchTest.scala
new file mode 100644
index 0000000..b8ab9a4
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerBranchTest.scala
@@ -0,0 +1,216 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class CommandInvokerBranchTest extends AnyFunSuite with Matchers:
+  
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+
+  // ──── Helper: Command that always fails ────
+  private case class FailingCommand() extends Command:
+    override def execute(): Boolean = false
+    override def undo(): Boolean = false
+    override def description: String = "Failing command"
+
+  // ──── Helper: Command that conditionally fails on undo or execute ────
+  private case class ConditionalFailCommand(var shouldFailOnUndo: Boolean = false, var shouldFailOnExecute: Boolean = false) extends Command:
+    override def execute(): Boolean = !shouldFailOnExecute
+    override def undo(): Boolean = !shouldFailOnUndo
+    override def description: String = "Conditional fail"
+
+  private def createMoveCommand(from: Square, to: Square, executeSucceeds: Boolean = true): MoveCommand =
+    val cmd = MoveCommand(
+      from = from,
+      to = to,
+      moveResult = if executeSucceeds then Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)) else None,
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+    cmd
+
+  // ──── BRANCH: execute() returns false ────
+  test("CommandInvoker.execute() with failing command returns false"):
+    val invoker = new CommandInvoker()
+    val cmd = FailingCommand()
+    invoker.execute(cmd) shouldBe false
+    invoker.history.size shouldBe 0
+    invoker.getCurrentIndex shouldBe -1
+
+  test("CommandInvoker.execute() does not add failed command to history"):
+    val invoker = new CommandInvoker()
+    val failingCmd = FailingCommand()
+    val successCmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    
+    invoker.execute(failingCmd) shouldBe false
+    invoker.history.size shouldBe 0
+    
+    invoker.execute(successCmd) shouldBe true
+    invoker.history.size shouldBe 1
+    invoker.history(0) shouldBe successCmd
+
+  // ──── BRANCH: undo() with invalid index (currentIndex < 0) ────
+  test("CommandInvoker.undo() returns false when currentIndex < 0"):
+    val invoker = new CommandInvoker()
+    // currentIndex starts at -1
+    invoker.undo() shouldBe false
+
+  test("CommandInvoker.undo() returns false when empty history"):
+    val invoker = new CommandInvoker()
+    invoker.canUndo shouldBe false
+    invoker.undo() shouldBe false
+
+  // ──── BRANCH: undo() with invalid index (currentIndex >= size) ────
+  test("CommandInvoker.undo() returns false when currentIndex >= history size"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex now = 1, history.size = 2
+    
+    invoker.undo()  // currentIndex becomes 0
+    invoker.undo()  // currentIndex becomes -1
+    invoker.undo()  // currentIndex still -1, should fail
+
+  // ──── BRANCH: undo() command returns false ────
+  test("CommandInvoker.undo() returns false when command.undo() fails"):
+    val invoker = new CommandInvoker()
+    val failingCmd = ConditionalFailCommand(shouldFailOnUndo = true)
+    
+    invoker.execute(failingCmd) shouldBe true
+    invoker.canUndo shouldBe true
+    
+    invoker.undo() shouldBe false
+    // Index should not change when undo fails
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker.undo() returns true when command.undo() succeeds"):
+    val invoker = new CommandInvoker()
+    val successCmd = ConditionalFailCommand(shouldFailOnUndo = false)
+    
+    invoker.execute(successCmd) shouldBe true
+    invoker.undo() shouldBe true
+    invoker.getCurrentIndex shouldBe -1
+
+  // ──── BRANCH: redo() with invalid index (currentIndex + 1 >= size) ────
+  test("CommandInvoker.redo() returns false when nothing to redo"):
+    val invoker = new CommandInvoker()
+    invoker.redo() shouldBe false
+
+  test("CommandInvoker.redo() returns false when at end of history"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    
+    invoker.execute(cmd)
+    // currentIndex = 0, history.size = 1
+    invoker.canRedo shouldBe false
+    invoker.redo() shouldBe false
+
+  test("CommandInvoker.redo() returns false when currentIndex + 1 >= size"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex = 1, size = 2, currentIndex + 1 = 2, so 2 < 2 is false
+    invoker.canRedo shouldBe false
+    invoker.redo() shouldBe false
+
+  // ──── BRANCH: redo() command returns false ────
+  test("CommandInvoker.redo() returns false when command.execute() fails"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val redoFailCmd = ConditionalFailCommand(shouldFailOnExecute = false)  // Succeeds on first execute
+    
+    invoker.execute(cmd1)
+    invoker.execute(redoFailCmd)  // Succeeds and added to history
+    
+    invoker.undo()
+    // currentIndex = 0, redoFailCmd is at index 1
+    invoker.canRedo shouldBe true
+    
+    // Now modify to fail on next execute (redo)
+    redoFailCmd.shouldFailOnExecute = true
+    invoker.redo() shouldBe false
+    // currentIndex should not change
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker.redo() returns true when command.execute() succeeds"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    
+    invoker.execute(cmd) shouldBe true
+    invoker.undo() shouldBe true
+    invoker.redo() shouldBe true
+    invoker.getCurrentIndex shouldBe 0
+
+  // ──── BRANCH: execute() with redo history discarding (while loop) ────
+  test("CommandInvoker.execute() discards redo history via while loop"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex = 1, size = 2
+    
+    invoker.undo()
+    // currentIndex = 0, size = 2
+    // Redo history exists: cmd2 is at index 1
+    invoker.canRedo shouldBe true
+    
+    invoker.execute(cmd3)
+    // while loop should discard cmd2
+    invoker.canRedo shouldBe false
+    invoker.history.size shouldBe 2
+    invoker.history(1) shouldBe cmd3
+
+  test("CommandInvoker.execute() discards multiple redo commands"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.G, Rank.R1), sq(File.F, Rank.R3))
+    val cmd4 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    invoker.execute(cmd3)
+    invoker.execute(cmd4)
+    // currentIndex = 3, size = 4
+    
+    invoker.undo()
+    invoker.undo()
+    // currentIndex = 1, size = 4
+    // Redo history: cmd3 (idx 2), cmd4 (idx 3)
+    invoker.canRedo shouldBe true
+    
+    val newCmd = createMoveCommand(sq(File.B, Rank.R2), sq(File.B, Rank.R4))
+    invoker.execute(newCmd)
+    // While loop should discard indices 2 and 3 (cmd3 and cmd4)
+    invoker.history.size shouldBe 3
+    invoker.canRedo shouldBe false
+
+  // ──── BRANCH: execute() with no redo history to discard ────
+  test("CommandInvoker.execute() with no redo history (while condition false)"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    // currentIndex = 1, size = 2
+    // currentIndex < size - 1 is 1 < 1 which is false, so while loop doesn't run
+    
+    invoker.canRedo shouldBe false
+    
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    invoker.execute(cmd3)  // While loop condition should be false, no iterations
+    invoker.history.size shouldBe 3
+
diff --git a/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerTest.scala b/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerTest.scala
new file mode 100644
index 0000000..2e06aac
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerTest.scala
@@ -0,0 +1,123 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class CommandInvokerTest extends AnyFunSuite with Matchers:
+
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+  
+  private def createMoveCommand(from: Square, to: Square): MoveCommand =
+    MoveCommand(
+      from = from,
+      to = to,
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+
