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Document mate distance pruning

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It is simple but somewhat tricky code that deserves
a bit of documentation. A bit of renaming while there.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba
2011-12-27 16:01:33 +01:00
parent 07f3f0384a
commit 750ac9ac50
2 changed files with 12 additions and 7 deletions
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15
src/search.cpp
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@@ -640,11 +640,16 @@ namespace {
|| ss->ply > PLY_MAX) && !RootNode)
return VALUE_DRAW;
// Step 3. Mate distance pruning
// Step 3. Mate distance pruning. Even if we mate at the next move our score
// would be at best mate_in(ss->ply+1), but if alpha is already bigger because
// a shorter mate was found upward in the tree then there is no need to search
// further, we will never beat current alpha. Same logic but with reversed signs
// applies also in the opposite condition of being mated instead of giving mate,
// in this case return a fail-high score.
if (!RootNode)
{
alpha = std::max(value_mated_in(ss->ply), alpha);
beta = std::min(value_mate_in(ss->ply+1), beta);
alpha = std::max(mated_in(ss->ply), alpha);
beta = std::min(mate_in(ss->ply+1), beta);
if (alpha >= beta)
return alpha;
}
@@ -1122,7 +1127,7 @@ split_point_start: // At split points actual search starts from here
// harmless because return value is discarded anyhow in the parent nodes.
// If we are in a singular extension search then return a fail low score.
if (!moveCount)
return excludedMove ? oldAlpha : inCheck ? value_mated_in(ss->ply) : VALUE_DRAW;
return excludedMove ? oldAlpha : inCheck ? mated_in(ss->ply) : VALUE_DRAW;
// If we have pruned all the moves without searching return a fail-low score
if (bestValue == -VALUE_INFINITE)
@@ -1365,7 +1370,7 @@ split_point_start: // At split points actual search starts from here
// All legal moves have been searched. A special case: If we're in check
// and no legal moves were found, it is checkmate.
if (inCheck && bestValue == -VALUE_INFINITE)
return value_mated_in(ss->ply);
return mated_in(ss->ply);
// Update transposition table
move = bestValue <= oldAlpha ? MOVE_NONE : ss->bestMove;