script.js

/* --- Chess Game Logic & AI --- */

// Game State
let game = new Chess();
let boardEl = document.getElementById('board');
let selectedSquare = null;
let aiDepth = 2; // Default Medium
let playerColor = 'w'; // Player is white
let gameOver = false;

// Piece Unicode Map
const pieces = {
    'p': '♟', 'n': '♞', 'b': '♝', 'r': '♜', 'q': '♛', 'k': '♚',
    'P': '♟', 'N': '♞', 'B': '♝', 'R': '♜', 'Q': '♛', 'K': '♚'
};

// Piece Values for AI
const pieceValues = {
    p: 10, n: 30, b: 30, r: 50, q: 90, k: 900,
    P: -10, N: -30, B: -30, R: -50, Q: -90, K: -900 // From Black perspective
};

// Initial Render
document.addEventListener('DOMContentLoaded', () => {
    renderBoard();
    updateStatus();
});

// --- Board Rendering ---

function renderBoard() {
    boardEl.innerHTML = '';
    const boardState = game.board();

    // Last move highlight
    const history = game.history({ verbose: true });
    const lastMove = history.length ? history[history.length - 1] : null;

    for (let i = 0; i < 8; i++) {
        for (let j = 0; j < 8; j++) {
            const squareDiv = document.createElement('div');
            const isLight = (i + j) % 2 === 0;
            const squareName = String.fromCharCode(97 + j) + (8 - i);
            const piece = boardState[i][j];

            squareDiv.className = `square ${isLight ? 'light' : 'dark'}`;
            squareDiv.dataset.square = squareName;
            squareDiv.onclick = () => handleSquareClick(squareName);

            // Highlight selected
            if (selectedSquare === squareName) {
                squareDiv.classList.add('selected');
            }

            // Highlight last move
            if (lastMove && (lastMove.from === squareName || lastMove.to === squareName)) {
                squareDiv.classList.add('last-move');
            }

            // Highlight King in Check
            if (piece && piece.type === 'k' && piece.color === game.turn() && game.in_check()) {
                squareDiv.classList.add('check');
            }

            // Highlight valid moves if a piece is selected
            if (selectedSquare) {
                const moves = game.moves({ square: selectedSquare, verbose: true });
                const move = moves.find(m => m.to === squareName);
                if (move) {
                    if (move.flags.includes('c') || move.flags.includes('e')) {
                        squareDiv.classList.add('valid-capture');
                    } else {
                        squareDiv.classList.add('valid-move');
                    }
                }
            }

            // Render Piece
            if (piece) {
                const pieceSpan = document.createElement('span');
                pieceSpan.className = `piece ${piece.color === 'w' ? 'white' : 'black'}`;
                // Using standard chess unicode
                pieceSpan.textContent = pieces[piece.type]; 
                // Note: Standard unicode usually comes in outline/filled variants. 
                // For simplicity in this constrained env, we use text color to differentiate.
                if(piece.color === 'w') pieceSpan.textContent = pieces[piece.type.toUpperCase()];
                else pieceSpan.textContent = pieces[piece.type];
                
                squareDiv.appendChild(pieceSpan);
            }

            boardEl.appendChild(squareDiv);
        }
    }
}

// --- Interaction ---

function handleSquareClick(square) {
    if (gameOver || game.turn() !== playerColor) return;

    const piece = game.get(square);

    // Select a piece
    if (piece && piece.color === playerColor) {
        selectedSquare = square;
        playSound('move');
        renderBoard();
        return;
    }

    // Move piece
    if (selectedSquare) {
        const moves = game.moves({ square: selectedSquare, verbose: true });
        const move = moves.find(m => m.to === square);

        if (move) {
            game.move(move.san);
            selectedSquare = null;
            renderBoard();
            updateStatus();
            updateHistory();
            
            // Trigger AI after short delay
            if (!game.game_over()) {
                setTimeout(makeAIMove, 250);
            }
        } else {
            // Deselect if clicking invalid empty square
            if(!piece) {
                selectedSquare = null;
                renderBoard();
            }
        }
    }
}

// --- AI Engine (Minimax) ---

function makeAIMove() {
    if (game.game_over()) return;

    updateStatus("AI is thinking...");

    // Use timeout to allow UI update before heavy calculation
    setTimeout(() => {
        const bestMove = calculateBestMove(game, aiDepth);
        game.move(bestMove);
        renderBoard();
        updateStatus();
        updateHistory();
        playSound('move');
    }, 100);
}

function calculateBestMove(gameInstance, depth) {
    const possibleMoves = gameInstance.moves();
    
    if (possibleMoves.length === 0) return null;

    // Alpha-Beta Pruning
    let bestMove = -Infinity;
    let bestMoveFound = null;

