Make low ply history size fixed

Size of low ply history should always be the same, so ensure it.

closes official-stockfish#5630

No functional change

src/movepick.cpp

@@ -179,7 +179,7 @@ void MovePicker::score() {
                         : pt == ROOK && bool(to & threatenedByMinor) ? 24335
                                                                      : 0);
 
-            if (ply < 4)
+            if (ply < LOW_PLY_HISTORY_SIZE)
                 m.value += 8 * (*lowPlyHistory)[ply][m.from_to()] / (1 + 2 * ply);
         }
 

src/movepick.h

@@ -37,6 +37,7 @@ namespace Stockfish {
 constexpr int PAWN_HISTORY_SIZE        = 512;    // has to be a power of 2
 constexpr int CORRECTION_HISTORY_SIZE  = 32768;  // has to be a power of 2
 constexpr int CORRECTION_HISTORY_LIMIT = 1024;
+constexpr int LOW_PLY_HISTORY_SIZE     = 4;
 
 static_assert((PAWN_HISTORY_SIZE & (PAWN_HISTORY_SIZE - 1)) == 0,
               "PAWN_HISTORY_SIZE has to be a power of 2");
@@ -137,7 +138,7 @@ using ButterflyHistory = Stats<int16_t, 7183, COLOR_NB, int(SQUARE_NB) * int(SQU
 
 // LowPlyHistory is adressed by play and move's from and to squares, used
 // to improve move ordering near the root
-using LowPlyHistory = Stats<int16_t, 7183, 4, int(SQUARE_NB) * int(SQUARE_NB)>;
+using LowPlyHistory = Stats<int16_t, 7183, LOW_PLY_HISTORY_SIZE, int(SQUARE_NB) * int(SQUARE_NB)>;
 
 // CapturePieceToHistory is addressed by a move's [piece][to][captured piece type]
 using CapturePieceToHistory = Stats<int16_t, 10692, PIECE_NB, SQUARE_NB, PIECE_TYPE_NB>;

src/search.cpp

@@ -1851,7 +1851,7 @@ void update_quiet_histories(
 
     Color us = pos.side_to_move();
     workerThread.mainHistory[us][move.from_to()] << bonus;
-    if (ss->ply < 4)
+    if (ss->ply < LOW_PLY_HISTORY_SIZE)
         workerThread.lowPlyHistory[ss->ply][move.from_to()] << bonus;
 
     update_continuation_histories(ss, pos.moved_piece(move), move.to_sq(), bonus);