+  test("CommandInvoker executes a command and adds it to history"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd) shouldBe true
+    invoker.history.size shouldBe 1
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker executes multiple commands in sequence"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    invoker.execute(cmd1) shouldBe true
+    invoker.execute(cmd2) shouldBe true
+    invoker.history.size shouldBe 2
+    invoker.getCurrentIndex shouldBe 1
+
+  test("CommandInvoker.canUndo returns false when empty"):
+    val invoker = new CommandInvoker()
+    invoker.canUndo shouldBe false
+
+  test("CommandInvoker.canUndo returns true after execution"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.canUndo shouldBe true
+
+  test("CommandInvoker.undo decrements current index"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.getCurrentIndex shouldBe 0
+    invoker.undo() shouldBe true
+    invoker.getCurrentIndex shouldBe -1
+
+  test("CommandInvoker.canRedo returns true after undo"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.undo()
+    invoker.canRedo shouldBe true
+
+  test("CommandInvoker.redo re-executes a command"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.undo() shouldBe true
+    invoker.redo() shouldBe true
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker.canUndo returns false when at beginning"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.undo()
+    invoker.canUndo shouldBe false
+
+  test("CommandInvoker clear removes all history"):
+    val invoker = new CommandInvoker()
+    val cmd = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    invoker.execute(cmd)
+    invoker.clear()
+    invoker.history.size shouldBe 0
+    invoker.getCurrentIndex shouldBe -1
+
+  test("CommandInvoker discards all history when executing after undoing all"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    invoker.undo()
+    invoker.undo()
+    // After undoing twice, we're at the beginning (before any commands)
+    invoker.getCurrentIndex shouldBe -1
+    invoker.canRedo shouldBe true
+    // Executing a new command from the beginning discards all redo history
+    invoker.execute(cmd3)
+    invoker.canRedo shouldBe false
+    invoker.history.size shouldBe 1
+    invoker.history(0) shouldBe cmd3
+    invoker.getCurrentIndex shouldBe 0
+
+  test("CommandInvoker discards redo history when executing mid-history"):
+    val invoker = new CommandInvoker()
+    val cmd1 = createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
+    val cmd2 = createMoveCommand(sq(File.E, Rank.R7), sq(File.E, Rank.R5))
+    val cmd3 = createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4))
+    invoker.execute(cmd1)
+    invoker.execute(cmd2)
+    invoker.undo()
+    // After one undo, we're at the end of cmd1
+    invoker.getCurrentIndex shouldBe 0
+    invoker.canRedo shouldBe true
+    // Executing a new command discards cmd2 (the redo history)
+    invoker.execute(cmd3)
+    invoker.canRedo shouldBe false
+    invoker.history.size shouldBe 2
+    invoker.history(0) shouldBe cmd1
+    invoker.history(1) shouldBe cmd3
+    invoker.getCurrentIndex shouldBe 1
+
diff --git a/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerThreadSafetyTest.scala b/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerThreadSafetyTest.scala
new file mode 100644
index 0000000..8b6215d
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/command/CommandInvokerThreadSafetyTest.scala
@@ -0,0 +1,131 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+import scala.collection.mutable
+
+class CommandInvokerThreadSafetyTest extends AnyFunSuite with Matchers:
+
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+
+  private def createMoveCommand(from: Square, to: Square): MoveCommand =
+    MoveCommand(
+      from = from,
+      to = to,
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+
+  test("CommandInvoker is thread-safe for concurrent execute and history reads"):
+    val invoker = new CommandInvoker()
+    @volatile var raceDetected = false
+    val exceptions = mutable.ListBuffer[Exception]()
+
+    // Thread 1: executes commands
+    val executorThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for i <- 1 to 1000 do
+            val cmd = createMoveCommand(
+              sq(File.E, Rank.R2),
+              sq(File.E, Rank.R4)
+            )
+            invoker.execute(cmd)
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    // Thread 2: reads history during execution
+    val readerThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 1000 do
+            val _ = invoker.history
+            val _ = invoker.getCurrentIndex
+            Thread.sleep(0) // Yield to increase contention
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    executorThread.start()
+    readerThread.start()
+    executorThread.join()
+    readerThread.join()
+
+    exceptions.isEmpty shouldBe true
+    raceDetected shouldBe false
+
+  test("CommandInvoker is thread-safe for concurrent execute, undo, and redo"):
+    val invoker = new CommandInvoker()
+    @volatile var raceDetected = false
+    val exceptions = mutable.ListBuffer[Exception]()
+
+    // Pre-populate with some commands
+    for _ <- 1 to 5 do
+      invoker.execute(createMoveCommand(sq(File.E, Rank.R2), sq(File.E, Rank.R4)))
+
+    // Thread 1: executes new commands
+    val executorThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            invoker.execute(createMoveCommand(sq(File.D, Rank.R2), sq(File.D, Rank.R4)))
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    // Thread 2: undoes commands
+    val undoThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            if invoker.canUndo then
+              invoker.undo()
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    // Thread 3: redoes commands
+    val redoThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            if invoker.canRedo then
+              invoker.redo()
+        } catch {
+          case e: Exception =>
+            exceptions += e
+            raceDetected = true
+        }
+      }
+    })
+
+    executorThread.start()
+    undoThread.start()
+    redoThread.start()
+    executorThread.join()
+    undoThread.join()
+    redoThread.join()
+
+    exceptions.isEmpty shouldBe true
+    raceDetected shouldBe false
diff --git a/modules/core/src/test/scala/de/nowchess/chess/command/CommandTest.scala b/modules/core/src/test/scala/de/nowchess/chess/command/CommandTest.scala
new file mode 100644
index 0000000..ca71cdc
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/command/CommandTest.scala
@@ -0,0 +1,52 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class CommandTest extends AnyFunSuite with Matchers:
+
+  test("QuitCommand can be created"):
+    val cmd = QuitCommand()
+    cmd shouldNot be(null)
+
+  test("QuitCommand execute returns true"):
+    val cmd = QuitCommand()
+    cmd.execute() shouldBe true
+
+  test("QuitCommand undo returns false (cannot undo quit)"):
+    val cmd = QuitCommand()
+    cmd.undo() shouldBe false
+
+  test("QuitCommand description"):
+    val cmd = QuitCommand()
+    cmd.description shouldBe "Quit game"
+
+  test("ResetCommand with no prior state"):
+    val cmd = ResetCommand()
+    cmd.execute() shouldBe true
+    cmd.undo() shouldBe false
+
+  test("ResetCommand with prior state can undo"):
+    val cmd = ResetCommand(
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+    cmd.execute() shouldBe true
+    cmd.undo() shouldBe true
+
+  test("ResetCommand with partial state cannot undo"):
+    val cmd = ResetCommand(
+      previousBoard = Some(Board.initial),
+      previousHistory = None,  // missing
+      previousTurn = Some(Color.White)
+    )
+    cmd.execute() shouldBe true
