Fix an off-by-one bug in extract_pv_from_tt

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

src/search.cpp

@@ -1399,18 +1399,18 @@ void RootMove::extract_pv_from_tt(Position& pos) {
 
  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();
 
  do {
  pv.push_back(m);
 
- 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;
 
@@ -1419,11 +1419,11 @@ void RootMove::extract_pv_from_tt(Position& pos) {
  && 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
 
- while (ply) pos.undo_move(pv[--ply]);
+ while (--ply) pos.undo_move(pv[ply - 1]);
 }
 
 
@@ -1435,21 +1435,21 @@ void RootMove::insert_pv_in_tt(Position& pos) {
 
  StateInfo state[MAX_PLY_PLUS_6], *st = state;
  const TTEntry* tte;
- int ply = 0;
+ int idx = 0; // Ply starts from 1, we need to start from 0
 
  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]);
 }