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Avoid TT cutoffs at root of null zugzwang verification

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This patch fixes an issue with zugzwang well explained by Tord:

"Assume that a zugzwang position occurs at iteration N,
at a search depth d, with d < 6*OnePly. The null move search
fails high, and no verification search is done, because the
depth is too small. The position gets stored in the transposition
table with a good score and a depth of d.

Now, consider what happens when the same position occurs at iteration
N+1, this time with a depth of d+OnePly (i.e. one ply deeper than at
the previous iteration). Once again, the null move search fails
high. The point is that the verification search will also fail high,
because of an instant transposition table cutoff caused by the value
stored in the TT during the previous iteration."

With this patch we simply do not allow TT cutoffs at the root node
of a null move verification search if the TT value was found by a
null search.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
This commit is contained in:
Marco Costalba 2010-04-10 10:32:17 +01:00
parent 06a350f1ae
commit 626b1f8c6a
1 changed files with 11 additions and 7 deletions
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18
src/search.cpp
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@ -294,7 +294,7 @@ namespace {
Depth extension(const Position&, Move, bool, bool, bool, bool, bool, bool*);
bool ok_to_do_nullmove(const Position& pos);
bool ok_to_prune(const Position& pos, Move m, Move threat);
bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply);
bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply, bool allowNullmove);
Value refine_eval(const TTEntry* tte, Value defaultEval, int ply);
void update_history(const Position& pos, Move move, Depth depth, Move movesSearched[], int moveCount);
void update_killers(Move m, SearchStack& ss);
@ -1299,7 +1299,7 @@ namespace {
tte = TT.retrieve(posKey);
ttMove = (tte ? tte->move() : MOVE_NONE);
if (tte && ok_to_use_TT(tte, depth, beta, ply))
if (tte && ok_to_use_TT(tte, depth, beta, ply, allowNullmove))
{
ss[ply].currentMove = ttMove; // Can be MOVE_NONE
return value_from_tt(tte->value(), ply);
@ -1625,7 +1625,7 @@ namespace {
tte = TT.retrieve(pos.get_key());
ttMove = (tte ? tte->move() : MOVE_NONE);
if (!pvNode && tte && ok_to_use_TT(tte, depth, beta, ply))
if (!pvNode && tte && ok_to_use_TT(tte, depth, beta, ply, true))
{
assert(tte->type() != VALUE_TYPE_EVAL);
@ -2306,14 +2306,18 @@ namespace {
}
// ok_to_use_TT() returns true if a transposition table score
// can be used at a given point in search.
// ok_to_use_TT() returns true if a transposition table score can be used at a
// given point in search. To avoid zugzwang issues TT cutoffs at the root node
// of a null move verification search are not allowed if the TT value was found
// by a null search, this is implemented testing allowNullmove and TT entry type.
bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply) {
bool ok_to_use_TT(const TTEntry* tte, Depth depth, Value beta, int ply, bool allowNullmove) {
Value v = value_from_tt(tte->value(), ply);
return ( tte->depth() >= depth
return (allowNullmove || !(tte->type() & VALUE_TYPE_NULL))
&& ( tte->depth() >= depth
|| v >= Max(value_mate_in(PLY_MAX), beta)
|| v < Min(value_mated_in(PLY_MAX), beta))