+    cmd.undo() shouldBe false
+
+  test("ResetCommand description"):
+    val cmd = ResetCommand()
+    cmd.description shouldBe "Reset board"
+
diff --git a/modules/core/src/test/scala/de/nowchess/chess/command/MoveCommandImmutabilityTest.scala b/modules/core/src/test/scala/de/nowchess/chess/command/MoveCommandImmutabilityTest.scala
new file mode 100644
index 0000000..b23350a
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/command/MoveCommandImmutabilityTest.scala
@@ -0,0 +1,65 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class MoveCommandImmutabilityTest extends AnyFunSuite with Matchers:
+
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+
+  test("MoveCommand should be immutable - fields cannot be mutated after creation"):
+    val cmd1 = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4)
+    )
+
+    // Create second command with filled state
+    val result = MoveResult.Successful(Board.initial, GameHistory.empty, Color.Black, None)
+    val cmd2 = cmd1.copy(
+      moveResult = Some(result),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+
+    // Original should be unchanged
+    cmd1.moveResult shouldBe None
+    cmd1.previousBoard shouldBe None
+    cmd1.previousHistory shouldBe None
+    cmd1.previousTurn shouldBe None
+
+    // New should have values
+    cmd2.moveResult shouldBe Some(result)
+    cmd2.previousBoard shouldBe Some(Board.initial)
+    cmd2.previousHistory shouldBe Some(GameHistory.empty)
+    cmd2.previousTurn shouldBe Some(Color.White)
+
+  test("MoveCommand equals and hashCode respect immutability"):
+    val cmd1 = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = None,
+      previousBoard = None,
+      previousHistory = None,
+      previousTurn = None
+    )
+
+    val cmd2 = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = None,
+      previousBoard = None,
+      previousHistory = None,
+      previousTurn = None
+    )
+
+    // Same values should be equal
+    cmd1 shouldBe cmd2
+    cmd1.hashCode shouldBe cmd2.hashCode
+
+    // Hash should be consistent (required for use as map keys)
+    val hash1 = cmd1.hashCode
+    val hash2 = cmd1.hashCode
+    hash1 shouldBe hash2
diff --git a/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala b/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
index 8124005..f5493b0 100644
--- a/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
+++ b/modules/core/src/test/scala/de/nowchess/chess/controller/GameControllerTest.scala
@@ -6,17 +6,12 @@ import de.nowchess.chess.logic.{CastleSide, GameHistory}
 import org.scalatest.funsuite.AnyFunSuite
 import org.scalatest.matchers.should.Matchers
 
-import java.io.ByteArrayInputStream
-
 class GameControllerTest extends AnyFunSuite with Matchers:
 
   private def sq(f: File, r: Rank): Square = Square(f, r)
   private def processMove(board: Board, history: GameHistory, turn: Color, raw: String): MoveResult =
     GameController.processMove(board, history, turn, raw)
 
-  private def gameLoop(board: Board, history: GameHistory, turn: Color): Unit =
-    GameController.gameLoop(board, history, turn)
-
   private def castlingRights(history: GameHistory, color: Color): CastlingRights =
     de.nowchess.chess.logic.CastlingRightsCalculator.deriveCastlingRights(history, color)
 
@@ -69,59 +64,6 @@ class GameControllerTest extends AnyFunSuite with Matchers:
         newTurn shouldBe Color.Black
       case other => fail(s"Expected Moved, got $other")
 
-  // ──── gameLoop ───────────────────────────────────────────────────────
-
-  private def withInput(input: String)(block: => Unit): Unit =
-    val stream = ByteArrayInputStream(input.getBytes("UTF-8"))
-    scala.Console.withIn(stream)(block)
-
-  test("gameLoop: 'quit' exits cleanly without exception"):
-    withInput("quit\n"):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: EOF (null readLine) exits via quit fallback"):
-    withInput(""):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: invalid format prints message and recurses until quit"):
-    withInput("badmove\nquit\n"):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: NoPiece prints message and recurses until quit"):
-    // E3 is empty in the initial position
-    withInput("e3e4\nquit\n"):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: WrongColor prints message and recurses until quit"):
-    // E7 has a Black pawn; it is White's turn
-    withInput("e7e6\nquit\n"):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: IllegalMove prints message and recurses until quit"):
-    withInput("e2e5\nquit\n"):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: legal non-capture move recurses with new board then quits"):
-    withInput("e2e4\nquit\n"):
-      gameLoop(Board.initial, GameHistory.empty, Color.White)
-
-  test("gameLoop: capture move prints capture message then recurses and quits"):
-    val captureBoard = Board(Map(
-      sq(File.E, Rank.R5) -> Piece.WhitePawn,
-      sq(File.D, Rank.R6) -> Piece.BlackPawn,
-      sq(File.H, Rank.R1) -> Piece.BlackKing,
-      sq(File.H, Rank.R8) -> Piece.WhiteKing
-    ))
-    withInput("e5d6\nquit\n"):
-      gameLoop(captureBoard, GameHistory.empty, Color.White)
-
-  // ──── helpers ────────────────────────────────────────────────────────
-
-  private def captureOutput(block: => Unit): String =
-    val out = java.io.ByteArrayOutputStream()
-    scala.Console.withOut(out)(block)
-    out.toString("UTF-8")
-
   // ──── processMove: check / checkmate / stalemate ─────────────────────
 
   test("processMove: legal move that delivers check returns MovedInCheck"):
@@ -161,56 +103,6 @@ class GameControllerTest extends AnyFunSuite with Matchers:
       case MoveResult.Stalemate => succeed
       case other => fail(s"Expected Stalemate, got $other")
 
-  // ──── gameLoop: check / checkmate / stalemate ─────────────────────────
-
-  test("gameLoop: checkmate prints winner message and resets to new game"):
-    // After Qa1-Qh8, position is checkmate; second "quit" exits the new game
-    val b = Board(Map(
-      sq(File.A, Rank.R1) -> Piece.WhiteQueen,
-      sq(File.A, Rank.R6) -> Piece.WhiteKing,
-      sq(File.A, Rank.R8) -> Piece.BlackKing
-    ))
-    val output = captureOutput:
-      withInput("a1h8\nquit\n"):
-        gameLoop(b, GameHistory.empty, Color.White)
-    output should include("Checkmate! White wins.")
-
-  test("gameLoop: stalemate prints draw message and resets to new game"):
-    val b = Board(Map(
-      sq(File.B, Rank.R1) -> Piece.WhiteQueen,
-      sq(File.C, Rank.R6) -> Piece.WhiteKing,
-      sq(File.A, Rank.R8) -> Piece.BlackKing
-    ))
-    val output = captureOutput:
-      withInput("b1b6\nquit\n"):
-        gameLoop(b, GameHistory.empty, Color.White)
-    output should include("Stalemate! The game is a draw.")
-
-  test("gameLoop: MovedInCheck without capture prints check message"):
-    val b = Board(Map(
-      sq(File.A, Rank.R1) -> Piece.WhiteRook,
-      sq(File.C, Rank.R3) -> Piece.WhiteKing,
-      sq(File.H, Rank.R8) -> Piece.BlackKing
-    ))
-    val output = captureOutput:
-      withInput("a1a8\nquit\n"):
-        gameLoop(b, GameHistory.empty, Color.White)
-    output should include("Black is in check!")
-
-  test("gameLoop: MovedInCheck with capture prints both capture and check message"):
-    // White Rook A1 captures Black Pawn on A8, Ra8 then attacks rank 8 putting Kh8 in check
-    val b = Board(Map(
-      sq(File.A, Rank.R1) -> Piece.WhiteRook,
-      sq(File.C, Rank.R3) -> Piece.WhiteKing,
-      sq(File.A, Rank.R8) -> Piece.BlackPawn,
-      sq(File.H, Rank.R8) -> Piece.BlackKing
-    ))
-    val output = captureOutput:
-      withInput("a1a8\nquit\n"):
-        gameLoop(b, GameHistory.empty, Color.White)
-    output should include("captures")
-    output should include("Black is in check!")
