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Better handling of quiet moves for move sorting.
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Code simplification.
Search parameters retuned.
Remove draw/mate handling in quiescence search.
lmr enhanced
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Richard Delorme committed Apr 13, 2024
1 parent c1b3129 commit b8ac6fb
Showing 1 changed file with 35 additions and 36 deletions.
71 changes: 35 additions & 36 deletions src/search.d
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Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
/*
* File search.d
* Best move search.
* © 2017-2022 Richard Delorme
* © 2017-2024 Richard Delorme
*/

module search;
Expand Down Expand Up @@ -71,7 +71,7 @@ final class TranspositionTable {
}

/* Look for a position in the hash table, return true if one is found and fill the /found/ entry */
bool probe(const Key k, ref Entry found) {
bool probe(const Key k, ref Entry found) const {
const size_t i = cast (size_t) (k.code & mask);
foreach (ref h; entry[i .. i + bucketSize]) if (h.code == k.code) {
found = h;
Expand Down Expand Up @@ -147,10 +147,12 @@ final class Search {

/* Store the move m into various heuristics: killers, refutation move, history */
void store(Move m, const int d, const ref Moves moves) {
if (m != killer[ply][0]) { killer[ply][1] = killer[ply][0]; killer[ply][0] = m; }
refutation[line[ply - 1] & Limits.move.mask] = m;
history.update(board, m, d * d, history.good);
for (int k = 0; m != moves[k]; ++k) history.update(board, moves[k], d * d, history.bad);
if (!m.isTactical(board)) {
if (m != killer[ply][0]) { killer[ply][1] = killer[ply][0]; killer[ply][0] = m; }
if (ply > 0) refutation[line[ply - 1] & Limits.move.mask] = m;
history.update(board, m, d * d, history.good);
for (int k = 0; m != moves[k]; ++k) history.update(board, moves[k], d * d, history.bad);
}
}

/* return true if enough time is available to search */
Expand All @@ -166,7 +168,8 @@ final class Search {
} else if (event.has("stop")) {
stop = true;
} else if (event.has("isready")) {
writeln("readyok"); event.peek();
writeln("readyok");
event.peek();
}
}
if (!checkTime(option.time.max)) stop = true;
Expand Down Expand Up @@ -211,27 +214,20 @@ final class Search {

++qsNodes;

// return 0 when the position is a Draw */
if (board.isDraw) return 0;

// compute mate score limits based on the search ply and return a mate score if the search is useless
bs = ply - Score.mate;
s = Score.mate - ply - 1;
if (s < β && (β = s) <= α) return s;

// stand pat: if not in check, compute the eval as best "quiet" score, and return it if it is good enough
if (!board.inCheck) {
bs = eval(board);
if ((bs > α) && (α = bs) >= β) return bs;
}
// else compute mate score limits based on the search ply as best score
} else bs = ply - Score.mate;

// compute the eval when the ply limit is reached
if (ply == Limits.ply.max) return eval(board);

// Generate all turbulent moves: capture, promotion, pawn push to 7th rank, check evasion, ...
moves.generate!false(board, history);

// loop over all good capture or chech evasion move
// loop over all good capture or check evasion move
while ((m = (i = moves.next).move) != 0) if (board.inCheck || i.isTactical) {
if (update!false(m)) s = -qs(-β, -α); else s = bs;
restore(m);
Expand All @@ -257,7 +253,7 @@ final class Search {
// Check if the search need to be stopped
if (abort()) return α;

// Once the search horizon is reached, call the quiescece search
// Once the search horizon is reached, call the quiescence search
if (d <= 0) return qs(α, β);

++pvsNodes;
Expand All @@ -268,7 +264,11 @@ final class Search {
// compute mate score limits based on the search ply and return a mate score if the search is useless
bs = ply - Score.mate;
s = Score.mate - ply - 1;
if (s < β && (β = s) <= α) return α;
if (s < β && (β = s) <= α) return s;

// Evaluate the position and return if the search ply limit is reached
v = sv[ply] = eval(board);
if (ply == Limits.ply.max) return v;

