feat: add AlphaBetaSearch and bot implementation with difficulty levels

modules/bot/build.gradle.kts

@@ -0,0 +1,65 @@
+plugins {
+    id("scala")
+    id("org.scoverage") version "8.1"
+}
+
+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"]!!)
+}
+
+tasks.withType<ScalaCompile> {
+    scalaCompileOptions.additionalParameters = listOf("-encoding", "UTF-8")
+}
+
+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:api"))
+    implementation(project(":modules:io"))
+    implementation(project(":modules:rule"))
+    implementation("com.microsoft.onnxruntime:onnxruntime:1.19.2")
+
+    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("skipped", "failed")
+        }
+    }
+    finalizedBy(tasks.reportScoverage)
+}
+tasks.reportScoverage {
+    dependsOn(tasks.test)
+}

modules/bot/src/main/scala/de/nowchess/bot/Bot.scala

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+package de.nowchess.bot
+
+import de.nowchess.api.game.GameContext
+import de.nowchess.api.move.Move
+
+trait Bot {
+
+  def name: String
+  def nextMove(context: GameContext): Option[Move]
+
+}

modules/bot/src/main/scala/de/nowchess/bot/BotController.scala

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+package de.nowchess.bot
+
+object BotController {
+  
+  private var bots: Map[String, Bot] = Map.empty
+
+}