-
   // ──── castling execution ─────────────────────────────────────────────
 
   test("processMove: e1g1 returns Moved with king on g1 and rook on f1"):
diff --git a/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineEdgeCasesTest.scala b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineEdgeCasesTest.scala
new file mode 100644
index 0000000..6843e11
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineEdgeCasesTest.scala
@@ -0,0 +1,214 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import de.nowchess.chess.observer.{Observer, GameEvent, MoveExecutedEvent, CheckDetectedEvent, BoardResetEvent, InvalidMoveEvent}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+/** Tests for GameEngine edge cases and uncovered paths */
+class GameEngineEdgeCasesTest extends AnyFunSuite with Matchers:
+
+  test("GameEngine handles empty input"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("Please enter a valid move or command")
+
+  test("GameEngine processes quit command"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("quit")
+    // Quit just returns, no events
+    observer.events.isEmpty shouldBe true
+
+  test("GameEngine processes q command (short form)"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("q")
+    observer.events.isEmpty shouldBe true
+
+  test("GameEngine handles uppercase quit"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("QUIT")
+    observer.events.isEmpty shouldBe true
+
+  test("GameEngine handles undo on empty history"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.canUndo shouldBe false
+    engine.processUserInput("undo")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("Nothing to undo")
+
+  test("GameEngine handles redo on empty redo history"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.canRedo shouldBe false
+    engine.processUserInput("redo")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("Nothing to redo")
+
+  test("GameEngine parses invalid move format"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("invalid_move_format")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("Invalid move format")
+
+  test("GameEngine handles lowercase input normalization"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("  UNDO  ")  // With spaces and uppercase
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]  // No moves to undo yet
+
+  test("GameEngine preserves board state on invalid move"):
+    val engine = new GameEngine()
+    val initialBoard = engine.board
+    
+    engine.processUserInput("invalid")
+    
+    engine.board shouldBe initialBoard
+
+  test("GameEngine preserves turn on invalid move"):
+    val engine = new GameEngine()
+    val initialTurn = engine.turn
+    
+    engine.processUserInput("invalid")
+    
+    engine.turn shouldBe initialTurn
+
+  test("GameEngine undo with no commands available"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // Make a valid move
+    engine.processUserInput("e2e4")
+    observer.events.clear()
+    
+    // Undo it
+    engine.processUserInput("undo")
+    
+    // Board should be reset
+    engine.board shouldBe Board.initial
+    engine.turn shouldBe Color.White
+
+  test("GameEngine redo after undo"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("e2e4")
+    val boardAfterMove = engine.board
+    val turnAfterMove = engine.turn
+    observer.events.clear()
+    
+    engine.processUserInput("undo")
+    engine.processUserInput("redo")
+    
+    engine.board shouldBe boardAfterMove
+    engine.turn shouldBe turnAfterMove
+
+  test("GameEngine canUndo flag tracks state correctly"):
+    val engine = new GameEngine()
+    
+    engine.canUndo shouldBe false
+    engine.processUserInput("e2e4")
+    engine.canUndo shouldBe true
+    engine.processUserInput("undo")
+    engine.canUndo shouldBe false
+
+  test("GameEngine canRedo flag tracks state correctly"):
+    val engine = new GameEngine()
+    
+    engine.canRedo shouldBe false
+    engine.processUserInput("e2e4")
+    engine.canRedo shouldBe false
+    engine.processUserInput("undo")
+    engine.canRedo shouldBe true
+
+  test("GameEngine command history is accessible"):
+    val engine = new GameEngine()
+    
+    engine.commandHistory.isEmpty shouldBe true
+    engine.processUserInput("e2e4")
+    engine.commandHistory.size shouldBe 1
+
+  test("GameEngine processes multiple moves in sequence"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    observer.events.clear()
+    
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    
+    observer.events.size shouldBe 2
+    engine.commandHistory.size shouldBe 2
+
+  test("GameEngine can undo multiple moves"):
+    val engine = new GameEngine()
+    
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    
+    engine.processUserInput("undo")
+    engine.turn shouldBe Color.Black
+    
+    engine.processUserInput("undo")
+    engine.turn shouldBe Color.White
+
+  test("GameEngine thread-safe operations"):
+    val engine = new GameEngine()
+    
+    // Access from synchronized methods
+    val board = engine.board
+    val history = engine.history
+    val turn = engine.turn
+    val canUndo = engine.canUndo
+    val canRedo = engine.canRedo
+    
+    board shouldBe Board.initial
+    canUndo shouldBe false
+    canRedo shouldBe false
+
+
+private class MockObserver extends Observer:
+  val events = mutable.ListBuffer[GameEvent]()
+  
+  override def onGameEvent(event: GameEvent): Unit =
+    events += event
diff --git a/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineGameEndingTest.scala b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineGameEndingTest.scala
new file mode 100644
index 0000000..a6132c3
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineGameEndingTest.scala
@@ -0,0 +1,93 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import de.nowchess.chess.observer.{Observer, GameEvent, CheckDetectedEvent, CheckmateEvent, StalemateEvent}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+/** Tests for GameEngine check/checkmate/stalemate paths */
+class GameEngineGameEndingTest extends AnyFunSuite with Matchers:
+
+  test("GameEngine handles Checkmate (Fool's Mate)"):
+    val engine = new GameEngine()
+    val observer = new EndingMockObserver()
+    engine.subscribe(observer)
+    
+    // Play Fool's mate
+    engine.processUserInput("f2f3")
+    engine.processUserInput("e7e5")
+    engine.processUserInput("g2g4")
+    
+    observer.events.clear()
+    engine.processUserInput("d8h4")
+    
+    // Verify CheckmateEvent
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe a[CheckmateEvent]
+    
+    val event = observer.events.head.asInstanceOf[CheckmateEvent]
+    event.winner shouldBe Color.Black
+    
+    // Board should be reset after checkmate
+    engine.board shouldBe Board.initial
+    engine.turn shouldBe Color.White
+
+  test("GameEngine handles check detection"):
+    val engine = new GameEngine()
+    val observer = new EndingMockObserver()
+    engine.subscribe(observer)
+    
+    // Play a simple check
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    engine.processUserInput("f1c4")
+    engine.processUserInput("g8f6")
+    
+    observer.events.clear()
+    engine.processUserInput("c4f7") // Check!
+    
+    val checkEvents = observer.events.collect { case e: CheckDetectedEvent => e }
+    checkEvents.size shouldBe 1
+    checkEvents.head.turn shouldBe Color.Black // Black is now in check
+    
+  // Shortest known stalemate is 19 moves. Here is a faster one:
+  // e3 a5 Qh5 Ra6 Qxa5 h5 h4 Rah6 Qxc7 f6 Qxd7+ Kf7 Qxb7 Qd3 Qxb8 Qh7 Qxc8 Kg6 Qe6
+  // Wait, let's just use Sam Loyd's 10-move stalemate:
+  // 1. e3 a5 2. Qh5 Ra6 3. Qxa5 h5 4. h4 Rah6 5. Qxc7 f6 6. Qxd7+ Kf7 7. Qxb7 Qd3 8. Qxb8 Qh7 9. Qxc8 Kg6 10. Qe6
+  test("GameEngine handles Stalemate via 10-move known sequence"):
+    val engine = new GameEngine()
+    val observer = new EndingMockObserver()
+    engine.subscribe(observer)
+    
+    val moves = List(
+      "e2e3", "a7a5",
+      "d1h5", "a8a6",
+      "h5a5", "h7h5",
+      "h2h4", "a6h6",
+      "a5c7", "f7f6",
+      "c7d7", "e8f7",
+      "d7b7", "d8d3",
+      "b7b8", "d3h7",
+      "b8c8", "f7g6",
+      "c8e6"
+    )
+    
+    moves.dropRight(1).foreach(engine.processUserInput)
+    
+    observer.events.clear()
+    engine.processUserInput(moves.last)
+    
+    val stalemateEvents = observer.events.collect { case e: StalemateEvent => e }
+    stalemateEvents.size shouldBe 1
+    
+    // Board should be reset after stalemate
+    engine.board shouldBe Board.initial
+    engine.turn shouldBe Color.White
+
+private class EndingMockObserver extends Observer:
+  val events = mutable.ListBuffer[GameEvent]()
+  
+  override def onGameEvent(event: GameEvent): Unit =
+    events += event
\ No newline at end of file
diff --git a/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineHandleFailedMoveTest.scala b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineHandleFailedMoveTest.scala
new file mode 100644
index 0000000..6401cae
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineHandleFailedMoveTest.scala
