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If LMR search fails high research at intemediate depth

Do not search immediately at full depth, but try a second
chance at lower depth. This is a feature that should scale
well because become important at high depths where we have
big reductions and also big savings in avoiding a costly
full depth search.

After 942 games at 1+0
Mod vs Orig +158 =645 -139  +7 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
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1 parent 0719470 commit ec0f0eba6b0dde1774169a8cea07c5dfd9222ab3 @mcostalba committed
Showing with 24 additions and 3 deletions.
  1. +24 −3 src/search.cpp
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27 src/search.cpp
@@ -1309,8 +1309,8 @@ namespace {
value = -search<PV>(pos, ss, -beta, -alpha, newDepth, ply+1, false, threadID);
else
{
- // Step 14. Reduced search
- // if the move fails high will be re-searched at full depth.
+ // Step 14. Reduced depth search
+ // If the move fails high will be re-searched at full depth.
bool doFullDepthSearch = true;
if ( depth >= 3 * OnePly
@@ -1325,6 +1325,16 @@ namespace {
value = -search<NonPV>(pos, ss, -(alpha+1), -alpha, newDepth-ss[ply].reduction, ply+1, true, threadID);
doFullDepthSearch = (value > alpha);
}
+
+ // The move failed high, but if reduction is very big we could
+ // face a false positive, retry with a less aggressive reduction,
+ // if the move fails high again then go with full depth search.
+ if (doFullDepthSearch && ss[ply].reduction > 2 * OnePly)
+ {
+ ss[ply].reduction = OnePly;
+ value = -search<NonPV>(pos, ss, -(alpha+1), -alpha, newDepth-ss[ply].reduction, ply+1, true, threadID);
+ doFullDepthSearch = (value > alpha);
+ }
}
// Step 15. Full depth search
@@ -1687,7 +1697,7 @@ namespace {
pos.do_move(move, st, ci, moveIsCheck);
// Step 14. Reduced search
- // if the move fails high will be re-searched at full depth.
+ // If the move fails high will be re-searched at full depth.
bool doFullDepthSearch = true;
if ( !dangerous
@@ -1702,6 +1712,17 @@ namespace {
value = -search<NonPV>(pos, ss, -(localAlpha+1), -localAlpha, newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID);
doFullDepthSearch = (value > localAlpha);
}
+
+ // The move failed high, but if reduction is very big we could
+ // face a false positive, retry with a less aggressive reduction,
+ // if the move fails high again then go with full depth search.
+ if (doFullDepthSearch && ss[sp->ply].reduction > 2 * OnePly)
+ {
+ ss[sp->ply].reduction = OnePly;
+ Value localAlpha = sp->alpha;
+ value = -search<NonPV>(pos, ss, -(localAlpha+1), -localAlpha, newDepth-ss[sp->ply].reduction, sp->ply+1, true, threadID);
+ doFullDepthSearch = (value > localAlpha);
+ }
}
// Step 15. Full depth search

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