modules/bot/src/test/scala/de/nowchess/bot/AlphaBetaSearchTest.scala

@@ -0,0 +1,162 @@
+package de.nowchess.bot
+
+import de.nowchess.api.board.{Board, Color, File, Piece, PieceType, Rank, Square}
+import de.nowchess.api.game.GameContext
+import de.nowchess.api.move.{Move, MoveType}
+import de.nowchess.rules.RuleSet
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+import de.nowchess.rules.sets.DefaultRules
+
+class AlphaBetaSearchTest extends AnyFunSuite with Matchers:
+
+  test("bestMove on initial position returns a move"):
+    val search = AlphaBetaSearch(DefaultRules)
+    val move = search.bestMove(GameContext.initial, maxDepth = 2)
+    move should not be None
+
+  test("bestMove on a position with one legal move returns that move"):
+    // Create a simple position: White king on h1, Black rook on a2
+    // (set up so there's only one legal move available)
+    // For simplicity, just test that a position with forced mate returns a move
+    val search = AlphaBetaSearch(DefaultRules)
+    val context = GameContext.initial
+    val move = search.bestMove(context, maxDepth = 1)
+    move should not be None
+
+  test("bestMove returns None for initial position has no legal moves"):
+    // Use a stub RuleSet that returns empty legal moves
+    val stubRules = new RuleSet:
+      def candidateMoves(context: GameContext)(square: Square) = List()
+      def legalMoves(context: GameContext)(square: Square) = List()
+      def allLegalMoves(context: GameContext) = List()
+      def isCheck(context: GameContext) = false
+      def isCheckmate(context: GameContext) = true
+      def isStalemate(context: GameContext) = false
+      def isInsufficientMaterial(context: GameContext) = false
+      def isFiftyMoveRule(context: GameContext) = false
+      def applyMove(context: GameContext)(move: Move) = context
+
+    val search = AlphaBetaSearch(stubRules)
+    val move = search.bestMove(GameContext.initial, maxDepth = 2)
+    move should be(None)
+
+  test("transposition table is cleared at start of bestMove"):
+    val search = AlphaBetaSearch(DefaultRules)
+    val context = GameContext.initial
+    // Call bestMove twice and verify both work independently
+    val move1 = search.bestMove(context, maxDepth = 1)
+    val move2 = search.bestMove(context, maxDepth = 1)
+    move1 should be(move2)
+
+  test("quiescence captures are ordered"):
+    val search = AlphaBetaSearch(DefaultRules)
+    // A position with multiple captures to verify quiescence orders them
+    val context = GameContext.initial
+    val move = search.bestMove(context, maxDepth = 2)
+    // Just verify it completes without error
+    move.isDefined should be(true)
+
+  test("search respects alpha-beta bounds"):
+    // This is implicit in the structure, but we test via behavior
+    val search = AlphaBetaSearch(DefaultRules)
+    val context = GameContext.initial
+    val move = search.bestMove(context, maxDepth = 3)
+    move should not be None
+
+  test("iterative deepening finds a move at each depth"):
+    val search = AlphaBetaSearch(DefaultRules)
+    val context = GameContext.initial
+    // Searching to depth 3 should use iterative deepening (depths 1, 2, 3)
+    val move = search.bestMove(context, maxDepth = 3)
+    move should not be None
+
+  test("stalemate position returns score 0"):
+    // Create a stalemate stub: white to move, no legal moves, not checkmate
+    val stalematRules = new RuleSet:
+      def candidateMoves(context: GameContext)(square: Square) = List()
+      def legalMoves(context: GameContext)(square: Square) = List()
+      def allLegalMoves(context: GameContext) = List()
+      def isCheck(context: GameContext) = false
+      def isCheckmate(context: GameContext) = false
+      def isStalemate(context: GameContext) = true
+      def isInsufficientMaterial(context: GameContext) = false
+      def isFiftyMoveRule(context: GameContext) = false
+      def applyMove(context: GameContext)(move: Move) = context
+
+    val search = AlphaBetaSearch(stalematRules)
+    val move = search.bestMove(GameContext.initial, maxDepth = 1)
+    move should be(None)
+
+  test("insufficient material returns score 0"):
+    val insufficientRules = new RuleSet:
+      def candidateMoves(context: GameContext)(square: Square) = List()
+      def legalMoves(context: GameContext)(square: Square) = List()
+      def allLegalMoves(context: GameContext) = List()
+      def isCheck(context: GameContext) = false
+      def isCheckmate(context: GameContext) = false
+      def isStalemate(context: GameContext) = false
+      def isInsufficientMaterial(context: GameContext) = true
+      def isFiftyMoveRule(context: GameContext) = false
+      def applyMove(context: GameContext)(move: Move) = context
+
+    val search = AlphaBetaSearch(insufficientRules)
+    val move = search.bestMove(GameContext.initial, maxDepth = 1)
+    move should be(None)
+
+  test("fifty move rule returns score 0"):
+    val fiftyMoveRules = new RuleSet:
+      def candidateMoves(context: GameContext)(square: Square) = List()
+      def legalMoves(context: GameContext)(square: Square) = List()
+      def allLegalMoves(context: GameContext) = List()
+      def isCheck(context: GameContext) = false
+      def isCheckmate(context: GameContext) = false
+      def isStalemate(context: GameContext) = false
+      def isInsufficientMaterial(context: GameContext) = false
+      def isFiftyMoveRule(context: GameContext) = true
+      def applyMove(context: GameContext)(move: Move) = context
+
+    val search = AlphaBetaSearch(fiftyMoveRules)
+    val move = search.bestMove(GameContext.initial, maxDepth = 1)
+    move should be(None)
+
+  test("capture moves are recognized in quiescence search"):
+    // Create a position with a capture available
+    val board = Board(Map(
+      Square(File.E, Rank.R4) -> Piece.WhiteQueen,
+      Square(File.E, Rank.R5) -> Piece.BlackPawn
+    ))
+    val context = GameContext.initial.withBoard(board)
+
+    val captureMove = Move(Square(File.E, Rank.R4), Square(File.E, Rank.R5), MoveType.Normal(true))
+    val rulesWithCapture = new RuleSet:
+      def candidateMoves(context: GameContext)(square: Square) = List()
+      def legalMoves(context: GameContext)(square: Square) = List()
+      def allLegalMoves(context: GameContext) = List(captureMove)
+      def isCheck(context: GameContext) = false
+      def isCheckmate(context: GameContext) = false
+      def isStalemate(context: GameContext) = false
+      def isInsufficientMaterial(context: GameContext) = false
+      def isFiftyMoveRule(context: GameContext) = false
+      def applyMove(context: GameContext)(move: Move) = context
+
+    val search = AlphaBetaSearch(rulesWithCapture)
+    val move = search.bestMove(context, maxDepth = 1)
+    move should be(Some(captureMove))
+
+  test("non-capture moves are not included in quiescence"):
+    val quietMove = Move(Square(File.E, Rank.R4), Square(File.E, Rank.R5), MoveType.Normal(false))
+    val rulesQuiet = new RuleSet:
+      def candidateMoves(context: GameContext)(square: Square) = List()
+      def legalMoves(context: GameContext)(square: Square) = List()
+      def allLegalMoves(context: GameContext) = List(quietMove)
+      def isCheck(context: GameContext) = false
+      def isCheckmate(context: GameContext) = false
+      def isStalemate(context: GameContext) = false
+      def isInsufficientMaterial(context: GameContext) = false
+      def isFiftyMoveRule(context: GameContext) = false
+      def applyMove(context: GameContext)(move: Move) = context
+
+    val search = AlphaBetaSearch(rulesQuiet)
+    val move = search.bestMove(GameContext.initial, maxDepth = 1)
+    move should be(Some(quietMove))  // bestMove returns the quiet move since it's the only legal move