This fixes an old issue where we want to make a position unique but only
change a small number of bits in the key instead of all 64 of them randomly.
This is fragile and can lead to non uniqueness issues in the TT.
Key make_key(uint64_t seed) takes any integer and produces a unique random 64 bit key.
It is computationally efficient and is based on a congruential pseudo random number
generator using well tested constants by Donald Knuth
(see https://en.wikipedia.org/wiki/Linear_congruential_generator)
STC https://tests.stockfishchess.org/tests/view/
5ef6c78f761b685b4c724bb6
LLR: 2.95 (-2.94,2.94) {-1.50,0.50}
Total: 154320 W: 29343 L: 29376 D: 95601
Ptnml(0-2): 2543, 18170, 35891, 17889, 2667
LTC https://tests.stockfishchess.org/tests/view/
5ef7d1a9020eec13834a940e
LLR: 2.95 (-2.94,2.94) {-1.50,0.50}
Total: 53488 W: 6629 L: 6584 D: 40275
Ptnml(0-2): 372, 4878, 16183, 4955, 356
closes https://github.com/official-stockfish/Stockfish/pull/2773
bench:
4626776
// search to overwrite a previous full search TT value, so we use a different
// position key in case of an excluded move.
excludedMove = ss->excludedMove;
- posKey = pos.key() ^ (Key(excludedMove) << 48); // Isn't a very good hash
+ posKey = excludedMove == MOVE_NONE ? pos.key() : pos.key() ^ make_key(excludedMove);
tte = TT.probe(posKey, ttHit);
ttValue = ttHit ? value_from_tt(tte->value(), ss->ply, pos.rule50_count()) : VALUE_NONE;
ttMove = rootNode ? thisThread->rootMoves[thisThread->pvIdx].pv[0]
return from_sq(m) != to_sq(m); // Catch MOVE_NULL and MOVE_NONE
}
+/// Based on a congruential pseudo random number generator
+constexpr Key make_key(uint64_t seed) {
+ return seed * 6364136223846793005ULL + 1442695040888963407ULL;
+}
+
#endif // #ifndef TYPES_H_INCLUDED
#include "tune.h" // Global visibility to tuning setup