xchess/lib/pawn.js

Chess Engine

export {test, allMoves}

import {white, black} from './color.js'

import {
	Move,
	Capture,
	PawnDoubleMove,
	EnPassant,
	PromotionRequest,
	CaptureAndPromotionRequest,
} from './move.js'

const INIT_INDEX = 2;
const EN_PASSANT_INDEX = 5;
const PROMOTION_INDEX = 7;
const leftDX = -1;
const rightDX = 1;

function test(pawn, target, board){
	const source = board.find(pawn);
	if(source){
		if(pawn.color == white)
			return source.isTopD(target);
		if(pawn.color == black)
			return source.isBottomD(target);
	}	return false;
}

function * allMoves(pawn, context){

	const {board} = context;

	// Position
	const from = board.find(pawn);
	if(!from) return;
	const ownY = pawn.color.ownIndex(from.y);
	if(ownY > PROMOTION_INDEX) return;

	// Forward
	const forwardDY = pawn.color.ownDir(1);
	const doubleForwardDY = pawn.color.ownDir(2);
	const F = from.to(0, forwardDY);
	if(board.isPassable(F, pawn.color)){
		if(ownY == PROMOTION_INDEX)
			yield new PromotionRequest(pawn, from, F);
		else
			yield new Move(pawn, from, F);
		const D = from.to(0, doubleForwardDY);
		if(ownY == INIT_INDEX && board.isPassable(D, pawn.color))
			yield new PawnDoubleMove(pawn, from, D);
	}

	// Captures right / left
	yield * CaptureMoves(pawn, from, rightDX, forwardDY, ownY, context);
	yield * CaptureMoves(pawn, from, leftDX, forwardDY, ownY, context);
}

function * CaptureMoves(pawn, from, dx, dy, ownY, context){
	const {board} = context;
	const to = from.to(dx, dy);
	if(to){
		const capture = board.get(to);
		if(capture){
			if(capture.isCapturable(pawn.color, board)){
				if(ownY == PROMOTION_INDEX)
					yield new CaptureAndPromotionRequest(pawn, from, to, capture);
				else
					yield new Capture(pawn, from, to, capture);
			}
		} else if(ownY == EN_PASSANT_INDEX){
			const captureAt = from.to(dx, 0);
			const capture = board.get(captureAt);
			if(capture && context.doubleMovePawn === capture)
				yield new EnPassant(pawn, from, to, capture, captureAt);
		}
	}
}