    // Shuffle moves to add randomness to equal positions
    possibleMoves.sort(() => Math.random() - 0.5);

    for (const move of possibleMoves) {
        gameInstance.move(move);
        const value = minimax(gameInstance, depth - 1, -10000, 10000, false);
        gameInstance.undo();

        if (value >= bestMove) {
            bestMove = value;
            bestMoveFound = move;
        }
    }

    return bestMoveFound || possibleMoves[Math.floor(Math.random() * possibleMoves.length)];
}

function minimax(gameInstance, depth, alpha, beta, isMaximizingPlayer) {
    if (depth === 0) {
        return -evaluateBoard(gameInstance.board());
    }

    const possibleMoves = gameInstance.moves();

    if (possibleMoves.length === 0) {
        if (gameInstance.in_checkmate()) return isMaximizingPlayer ? -10000 : 10000; // Depth * value
        return 0; // Stalemate
    }

    if (isMaximizingPlayer) {
        let bestMove = -9999;
        for (const move of possibleMoves) {
            gameInstance.move(move);
            bestMove = Math.max(bestMove, minimax(gameInstance, depth - 1, alpha, beta, !isMaximizingPlayer));
            gameInstance.undo();
            alpha = Math.max(alpha, bestMove);
            if (beta <= alpha) return bestMove;
        }
        return bestMove;
    } else {
        let bestMove = 9999;
        for (const move of possibleMoves) {
            gameInstance.move(move);
            bestMove = Math.min(bestMove, minimax(gameInstance, depth - 1, alpha, beta, !isMaximizingPlayer));
            gameInstance.undo();
            beta = Math.min(beta, bestMove);
            if (beta <= alpha) return bestMove;
        }
        return bestMove;
    }
}

function evaluateBoard(board) {
    let totalEvaluation = 0;
    for (let i = 0; i < 8; i++) {
        for (let j = 0; j < 8; j++) {
            totalEvaluation += getPieceValue(board[i][j]);
        }
    }
    return totalEvaluation;
}

function getPieceValue(piece) {
    if (piece === null) return 0;
    // Simple logic: AI is Black, wants to minimize positive score (which favors white)
    // We want AI (Black) to maximize its own value.
    // Let's standardise: Positive = Good for Black, Negative = Good for White
    
    // Re-map for standard Minimax
    // If I am playing as Black (AI), I want positive score.
    let value = 0;
    if (piece.color === 'b') {
        value = pieceValues[piece.type];
    } else {
        value = pieceValues[piece.type.toUpperCase()]; // Negative values
    }
    
    // Add position bonuses (Simplified: center control)
    const centerBonus = (i, j) => {
        if (i >= 3 && i <= 4 && j >= 3 && j <= 4) return 1; 
        return 0;
    }

    // Adjust based on simple position
    if(piece.color === 'b') return value + (piece.type === 'n' ? centerBonus(i,j) : 0);
    return value;
}

// --- UI Helpers ---

function updateStatus(msg = null) {
    const statusEl = document.getElementById('game-status');
    
    if (msg) {
        statusEl.textContent = msg;
        return;
    }

    let status = '';
    const turn = game.turn() === 'w' ? 'White' : 'Black';

    if (game.in_checkmate()) {
        status = `Game Over: ${turn === 'White' ? 'Black' : 'White'} wins by checkmate!`;
        gameOver = true;
        playSound('capture');
    } else if (game.in_draw()) {
        status = 'Game Over: Draw!';
        gameOver = true;
    } else {
        status = `${turn} to move`;
        if (game.in_check()) status += ' (Check!)';
    }
    statusEl.textContent = status;
}

function updateHistory() {
    const historyEl = document.getElementById('move-history');
    const history = game.history();
    let html = '';
    
    for (let i = 0; i < history.length; i += 2) {
        const moveNum = Math.floor(i / 2) + 1;
        html += `<div class="flex gap-2 text-xs">
                    <span class="text-gray-500">${moveNum}.</span>
                    <span class="text-gray-200">${history[i]}</span>
                    ${history[i+1] ? `<span class="text-gray-200">${history[i+1]}</span>` : '<span class="opacity-20">...</span>'}
                 </div>`;
    }
    historyEl.innerHTML = html;
    historyEl.scrollTop = historyEl.scrollHeight;
}

function resetGame() {
    game.reset();
    gameOver = false;
    selectedSquare = null;
    updateStatus();
    renderBoard();
    document.getElementById('move-history').innerHTML = '<p class="text-center italic opacity-50">Game started...</p>';
    playSound('move');
}

function setDifficulty(level) {
    aiDepth = level;
    document.querySelectorAll('.diff-btn').forEach(btn => {
        btn.classList.remove('bg-primary', 'text-white', 'active');
        btn.classList.add('text-gray-400');
    });
    
    // Simple active state logic based on click text
    const buttons = document.querySelectorAll('.diff-btn');
    const index = level - 1;
    buttons[index].classList.add('bg-primary', 'text-white', 'active');
    buttons[index].classList.remove('text-gray-400');
    
    resetGame();
}

// Simple Sound Synth (Beeps)
const audioCtx = new (window.AudioContext || window.webkitAudioContext)();
function playSound(type) {
    if (audioCtx.state === 'suspended') audioCtx.resume();
    
    const osc = audioCtx.createOscillator();
    const gainNode = audioCtx.createGain();
    
    osc.connect(gainNode);
    gainNode.connect(audioCtx.destination);
    
    if (type === 'move') {
        osc.type = 'sine';
        osc.frequency.setValueAtTime(300, audioCtx.currentTime);
        osc.frequency.exponentialRampToValueAtTime(100, audioCtx.currentTime + 0.1);
        gainNode.gain.setValueAtTime(0.1, audioCtx.currentTime);
        gainNode.gain.exponentialRampToValueAtTime(0.01, audioCtx.currentTime + 0.1);
        osc.start();
        osc.stop(audioCtx.currentTime + 0.1);
    } else if (type === 'capture') {
        osc.type = 'triangle';
        osc.frequency.setValueAtTime(150, audioCtx.currentTime);
        gainNode.gain.setValueAtTime(0.2, audioCtx.currentTime);
        gainNode.gain.exponentialRampToValueAtTime(0.01, audioCtx.currentTime + 0.2);
        osc.start();
        osc.stop(audioCtx.currentTime + 0.2);
    }
}