Computer Chess Club Archives


Search

Terms

Messages

Subject: Question about details of hashing (olithink)

Author: Michel Langeveld

Date: 11:31:24 01/22/04


Olithink uses hashing as follow in its search.c.
Questions in the code marked with //

int search(int on_move, int ply, int depth, int alpha, int beta, int do_null)
{
	int n, i, j, w, c, hflag, nolegal;
	Move bestmove = 0, move = 0;

	if ((++nodes & 0x1fff) == 0) {
		if (bioskey()) inputmove(pmove ? ONMOVE(pmove) : COL_N);
		if (!pmove && (nodes*10)/nps*250 > mytime) { sabort = 1; return alpha; }
	}

	if (ply && depth && checkfordraw(1)) return 0;
	pvlength[ply] = ply;

	c = attacked(kingpos[on_move], on_move);
	if (c) depth++;
	else if (nstack && IS_PROM(movestack[nstack-1])) depth++;
	if (depth < 0) depth = 0;

        //*** Ply gets updated each move ... Why does it not look in the hash
        //*** If ply == 0?

	hflag = lookUpH(depth, &w, &move);
	if (ply && hflag) {
		if (hflag == 1) if (w >= beta) return beta;
		if (hflag == 2) { do_null = 0; if (w < alpha) return alpha; }
	}

        //*** hflag == 0 means NULLMOVE
        //*** hflag == 1 means hashscore was >= beta
        //*** hflag == 2 means hashscore was <= alpha
        //*** hflag == 4 means EXACT bound

        //*** why doesn't it use EXACT and NULLMOVES, is this non-optimal impl?

	if (!c && do_null && depth && ply && bitcount(R000BitB[on_move] &
(~R000BitB[PAWN[on_move]])) > 3) {
		donullmove();
		w = -search(on_move^1, ply+1, depth - 3 - (depth > 5), -beta, -beta + 1, 0);
		undonullmove();
		if (w >= beta) {
			storeH(0, depth, w, 1);
			return w;
		}
	}

	if (!ply) move = pv[0][0];
	hashmv[ply] = move;
	hflag = move ? 1 : 0;
	nolegal = 1;
	n = 0;

	if (depth == 0 && !c) {
		w = eval(on_move, alpha, beta);
		if (w > alpha) alpha = w;
		if (alpha >= beta) return beta;

                //Funny QSearch implementation ... this generates
                //captures
		n = generate_moves(on_move, ply, 1);
		if (n == 0) return w;
		hflag = 0;
	}

	i = 0;
	do {
		if (i == 1) hflag = 0;
		if (n == 0 && !hflag) {
			if (c) 	n = generate_check_esc(on_move, ply, 0);
			else n = generate_moves(on_move, ply, 0);

			if (i == 1) {
					for (j = 0; j < n; j++) {
						if (IDENTMV(movelist[j][ply], hashmv[ply])) {
							movelist[j][ply] = movelist[n-1][ply];
							break;
						}
					}
				}
			if (n == 0) break;
			}
		if (!hflag) move = pick(ply, n-i);
		if (!depth && !c && P_TO(move) != EMPTY && swap(move) < -150) continue;

		domove(move);
		if (!c && attacked(kingpos[on_move], on_move)) { undomove(); continue;}
		w = -search(on_move^1, ply+1, depth-1, -beta, -alpha, 1);
		if (nolegal) nolegal=0;
		undomove();
		if (sabort) break;

		if (w >= beta)  {
			if (P_TO(move) == EMPTY)  {
			  killer2[ply] = killer[ply];
			  killer[ply] = move;
			}
			history[move & 0xFFFF] += depth;
			storeH(move, depth, w, 1);
			return beta;

                        //*** Why store score w, and return beta
                        //*** Doesn't this work bugs in the hand?
		}
		if (w > alpha) {
			pv[ply][ply] = bestmove = move;
			for (j = ply+1; j < pvlength[ply+1]; j++) pv[ply][j] = pv[ply+1][j];
			pvlength[ply] = pvlength[ply+1];
			alpha = w;
			if (alpha == 32499 - ply) break;
		}
	} while (++i < n || hflag);
	if (sabort) return alpha;

	if (nolegal) {
		if (c) return -32500 + ply;
		else if (depth) return 0;
	} else
	storeH(bestmove, depth, alpha, bestmove ? 4 : 2);
	return alpha;
}




This page took 0.04 seconds to execute

Last modified: Thu, 15 Apr 21 08:11:13 -0700

Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.