chess-ai-kong/src/evaluation/evaluator-board.js

Chess AI based on Alpha-Beta pruning algorithm.

'use-strict';

var strategy = require('./strategy');
var _monitor = require('./../monitoring/monitoring');

//Settings
var castlingRate = 40;
var checkRate = 100;
//var checkMateRate = 200000;
//var staleMateRate = 150000;
//var movabilityRate = 5;

/**
 * Evaluate the current position for the current player (turn).
 *
 * @param position The current position and turn
 * @param strategyName The name of the strategy to use
 * @returns {number} The score (regarding the strategy currently set)
 */
function ratePositionAndPieces(position, strategyName) {

    var score = 0;

    var ind;
    for (ind = 0; ind < position.board.length; ind++) {
        var currentPiece = position.board[ind];
        if (currentPiece != null && currentPiece.side == position.turn) {
            score += strategy.getPieceScore(currentPiece, strategyName);
            score += strategy.getPositionScore(currentPiece, ind, strategyName);
        }
    }
    return score;
}

/**
 * Rate the defense (castlings).
 *
 * @param position The current position and turn
 * @returns {number} The score (regarding the strategy currently set)
 */
function rateKingSafety(position) {

    var score = 0;

    /**
     * TODO: increase the score for castled kings.
     */
    //Castlings: if castled, increase the score
    //if(castled) {
    //    score += castlingRate;
    //} else {
        //Castlings: if castling is not possible anymore, reduce the score
        if(!position.castlingFlags[position.turn].K) {
            score -= castlingRate;
        }

        if(!position.castlingFlags[position.turn].Q) {
            score -= castlingRate;
        }
    //}

    return score;
}

/**
 * Rate the Movability including checks and stale situations.
 *
 * @param position The current position and turn
 * @param depth The depth in the search algorithm
 * @param playerTurn True if the function is called to rate the player's turn, false if opponent
 * @returns {number} The score (regarding the strategy currently set)
 */
function rateMovability(position, depth, playerTurn) {

    var score = 0;

    //score += movesLength*movabilityRate;
    if(playerTurn) {
        //if (movesLength == 0) {
        //    if (position.check) {
        //        score -= checkMateRate * (depth+1);
        //    } else {
        //        score -= staleMateRate * (depth+1);
        //    }
        //} else
        if (position.check) {
            score -= checkRate * (depth+1);
        }
    }

    return score;
}

/**
 * Evaluate the board for the current player (turn).
 *
 * @param currentPosition The current position and turn
 * @param depth the depth in the search algorithm
 * @param strategyName The name of the strategy to use
 * @returns {number} The score (regarding the strategy currently set)
 */
function evaluateBoard(currentPosition, depth, strategyName) {
    _monitor.startWatch('evaluateBoard');
    var score = ratePositionAndPieces(currentPosition, strategyName);
    score += rateKingSafety(currentPosition);
    score += rateMovability(currentPosition, depth, true);
    currentPosition.turn = currentPosition.turn === 'W' ? 'B' : 'W';
    score -= ratePositionAndPieces(currentPosition, strategyName);
    score -= rateKingSafety(currentPosition);
    score -= rateMovability(currentPosition, depth, false);
    currentPosition.turn = currentPosition.turn === 'W' ? 'B' : 'W';
    _monitor.stopWatch('evaluateBoard');
    return score;
}

module.exports.evaluateBoard = evaluateBoard;