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Do not prune the move if we are still under mate

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If after the first tried 2 + int(depth) moves we still
have no any move that takes us out of a mate then do
not prune the following move, it is more important to
escape mate then speed up search.

This fixes an odd behaviour regarding mates, as example
the following diagram is a mate in 4, not in 3 as bogusly
reported before this patch.

1B2n3/8/2R5/5p2/3kp1n1/4p3/B3K3/8 w - - bm #4;

The performance impact should be minimal, the increment
in searched nodes is less then 0.1 %%

Idea and patch by Joona

After 999 games at 1+0
Mod vs Orig +193 =604 -202  -3 ELO

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2009-10-14 09:18:16 +01:00
parent 53f882ff1a
commit c1ea5ed6f7
1 changed files with 2 additions and 1 deletions
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3
src/search.cpp
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@ -1369,7 +1369,8 @@ namespace {
{
// History pruning. See ok_to_prune() definition
if ( moveCount >= 2 + int(depth)
&& ok_to_prune(pos, move, ss[ply].threatMove, depth))
&& ok_to_prune(pos, move, ss[ply].threatMove, depth)
&& bestValue > value_mated_in(PLY_MAX))
continue;
// Value based pruning