@@ -0,0 +1,110 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import de.nowchess.chess.observer.{Observer, GameEvent, InvalidMoveEvent}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+/** Tests to maximize handleFailedMove coverage */
+class GameEngineHandleFailedMoveTest extends AnyFunSuite with Matchers:
+
+  test("GameEngine handles InvalidFormat error type"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("not_a_valid_move_format")
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val msg1 = observer.events.head.asInstanceOf[InvalidMoveEvent].reason
+    msg1 should include("Invalid move format")
+
+  test("GameEngine handles NoPiece error type"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("h3h4")
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val msg2 = observer.events.head.asInstanceOf[InvalidMoveEvent].reason
+    msg2 should include("No piece on that square")
+
+  test("GameEngine handles WrongColor error type"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("e2e4")  // White move
+    observer.events.clear()
+    
+    engine.processUserInput("a1b2")  // Try to move black's rook position with white's move (wrong color)
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val msg3 = observer.events.head.asInstanceOf[InvalidMoveEvent].reason
+    msg3 should include("That is not your piece")
+
+  test("GameEngine handles IllegalMove error type"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("e2e1")  // Try pawn backward
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val msg4 = observer.events.head.asInstanceOf[InvalidMoveEvent].reason
+    msg4 should include("Illegal move")
+
+  test("GameEngine invalid move message for InvalidFormat"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("xyz123")
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("coordinate notation")
+
+  test("GameEngine invalid move message for NoPiece"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("a3a4")  // a3 is empty
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("No piece")
+
+  test("GameEngine invalid move message for WrongColor"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("e2e4")
+    observer.events.clear()
+    
+    engine.processUserInput("e4e5")  // e4 has white pawn, it's black's turn
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("not your piece")
+
+  test("GameEngine invalid move message for IllegalMove"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    engine.processUserInput("e2e1")  // Pawn can't move backward
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("Illegal move")
+
+  test("GameEngine board unchanged after each type of invalid move"):
+    val engine = new GameEngine()
+    val initial = engine.board
+    
+    engine.processUserInput("invalid")
+    engine.board shouldBe initial
+    
+    engine.processUserInput("h3h4")
+    engine.board shouldBe initial
+    
+    engine.processUserInput("e2e1")
+    engine.board shouldBe initial
diff --git a/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineInvalidMovesTest.scala b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineInvalidMovesTest.scala
new file mode 100644
index 0000000..2be9947
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineInvalidMovesTest.scala
@@ -0,0 +1,114 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import de.nowchess.chess.observer.{Observer, GameEvent, InvalidMoveEvent, MoveExecutedEvent}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+/** Tests for GameEngine invalid move handling via handleFailedMove */
+class GameEngineInvalidMovesTest extends AnyFunSuite with Matchers:
+
+  test("GameEngine handles no piece at source square"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // Try to move from h1 which may be empty or not have our piece
+    // We'll try from a clearly empty square
+    engine.processUserInput("h1h2")
+    
+    // Should get an InvalidMoveEvent about NoPiece
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+
+  test("GameEngine handles moving wrong color piece"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // White moves first
+    engine.processUserInput("e2e4")
+    observer.events.clear()
+    
+    // White tries to move again (should fail - it's black's turn)
+    // But we need to try a move that looks legal but has wrong color
+    // This is hard to test because we'd need to be black and move white's piece
+    // Let's skip this for now and focus on testable cases
+    
+    // Actually, let's try moving a square that definitely has the wrong piece
+    // Move a white pawn as black by reaching that position
+    engine.processUserInput("e7e5")
+    observer.events.clear()
+    
+    // Now try to move white's e4 pawn as black (it's black's turn but e4 is white)
+    engine.processUserInput("e4e5")
+    
+    observer.events.size shouldBe 1
+    val event = observer.events.head
+    event shouldBe an[InvalidMoveEvent]
+
+  test("GameEngine handles illegal move"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // A pawn can't move backward
+    engine.processUserInput("e2e1")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("Illegal move")
+
+  test("GameEngine handles pawn trying to move 3 squares"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // Pawn can only move 1 or 2 squares on first move, not 3
+    engine.processUserInput("e2e5")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+
+  test("GameEngine handles moving from empty square"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // h3 is empty in starting position
+    engine.processUserInput("h3h4")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[InvalidMoveEvent]
+    val event = observer.events.head.asInstanceOf[InvalidMoveEvent]
+    event.reason should include("No piece on that square")
+
+  test("GameEngine processes valid move after invalid attempt"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    
+    // Try invalid move
+    engine.processUserInput("h3h4")
+    observer.events.clear()
+    
+    // Make valid move
+    engine.processUserInput("e2e4")
+    
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe an[MoveExecutedEvent]
+
+  test("GameEngine maintains state after failed move attempt"):
+    val engine = new GameEngine()
+    val initialTurn = engine.turn
+    val initialBoard = engine.board
+    
+    // Try invalid move
+    engine.processUserInput("h3h4")
+    
+    // State should not change
+    engine.turn shouldBe initialTurn
+    engine.board shouldBe initialBoard
diff --git a/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala
new file mode 100644
index 0000000..755ddb8
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/engine/GameEngineTest.scala
@@ -0,0 +1,310 @@
+package de.nowchess.chess.engine
+
+import scala.collection.mutable
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import de.nowchess.chess.observer.{Observer, GameEvent, MoveExecutedEvent, CheckDetectedEvent, BoardResetEvent, InvalidMoveEvent}
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class GameEngineTest extends AnyFunSuite with Matchers:
+
+  test("GameEngine starts with initial board state"):
+    val engine = new GameEngine()
+    engine.board shouldBe Board.initial
+    engine.history shouldBe GameHistory.empty
+    engine.turn shouldBe Color.White
+
+  test("GameEngine accepts Observer subscription"):
+    val engine = new GameEngine()
+    val mockObserver = new MockObserver()
+    engine.subscribe(mockObserver)
+    engine.observerCount shouldBe 1
+
+  test("GameEngine notifies observers on valid move"):
+    val engine = new GameEngine()
+    val mockObserver = new MockObserver()
+    engine.subscribe(mockObserver)
+    engine.processUserInput("e2e4")
+    mockObserver.events.size shouldBe 1
+    mockObserver.events.head shouldBe a[MoveExecutedEvent]
+
+  test("GameEngine updates state after valid move"):
+    val engine = new GameEngine()
+    val initialTurn = engine.turn
+    engine.processUserInput("e2e4")
+    engine.turn shouldNot be(initialTurn)
+    engine.turn shouldBe Color.Black
+
+  test("GameEngine notifies observers on invalid move"):
+    val engine = new GameEngine()
+    val mockObserver = new MockObserver()
+    engine.subscribe(mockObserver)
+    engine.processUserInput("invalid_move")
+    mockObserver.events.size shouldBe 1
+
+  test("GameEngine notifies multiple observers"):
+    val engine = new GameEngine()
+    val observer1 = new MockObserver()
+    val observer2 = new MockObserver()
+    engine.subscribe(observer1)
+    engine.subscribe(observer2)
+    engine.processUserInput("e2e4")
+    observer1.events.size shouldBe 1
+    observer2.events.size shouldBe 1
+
+  test("GameEngine allows observer unsubscription"):
+    val engine = new GameEngine()
+    val mockObserver = new MockObserver()
+    engine.subscribe(mockObserver)
+    engine.unsubscribe(mockObserver)
+    engine.observerCount shouldBe 0
+
+  test("GameEngine unsubscribed observer receives no events"):
+    val engine = new GameEngine()
+    val mockObserver = new MockObserver()
+    engine.subscribe(mockObserver)
+    engine.unsubscribe(mockObserver)
+    engine.processUserInput("e2e4")
+    mockObserver.events.size shouldBe 0
+
+  test("GameEngine reset notifies observers and resets state"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    engine.reset()
