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Start searching for a repetition from the 4th ply behind

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A position can never repeat the one on the previous move.
Thus we can start searching for a repetition from the 4th
ply behind. In the case:

 std::min(st->rule50, st->pliesFromNull) < 4

We don't need to do any more calculations. This case happens
very often - in more than a half of all calls of the function.

No functional change.
This commit is contained in:
Aram Tumanian 2016-11-11 17:46:21 +02:00 • committed by Marco Costalba
parent 76d113f5f0
commit 797602938d
1 changed files with 10 additions and 4 deletions
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14
src/position.cpp
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@ -1079,14 +1079,20 @@ bool Position::is_draw() const {
if (st->rule50 > 99 && (!checkers() || MoveList<LEGAL>(*this).size()))
return true;
StateInfo* stp = st;
for (int i = 2, e = std::min(st->rule50, st->pliesFromNull); i <= e; i += 2)
{
int e = std::min(st->rule50, st->pliesFromNull);
if (e < 4)
return false;
StateInfo* stp = st->previous->previous;
do {
stp = stp->previous->previous;
if (stp->key == st->key)
return true; // Draw at first repetition
}
} while ((e -= 2) >= 4);
return false;
}