At root we start counting plies from 1,
instead pv[] array starts from 0. So
the variable 'ply' we use in extract_pv_from_tt
to index pv[] is misnamed, indeed it is
not the real ply, but ply-1.
The fix is to leave ply name in extract_pv_from_tt
but assign it the correct start value and
consequentely change all the references to pv[].
Instead in insert_pv_in_tt it's simpler to rename
the misnamed 'ply' in 'idx'.
The off-by-one bug was unhidden when trying to use
'ply' for what it should have been, for instance in
this position:
position fen 8/6R1/8/3k4/8/8/8/2K5 w - - 0 1
at depth 24 mate line is erroneusly truncated due
to value_from_tt() using the wrong ply.
Spotted by Ronald de Man.
bench:
8732553
StateInfo state[MAX_PLY_PLUS_6], *st = state;
const TTEntry* tte;
StateInfo state[MAX_PLY_PLUS_6], *st = state;
const TTEntry* tte;
- int ply = 0;
- Move m = pv[0];
+ int ply = 1; // At root ply is 1...
+ Move m = pv[0]; // ...instead pv[] array starts from 0
Value expectedScore = score;
pv.clear();
Value expectedScore = score;
pv.clear();
- assert(MoveList<LEGAL>(pos).contains(pv[ply]));
+ assert(MoveList<LEGAL>(pos).contains(pv[ply - 1]));
- pos.do_move(pv[ply++], *st++);
+ pos.do_move(pv[ply++ - 1], *st++);
tte = TT.probe(pos.key());
expectedScore = -expectedScore;
tte = TT.probe(pos.key());
expectedScore = -expectedScore;
&& pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
&& pos.legal(m, pos.pinned_pieces(pos.side_to_move()))
&& ply < MAX_PLY
&& pos.pseudo_legal(m = tte->move()) // Local copy, TT could change
&& pos.legal(m, pos.pinned_pieces(pos.side_to_move()))
&& ply < MAX_PLY
- && (!pos.is_draw() || ply < 2));
+ && (!pos.is_draw() || ply <= 2));
pv.push_back(MOVE_NONE); // Must be zero-terminating
pv.push_back(MOVE_NONE); // Must be zero-terminating
- while (ply) pos.undo_move(pv[--ply]);
+ while (--ply) pos.undo_move(pv[ply - 1]);
StateInfo state[MAX_PLY_PLUS_6], *st = state;
const TTEntry* tte;
StateInfo state[MAX_PLY_PLUS_6], *st = state;
const TTEntry* tte;
+ int idx = 0; // Ply starts from 1, we need to start from 0
do {
tte = TT.probe(pos.key());
do {
tte = TT.probe(pos.key());
- if (!tte || tte->move() != pv[ply]) // Don't overwrite correct entries
- TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[ply], VALUE_NONE);
+ if (!tte || tte->move() != pv[idx]) // Don't overwrite correct entries
+ TT.store(pos.key(), VALUE_NONE, BOUND_NONE, DEPTH_NONE, pv[idx], VALUE_NONE);
- assert(MoveList<LEGAL>(pos).contains(pv[ply]));
+ assert(MoveList<LEGAL>(pos).contains(pv[idx]));
- pos.do_move(pv[ply++], *st++);
+ pos.do_move(pv[idx++], *st++);
- } while (pv[ply] != MOVE_NONE);
+ } while (pv[idx] != MOVE_NONE);
- while (ply) pos.undo_move(pv[--ply]);
+ while (idx) pos.undo_move(pv[--idx]);