Remove moveCountPruning in search.cpp

The definition of moveCountPruning may cause confusion by implying that
the variable is unconstrained. However, once it is set to true, it
should not be reset to false, otherwise it would break the internal
logic of MovePicker.

Several patches have overlooked this constraint. For example:
https://tests.stockfishchess.org/tests/view/671e7c0486d5ee47d953d226
https://tests.stockfishchess.org/tests/view/66a1de7b4ff211be9d4eccea

The implementation approach was suggested by Disservin.

Passed non-regression STC:
LLR: 3.02 (-2.94,2.94) <-1.75,0.25>
Total: 180672 W: 47072 L: 47006 D: 86594
Ptnml(0-2): 536, 19482, 50247, 19522, 549
https://tests.stockfishchess.org/tests/view/6720df6f86d5ee47d953d542

closes official-stockfish#5661

No functional change

bench 1281912

src/movepick.cpp

@@ -216,7 +216,7 @@ Move MovePicker::select(Pred filter) {
 // This is the most important method of the MovePicker class. We emit one
 // new pseudo-legal move on every call until there are no more moves left,
 // picking the move with the highest score from a list of generated moves.
-Move MovePicker::next_move(bool skipQuiets) {
+Move MovePicker::next_move() {
 
     auto quiet_threshold = [](Depth d) { return -3560 * d; };
 
@@ -322,4 +322,6 @@ Move MovePicker::next_move(bool skipQuiets) {
     return Move::none();  // Silence warning
 }
 
+void MovePicker::skip_quiet_moves() { skipQuiets = true; }
+
 }  // namespace Stockfish

src/movepick.h

@@ -212,7 +212,8 @@ class MovePicker {
                const PawnHistory*,
                int);
     MovePicker(const Position&, Move, int, const CapturePieceToHistory*);
-    Move next_move(bool skipQuiets = false);
+    Move next_move();
+    void skip_quiet_moves();
 
    private:
     template<PickType T, typename Pred>
@@ -234,6 +235,7 @@ class MovePicker {
     int                          threshold;
     Depth                        depth;
     int                          ply;
+    bool                         skipQuiets = false;
     ExtMove                      moves[MAX_MOVES];
 };
 

src/search.cpp

@@ -942,12 +942,11 @@ Value Search::Worker::search(
 
     value = bestValue;
 
-    int  moveCount        = 0;
-    bool moveCountPruning = false;
+    int moveCount = 0;
 
     // Step 13. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
-    while ((move = mp.next_move(moveCountPruning)) != Move::none())
+    while ((move = mp.next_move()) != Move::none())
     {
         assert(move.is_ok());
 
@@ -993,7 +992,8 @@ Value Search::Worker::search(
         if (!rootNode && pos.non_pawn_material(us) && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
         {
             // Skip quiet moves if movecount exceeds our FutilityMoveCount threshold (~8 Elo)
-            moveCountPruning = moveCount >= futility_move_count(improving, depth);
+            if (moveCount >= futility_move_count(improving, depth))
+                mp.skip_quiet_moves();
 
             // Reduced depth of the next LMR search
             int lmrDepth = newDepth - r;