+    engine.board shouldBe Board.initial
+    engine.turn shouldBe Color.White
+    observer.events.size shouldBe 1
+
+  test("GameEngine processes sequence of moves"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    observer.events.size shouldBe 2
+    engine.turn shouldBe Color.White
+
+  test("GameEngine is thread-safe for synchronized operations"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    val t = new Thread(() => engine.processUserInput("e2e4"))
+    t.start()
+    t.join()
+    observer.events.size shouldBe 1
+
+  test("GameEngine canUndo returns false initially"):
+    val engine = new GameEngine()
+    engine.canUndo shouldBe false
+
+  test("GameEngine canUndo returns true after move"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.canUndo shouldBe true
+
+  test("GameEngine canRedo returns false initially"):
+    val engine = new GameEngine()
+    engine.canRedo shouldBe false
+
+  test("GameEngine undo restores previous state"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    val boardAfterMove = engine.board
+    engine.undo()
+    engine.board shouldBe Board.initial
+    engine.turn shouldBe Color.White
+
+  test("GameEngine undo notifies observers"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    observer.events.clear()
+    engine.undo()
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe a[BoardResetEvent]
+
+  test("GameEngine redo replays undone move"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    val boardAfterMove = engine.board
+    engine.undo()
+    engine.redo()
+    engine.board shouldBe boardAfterMove
+    engine.turn shouldBe Color.Black
+
+  test("GameEngine canUndo false when nothing to undo"):
+    val engine = new GameEngine()
+    engine.canUndo shouldBe false
+    engine.processUserInput("e2e4")
+    engine.undo()
+    engine.canUndo shouldBe false
+
+  test("GameEngine canRedo true after undo"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.undo()
+    engine.canRedo shouldBe true
+
+  test("GameEngine canRedo false after redo"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.undo()
+    engine.redo()
+    engine.canRedo shouldBe false
+
+  test("GameEngine undo on empty history sends invalid event"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    engine.undo()
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe a[InvalidMoveEvent]
+
+  test("GameEngine redo on empty redo sends invalid event"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    engine.redo()
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe a[InvalidMoveEvent]
+
+  test("GameEngine undo via processUserInput"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    val boardAfterMove = engine.board
+    engine.processUserInput("undo")
+    engine.board shouldBe Board.initial
+
+  test("GameEngine redo via processUserInput"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    val boardAfterMove = engine.board
+    engine.processUserInput("undo")
+    engine.processUserInput("redo")
+    engine.board shouldBe boardAfterMove
+
+  test("GameEngine handles empty input"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    engine.processUserInput("")
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe a[InvalidMoveEvent]
+
+  test("GameEngine multiple undo/redo sequence"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    engine.processUserInput("g1f3")
+    
+    engine.turn shouldBe Color.Black
+    
+    engine.undo()
+    engine.turn shouldBe Color.White
+    
+    engine.undo()
+    engine.turn shouldBe Color.Black
+    
+    engine.undo()
+    engine.turn shouldBe Color.White
+    engine.board shouldBe Board.initial
+
+  test("GameEngine redo after multiple undos"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    engine.processUserInput("g1f3")
+    
+    engine.undo()
+    engine.undo()
+    engine.undo()
+    
+    engine.redo()
+    engine.turn shouldBe Color.Black
+    
+    engine.redo()
+    engine.turn shouldBe Color.White
+    
+    engine.redo()
+    engine.turn shouldBe Color.Black
+
+  test("GameEngine new move after undo clears redo history"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    engine.undo()
+    engine.canRedo shouldBe true
+    
+    engine.processUserInput("e7e6")  // Different move
+    engine.canRedo shouldBe false
+
+  test("GameEngine command history tracking"):
+    val engine = new GameEngine()
+    engine.commandHistory.size shouldBe 0
+    
+    engine.processUserInput("e2e4")
+    engine.commandHistory.size shouldBe 1
+    
+    engine.processUserInput("e7e5")
+    engine.commandHistory.size shouldBe 2
+
+  test("GameEngine quit input"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    val initialEvents = observer.events.size
+    engine.processUserInput("quit")
+    // quit should not produce an event
+    observer.events.size shouldBe initialEvents
+
+  test("GameEngine quit via q"):
+    val engine = new GameEngine()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    val initialEvents = observer.events.size
+    engine.processUserInput("q")
+    observer.events.size shouldBe initialEvents
+  
+  test("GameEngine undo notifies with BoardResetEvent after successful undo"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    observer.events.clear()
+
+    engine.undo()
+
+    // Should have received a BoardResetEvent on undo
+    observer.events.size should be > 0
+    observer.events.exists(_.isInstanceOf[BoardResetEvent]) shouldBe true
+
+  test("GameEngine redo notifies with MoveExecutedEvent after successful redo"):
+    val engine = new GameEngine()
+    engine.processUserInput("e2e4")
+    engine.processUserInput("e7e5")
+    val boardAfterSecondMove = engine.board
+
+    engine.undo()
+    val observer = new MockObserver()
+    engine.subscribe(observer)
+    observer.events.clear()
+
+    engine.redo()
+
+    // Should have received a MoveExecutedEvent for the redo
+    observer.events.size shouldBe 1
+    observer.events.head shouldBe a[MoveExecutedEvent]
+    engine.board shouldBe boardAfterSecondMove
+    engine.turn shouldBe Color.White
+
+  // Mock Observer for testing
+  private class MockObserver extends Observer:
+    val events = mutable.ListBuffer[GameEvent]()
+    override def onGameEvent(event: GameEvent): Unit =
+      events += event
+
diff --git a/modules/core/src/test/scala/de/nowchess/chess/engine/MoveCommandDefaultsTest.scala b/modules/core/src/test/scala/de/nowchess/chess/engine/MoveCommandDefaultsTest.scala
new file mode 100644
index 0000000..46df874
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/engine/MoveCommandDefaultsTest.scala
@@ -0,0 +1,110 @@
+package de.nowchess.chess.command
+
+import de.nowchess.api.board.{Square, File, Rank, Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class MoveCommandDefaultsTest extends AnyFunSuite with Matchers:
+  
+  private def sq(f: File, r: Rank): Square = Square(f, r)
+
+  // Tests for MoveCommand with default parameter values
+  test("MoveCommand with no moveResult defaults to None"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4)
+    )
+    cmd.moveResult shouldBe None
+    cmd.execute() shouldBe false
+
+  test("MoveCommand with no previousBoard defaults to None"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4)
+    )
+    cmd.previousBoard shouldBe None
+    cmd.undo() shouldBe false
+
+  test("MoveCommand with no previousHistory defaults to None"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4)
+    )
+    cmd.previousHistory shouldBe None
+    cmd.undo() shouldBe false
+
+  test("MoveCommand with no previousTurn defaults to None"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4)
+    )
+    cmd.previousTurn shouldBe None
+    cmd.undo() shouldBe false
+
+  test("MoveCommand description is always returned"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4)
+    )
+    cmd.description shouldBe "Move from e2 to e4"
+
+  test("MoveCommand execute returns false when moveResult is None"):
+    val cmd = MoveCommand(
+      from = sq(File.A, Rank.R1),
+      to = sq(File.B, Rank.R3)
+    )
+    cmd.execute() shouldBe false
+
+  test("MoveCommand undo returns false when any previous state is None"):
+    // Missing previousBoard
+    val cmd1 = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = None,
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+    cmd1.undo() shouldBe false
+
+    // Missing previousHistory
+    val cmd2 = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = None,
+      previousTurn = Some(Color.White)
+    )
+    cmd2.undo() shouldBe false
+
+    // Missing previousTurn
+    val cmd3 = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = None
+    )
+    cmd3.undo() shouldBe false
+
+  test("MoveCommand execute returns true when moveResult is defined"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None))
+    )
+    cmd.execute() shouldBe true
+
+  test("MoveCommand undo returns true when all previous states are defined"):
+    val cmd = MoveCommand(
+      from = sq(File.E, Rank.R2),
+      to = sq(File.E, Rank.R4),
+      moveResult = Some(MoveResult.Successful(Board.initial, GameHistory.empty, Color.White, None)),