// Look at the transposition table and return a score if it is available
if (tt.probe(board.key, h) && !isPv) {
Expand All @@ -278,30 +278,28 @@ final class Search {
else if (h.bound == Bound.lower && s >= β) return s;
else if (h.bound == Bound.upper && s <= α) return s;
}
if ((h.bound != Bound.upper && s > v) || (h.bound != Bound.lower && s < v)) v = sv[ply] = s;
}

// Evaluate the position and return if the search ply limit is reached
v = sv[ply] = eval(board);
if (ply == Limits.ply.max) return v;

// Is the position tactical : in check or looking for a mate
// Is the position tactical: in check or looking for a mate?
const bool tactical = (board.inCheck || abs(v) >= Score.big || α >= Score.big || β <= -Score.big);
// Is the position suspicious : in the principal variation or the static evaluation increase at this ply
// Is the position suspicious: in the principal variation or the static evaluation increase at this ply?
const bool suspicious = (isPv || (ply >= 2 && sv[ply] > sv[ply - 2]));

// Some pruning on safe position
if (!tactical && !isPv) {
// futility pruning if the Eval is very good
if (v >= β + 243 * d - 124) return β;
// razoring if the eval is very bad
const int razor = α - 96 * d + 30;
const int razor = α - 80 * d + 30;
if (v <= razor) {
if (d <= 2) return qs(α, β);
else if (qs(razor, razor + 1) <= razor) return α;
}

// Null move pruning if the position is very good
// Pruning if the position is very good & player still has a figure at least
if (v >= β && (board.color[board.player] & ~(board.piece[Piece.pawn] | board.piece[Piece.king]))) {
// Futility pruning
if (v >= β + 180 * d - 110) return β;
// Null move
r = 3 + d / 4 + min((v - β) / 128, 3);
update(0);
s = -pvs(-β, -β + 1, d - r);
Expand Down Expand Up @@ -329,19 +327,20 @@ final class Search {
if (update(m)) {
++nLegal; // count legal moves
e = board.inCheck;
if (moves.isFirst(m)) s = -pvs(-β, -α, d + e - 1); // first move full width
if (nLegal == 1) s = -pvs(-β, -α, d + e - 1); // first move full width
else {
// reduce on quiet move & position.
r = (i.isTactical || e || tactical) ? 0 : reduce(d, ++quiet) + IIR;
// reduce less on suspicious position
// reduce less on suspicious position or good move
if (r && suspicious) --r;
// reduce ?
if (r && history.isGood(board[m.to], m.to)) --r;
// prune ?
if (r && quiet > (4 + d * d) / (2 - suspicious)) s = bs;
// get the score from the next ply
else {
// use a null window
s = -pvs(-α - 1, -α, d + e - r - 1);
// re-search using full window on best a bestmove or exact score
// re-search using full window on a bestmove or an exact score
if ((r && s > bs) || (α < s && s < β)) s = -pvs(-β, -α, d + e - 1);
}
}
Expand All @@ -353,8 +352,8 @@ final class Search {
if (s > bs && (bs = s) > α) {
// save it as best move
if (ply == 0) rootMoves.setBest(m, 0);
else tt.store(board.key, d + e, ply, tt.bound(bs, β), bs, m);
if (!i.isTactical) store(m, d + e, moves);
tt.store(board.key, d + e, ply, tt.bound(bs, β), bs, m);
store(m, d + e, moves);
if (isPv) pv[ply].set(m, pv[ply + 1]);
// return on β cutoff
if ((α = bs) >= β) break;
Expand Down Expand Up @@ -391,7 +390,7 @@ final class Search {
}

/* condition to keep searching the next ply */
bool persist(const int d, const int s) const { return !stop && checkTime(0.7 * option.time.max) && d <= option.depth.max && pvsNodes + qsNodes < option.nodes.max && mateIn(s) > option.mate.max; }
bool persist(const int d, const int s) const { return !stop && checkTime(0.6 * option.time.max) && d <= option.depth.max && pvsNodes + qsNodes < option.nodes.max && mateIn(s) > option.mate.max; }

/* clear the search state */
void clear() {
Expand Down

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