+      previousBoard = Some(Board.initial),
+      previousHistory = Some(GameHistory.empty),
+      previousTurn = Some(Color.White)
+    )
+    cmd.undo() shouldBe true
diff --git a/modules/core/src/test/scala/de/nowchess/chess/main/MainTest.scala b/modules/core/src/test/scala/de/nowchess/chess/main/MainTest.scala
deleted file mode 100644
index 22553e8..0000000
--- a/modules/core/src/test/scala/de/nowchess/chess/main/MainTest.scala
+++ /dev/null
@@ -1,12 +0,0 @@
-package de.nowchess.chess.main
-
-import de.nowchess.chess.Main
-import java.io.ByteArrayInputStream
-import org.scalatest.funsuite.AnyFunSuite
-import org.scalatest.matchers.should.Matchers
-
-class MainTest extends AnyFunSuite with Matchers:
-
-  test("main exits cleanly when 'quit' is entered"):
-    scala.Console.withIn(ByteArrayInputStream("quit\n".getBytes("UTF-8"))):
-      Main.main(Array.empty)
diff --git a/modules/core/src/test/scala/de/nowchess/chess/observer/ObservableThreadSafetyTest.scala b/modules/core/src/test/scala/de/nowchess/chess/observer/ObservableThreadSafetyTest.scala
new file mode 100644
index 0000000..0a6192e
--- /dev/null
+++ b/modules/core/src/test/scala/de/nowchess/chess/observer/ObservableThreadSafetyTest.scala
@@ -0,0 +1,168 @@
+package de.nowchess.chess.observer
+
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+import scala.collection.mutable
+
+class ObservableThreadSafetyTest extends AnyFunSuite with Matchers:
+
+  private class TestObservable extends Observable:
+    def testNotifyObservers(event: GameEvent): Unit =
+      notifyObservers(event)
+
+  private class CountingObserver extends Observer:
+    @volatile private var eventCount = 0
+    @volatile private var lastEvent: Option[GameEvent] = None
+
+    def onGameEvent(event: GameEvent): Unit =
+      eventCount += 1
+      lastEvent = Some(event)
+
+  private def createTestEvent(): GameEvent =
+    BoardResetEvent(
+      board = Board.initial,
+      history = GameHistory.empty,
+      turn = Color.White
+    )
+
+  test("Observable is thread-safe for concurrent subscribe and notify"):
+    val observable = new TestObservable()
+    val testEvent = createTestEvent()
+    @volatile var raceConditionCaught = false
+
+    // Thread 1: repeatedly notifies observers with long iteration
+    val notifierThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500000 do
+            observable.testNotifyObservers(testEvent)
+        } catch {
+          case _: java.util.ConcurrentModificationException =>
+            raceConditionCaught = true
+        }
+      }
+    })
+
+    // Thread 2: rapidly subscribes/unsubscribes observers during notify
+    val subscriberThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500000 do
+            val obs = new CountingObserver()
+            observable.subscribe(obs)
+            observable.unsubscribe(obs)
+        } catch {
+          case _: java.util.ConcurrentModificationException =>
+            raceConditionCaught = true
+        }
+      }
+    })
+
+    notifierThread.start()
+    subscriberThread.start()
+    notifierThread.join()
+    subscriberThread.join()
+
+    raceConditionCaught shouldBe false
+
+  test("Observable is thread-safe for concurrent subscribe, unsubscribe, and notify"):
+    val observable = new TestObservable()
+    val testEvent = createTestEvent()
+    val exceptions = mutable.ListBuffer[Exception]()
+    val observers = mutable.ListBuffer[CountingObserver]()
+
+    // Pre-subscribe some observers
+    for _ <- 1 to 10 do
+      val obs = new CountingObserver()
+      observers += obs
+      observable.subscribe(obs)
+
+    // Thread 1: notifies observers
+    val notifierThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 5000 do
+            observable.testNotifyObservers(testEvent)
+        } catch {
+          case e: Exception => exceptions += e
+        }
+      }
+    })
+
+    // Thread 2: subscribes new observers
+    val subscriberThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 5000 do
+            val obs = new CountingObserver()
+            observable.subscribe(obs)
+        } catch {
+          case e: Exception => exceptions += e
+        }
+      }
+    })
+
+    // Thread 3: unsubscribes observers
+    val unsubscriberThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for i <- 1 to 5000 do
+            if observers.nonEmpty then
+              val obs = observers(i % observers.size)
+              observable.unsubscribe(obs)
+        } catch {
+          case e: Exception => exceptions += e
+        }
+      }
+    })
+
+    notifierThread.start()
+    subscriberThread.start()
+    unsubscriberThread.start()
+    notifierThread.join()
+    subscriberThread.join()
+    unsubscriberThread.join()
+
+    exceptions.isEmpty shouldBe true
+
+  test("Observable.observerCount is thread-safe during concurrent modifications"):
+    val observable = new TestObservable()
+    val exceptions = mutable.ListBuffer[Exception]()
+    val countResults = mutable.ListBuffer[Int]()
+
+    // Thread 1: subscribes observers
+    val subscriberThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            observable.subscribe(new CountingObserver())
+        } catch {
+          case e: Exception => exceptions += e
+        }
+      }
+    })
+
+    // Thread 2: reads observer count
+    val readerThread = new Thread(new Runnable {
+      def run(): Unit = {
+        try {
+          for _ <- 1 to 500 do
+            val count = observable.observerCount
+            countResults += count
+        } catch {
+          case e: Exception => exceptions += e
+        }
+      }
+    })
+
+    subscriberThread.start()
+    readerThread.start()
+    subscriberThread.join()
+    readerThread.join()
+
+    exceptions.isEmpty shouldBe true
+    // Count should never go backwards
+    for i <- 1 until countResults.size do
+      countResults(i) >= countResults(i - 1) shouldBe true
diff --git a/modules/ui/build.gradle.kts b/modules/ui/build.gradle.kts
new file mode 100644
index 0000000..f70d8c1
--- /dev/null
+++ b/modules/ui/build.gradle.kts
@@ -0,0 +1,73 @@
+plugins {
+    id("scala")
+    id("org.scoverage") version "8.1"
+    application
+}
+
+group = "de.nowchess"
+version = "1.0-SNAPSHOT"
+
+@Suppress("UNCHECKED_CAST")
+val versions = rootProject.extra["VERSIONS"] as Map<String, String>
+
+repositories {
+    mavenCentral()
+}
+
+scala {
+    scalaVersion = versions["SCALA3"]!!
+}
+
+scoverage {
+    scoverageVersion.set(versions["SCOVERAGE"]!!)
+}
+
+application {
+    mainClass.set("de.nowchess.ui.Main")
+}
+
+tasks.withType<ScalaCompile> {
+    scalaCompileOptions.additionalParameters = listOf("-encoding", "UTF-8")
+}
+
+tasks.named<JavaExec>("run") {
+    jvmArgs("-Dfile.encoding=UTF-8", "-Dstdout.encoding=UTF-8", "-Dstderr.encoding=UTF-8")
+    standardInput = System.`in`
+}
+
+dependencies {
+
+    implementation("org.scala-lang:scala3-compiler_3") {
+        version {
+            strictly(versions["SCALA3"]!!)
+        }
+    }
+    implementation("org.scala-lang:scala3-library_3") {
+        version {
+            strictly(versions["SCALA3"]!!)
+        }
+    }
+
+    implementation(project(":modules:core"))
+    implementation(project(":modules:api"))
+
+    testImplementation(platform("org.junit:junit-bom:5.13.4"))
+    testImplementation("org.junit.jupiter:junit-jupiter")
+    testImplementation("org.scalatest:scalatest_3:${versions["SCALATEST"]!!}")
+    testImplementation("co.helmethair:scalatest-junit-runner:${versions["SCALATEST_JUNIT"]!!}")
+
+    testRuntimeOnly("org.junit.platform:junit-platform-launcher")
+}
+
+tasks.test {
+    useJUnitPlatform {
+        includeEngines("scalatest")
+        testLogging {
+            events("passed", "skipped", "failed")
+        }
+    }
+    finalizedBy(tasks.reportScoverage)
+}
+tasks.reportScoverage {
+    dependsOn(tasks.test)
+}
diff --git a/modules/ui/src/main/scala/de/nowchess/ui/Main.scala b/modules/ui/src/main/scala/de/nowchess/ui/Main.scala
new file mode 100644
index 0000000..c8f5562
--- /dev/null
+++ b/modules/ui/src/main/scala/de/nowchess/ui/Main.scala
@@ -0,0 +1,15 @@
+package de.nowchess.ui
+
+import de.nowchess.chess.engine.GameEngine
+import de.nowchess.ui.terminal.TerminalUI
+
+/** Application entry point - starts the Terminal UI for the chess game. */
+object Main:
+  def main(args: Array[String]): Unit =
+    // Create the core game engine (single source of truth)
+    val engine = new GameEngine()
+
+    // Create and start the terminal UI
+    val tui = new TerminalUI(engine)
+    tui.start()
+
diff --git a/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala b/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala
new file mode 100644
index 0000000..5fc32af
--- /dev/null
+++ b/modules/ui/src/main/scala/de/nowchess/ui/terminal/TerminalUI.scala
@@ -0,0 +1,76 @@
+package de.nowchess.ui.terminal
+
+import scala.io.StdIn
+import de.nowchess.chess.engine.GameEngine
+import de.nowchess.chess.observer.{Observer, GameEvent, *}
+import de.nowchess.chess.view.Renderer
+
+/** Terminal UI that implements Observer pattern.
+ *  Subscribes to GameEngine and receives state change events.
+ *  Handles all I/O and user interaction in the terminal.
+ */
+class TerminalUI(engine: GameEngine) extends Observer:
+  private var running = true
+
+  /** Called by GameEngine whenever a game event occurs. */
+  override def onGameEvent(event: GameEvent): Unit =
+    event match
+      case e: MoveExecutedEvent =>
+        println()
+        print(Renderer.render(e.board))
+        e.capturedPiece.foreach: cap =>
+          println(s"Captured: $cap on ${e.toSquare}")
+        printPrompt(e.turn)
+
+      case e: CheckDetectedEvent =>
+        println(s"${e.turn.label} is in check!")
+
+      case e: CheckmateEvent =>
+        println(s"Checkmate! ${e.winner.label} wins.")
+        println()
+        print(Renderer.render(e.board))
+
+      case e: StalemateEvent =>
+        println("Stalemate! The game is a draw.")
+        println()
+        print(Renderer.render(e.board))
+
+      case e: InvalidMoveEvent =>
+        println(s"⚠️  ${e.reason}")
+
+      case e: BoardResetEvent =>
+        println("Board has been reset to initial position.")
+        println()
+        print(Renderer.render(e.board))
+        printPrompt(e.turn)
+
+  /** Start the terminal UI game loop. */
+  def start(): Unit =
+    // Register as observer
+    engine.subscribe(this)
+
+    // Show initial board
+    println()
+    print(Renderer.render(engine.board))
+    printPrompt(engine.turn)
+
+    // Game loop
+    while running do
+      val input = Option(StdIn.readLine()).getOrElse("quit").trim
+      input.toLowerCase match
+        case "quit" | "q" =>
+          running = false
+          println("Game over. Goodbye!")
+        case "" =>
+          printPrompt(engine.turn)
+        case _ =>
+          engine.processUserInput(input)
+
+    // Unsubscribe when done
+    engine.unsubscribe(this)
+
+  private def printPrompt(turn: de.nowchess.api.board.Color): Unit =
+    val undoHint = if engine.canUndo then " [undo]" else ""
+    val redoHint = if engine.canRedo then " [redo]" else ""
+    print(s"${turn.label}'s turn. Enter move (or 'quit'/'q' to exit)$undoHint$redoHint: ")
+
diff --git a/modules/ui/src/test/scala/de/nowchess/ui/MainTest.scala b/modules/ui/src/test/scala/de/nowchess/ui/MainTest.scala
new file mode 100644
index 0000000..dea2b2f
--- /dev/null
+++ b/modules/ui/src/test/scala/de/nowchess/ui/MainTest.scala
@@ -0,0 +1,22 @@
+package de.nowchess.ui
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+import java.io.{ByteArrayInputStream, ByteArrayOutputStream}
+
+class MainTest extends AnyFunSuite with Matchers {
+
+  test("main should execute and quit immediately when fed 'quit'") {
+    val in = new ByteArrayInputStream("quit\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        Main.main(Array.empty)
+      }
+    }
+    
+    val output = out.toString
+    output should include ("Game over. Goodbye!")
+  }
+}
diff --git a/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala b/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala
new file mode 100644
index 0000000..16ccba4
--- /dev/null
+++ b/modules/ui/src/test/scala/de/nowchess/ui/terminal/TerminalUITest.scala
@@ -0,0 +1,189 @@
+package de.nowchess.ui.terminal
+
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+import java.io.{ByteArrayInputStream, ByteArrayOutputStream}
+import de.nowchess.chess.engine.GameEngine
+import de.nowchess.chess.observer.*
+import de.nowchess.api.board.{Board, Color}
+import de.nowchess.chess.logic.GameHistory
+
+class TerminalUITest extends AnyFunSuite with Matchers {
+
+  test("TerminalUI should start, print initial state, and correctly respond to 'q'") {
+    val in = new ByteArrayInputStream("q\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    val output = out.toString
+    output should include("White's turn.")
+    output should include("Game over. Goodbye!")
+  }
+
+  test("TerminalUI should ignore empty inputs and re-print prompt") {
+    val in = new ByteArrayInputStream("\nq\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    val output = out.toString
+    // Prompt appears three times: Initial, after empty, on exit.
+    output.split("White's turn.").length should be > 2
+  }
+
+  test("TerminalUI should explicitly handle empty input by re-prompting") {
+    val in = new ByteArrayInputStream("\n\nq\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    val output = out.toString
+    // With two empty inputs, prompt should appear at least 4 times:
+    // 1. Initial board display
+    // 2. After first empty input
+    // 3. After second empty input
+    // 4. Before quit
+    val promptCount = output.split("White's turn.").length
+    promptCount should be >= 4
+    output should include("Game over. Goodbye!")
+  }
+
+  test("TerminalUI printPrompt should include undo and redo hints if engine returns true") {
+    val in = new ByteArrayInputStream("\nq\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    val engine = new GameEngine() {
+      // Stub engine to force undo/redo to true
+      override def canUndo: Boolean = true
+      override def canRedo: Boolean = true
+    }
+    
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    val output = out.toString
+    output should include("[undo]")
+    output should include("[redo]")
+  }
+
+  test("TerminalUI onGameEvent should properly format InvalidMoveEvent") {
+    val out = new ByteArrayOutputStream()
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withOut(out) {
+      ui.onGameEvent(InvalidMoveEvent(Board(Map.empty), GameHistory(), Color.Black, "Invalid move format"))
+    }
+
+    out.toString should include("⚠️")
+    out.toString should include("Invalid move format")
+  }
+
+  test("TerminalUI onGameEvent should properly format CheckDetectedEvent") {
+    val out = new ByteArrayOutputStream()
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withOut(out) {
+      ui.onGameEvent(CheckDetectedEvent(Board(Map.empty), GameHistory(), Color.Black))
+    }
+
+    out.toString should include("Black is in check!")
+  }
+
+  test("TerminalUI onGameEvent should properly format CheckmateEvent") {
+    val out = new ByteArrayOutputStream()
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withOut(out) {
+      ui.onGameEvent(CheckmateEvent(Board(Map.empty), GameHistory(), Color.Black, Color.White))
+    }
+
+    val ostr = out.toString
+    ostr should include("Checkmate! White wins.")
+  }
+
+  test("TerminalUI onGameEvent should properly format StalemateEvent") {
+    val out = new ByteArrayOutputStream()
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withOut(out) {
+      ui.onGameEvent(StalemateEvent(Board(Map.empty), GameHistory(), Color.Black))
+    }
+
+    out.toString should include("Stalemate! The game is a draw.")
+  }
+
+  test("TerminalUI onGameEvent should properly format BoardResetEvent") {
+    val out = new ByteArrayOutputStream()
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withOut(out) {
+      ui.onGameEvent(BoardResetEvent(Board(Map.empty), GameHistory(), Color.White))
+    }
+
+    out.toString should include("Board has been reset to initial position.")
+  }
+
+  test("TerminalUI onGameEvent should properly format MoveExecutedEvent with capturing piece") {
+    val out = new ByteArrayOutputStream()
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withOut(out) {
+      ui.onGameEvent(MoveExecutedEvent(Board(Map.empty), GameHistory(), Color.Black, "A1", "A8", Some("Knight(White)")))
+    }
+
+    out.toString should include("Captured: Knight(White) on A8") // Depending on how piece/coord serialize
+  }
+
+  test("TerminalUI processes valid move input via processUserInput") {
+    val in = new ByteArrayInputStream("e2e4\nq\n".getBytes)
+    val out = new ByteArrayOutputStream()
+
+    val engine = new GameEngine()
+    val ui = new TerminalUI(engine)
+
+    Console.withIn(in) {
+      Console.withOut(out) {
+        ui.start()
+      }
+    }
+
+    val output = out.toString
+    output should include("White's turn.")
+    output should include("Game over. Goodbye!")
+    // The move should have been processed and the board displayed
+    engine.turn shouldBe Color.Black
+  }
+}
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 4259047..f164a80 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -1,2 +1,2 @@
 rootProject.name = "NowChessSystems"
-include("modules:core", "modules:api")
\ No newline at end of file
+include("modules:core", "modules:api", "modules:ui")
\ No newline at end of file