Join common code in the stages of next_move()

Rewrite the MovePicker class using lambda expressions for move filtering.
Includes code style changes by @mcostalba.

Verified for speed, passed STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 43191 W: 9391 L: 9312 D: 24488
http://tests.stockfishchess.org/tests/view/5a99b9df0ebc590297cc8f04

This rewrite of MovePicker.cpp seems to trigger less random crashes on Ryzen
machines than the version in previous master (reported by Bojun Guo).

Closes https://github.com/official-stockfish/Stockfish/pull/1454

No functional change.

src/movepick.cpp

@@ -25,12 +25,15 @@
 namespace {
 
   enum Stages {
-    MAIN_SEARCH, CAPTURES_INIT, GOOD_CAPTURES, KILLER0, KILLER1, COUNTERMOVE, QUIET_INIT, QUIET, BAD_CAPTURES,
+    MAIN_TT, CAPTURE_INIT, GOOD_CAPTURE, KILLER0, KILLER1, COUNTERMOVE, QUIET_INIT, QUIET, BAD_CAPTURE,
-    EVASION, EVASIONS_INIT, ALL_EVASIONS,
+    EVASION_TT, EVASION_INIT, EVASION,
-    PROBCUT, PROBCUT_CAPTURES_INIT, PROBCUT_CAPTURES,
+    PROBCUT_TT, PROBCUT_INIT, PROBCUT,
-    QSEARCH, QCAPTURES_INIT, QCAPTURES, QCHECKS
+    QSEARCH_TT, QCAPTURE_INIT, QCAPTURE, QCHECK_INIT, QCHECK
   };
 
+  // Helper filter used with select_move()
+  const auto Any = [](){ return true; };
+
   // partial_insertion_sort() sorts moves in descending order up to and including
   // a given limit. The order of moves smaller than the limit is left unspecified.
   void partial_insertion_sort(ExtMove* begin, ExtMove* end, int limit) {
@@ -46,15 +49,6 @@ namespace {
         }
   }
 
-  // pick_best() finds the best move in the range (begin, end) and moves it to
-  // the front. It's faster than sorting all the moves in advance when there
-  // are few moves, e.g., the possible captures.
-  Move pick_best(ExtMove* begin, ExtMove* end) {
-
-    std::swap(*begin, *std::max_element(begin, end));
-    return *begin;
-  }
-
 } // namespace
 
 
@@ -72,18 +66,19 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHist
 
   assert(d > DEPTH_ZERO);
 
-  stage = pos.checkers() ? EVASION : MAIN_SEARCH;
+  stage = pos.checkers() ? EVASION_TT : MAIN_TT;
   ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
   stage += (ttMove == MOVE_NONE);
 }
 
 /// MovePicker constructor for quiescence search
-MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,  const CapturePieceToHistory* cph, Square s)
+MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const ButterflyHistory* mh,
-           : pos(p), mainHistory(mh), captureHistory(cph), recaptureSquare(s), depth(d) {
+                       const CapturePieceToHistory* cph, Square rs)
+           : pos(p), mainHistory(mh), captureHistory(cph), recaptureSquare(rs), depth(d) {
 
   assert(d <= DEPTH_ZERO);
 
-  stage = pos.checkers() ? EVASION : QSEARCH;
+  stage = pos.checkers() ? EVASION_TT : QSEARCH_TT;
   ttMove =    ttm
            && pos.pseudo_legal(ttm)
            && (depth > DEPTH_QS_RECAPTURES || to_sq(ttm) == recaptureSquare) ? ttm : MOVE_NONE;
@@ -97,7 +92,7 @@ MovePicker::MovePicker(const Position& p, Move ttm, Value th, const CapturePiece
 
   assert(!pos.checkers());
 
-  stage = PROBCUT;
+  stage = PROBCUT_TT;
   ttMove =   ttm
           && pos.pseudo_legal(ttm)
           && pos.capture(ttm)
@@ -105,9 +100,9 @@ MovePicker::MovePicker(const Position& p, Move ttm, Value th, const CapturePiece
   stage += (ttMove == MOVE_NONE);
 }
 
-/// score() assigns a numerical value to each move in a list, used for sorting.
+/// MovePicker::score() assigns a numerical value to each move in a list, used
-/// Captures are ordered by Most Valuable Victim (MVV), preferring captures
+/// for sorting. Captures are ordered by Most Valuable Victim (MVV), preferring
-/// with a good history. Quiets are ordered using the histories.
+/// captures with a good history. Quiets moves are ordered using the histories.
 template<GenType Type>
 void MovePicker::score() {
 
@@ -134,57 +129,58 @@ void MovePicker::score() {
       }
 }
 
-/// next_move() is the most important method of the MovePicker class. It returns
+/// MovePicker::select_move() returns the next move satisfying a predicate function
-/// a new pseudo legal move every time it is called, until there are no more moves
+template<PickType T, typename Pred>
-/// left. It picks the move with the biggest value from a list of generated moves
+Move MovePicker::select_move(Pred filter) {
-/// taking care not to return the ttMove if it has already been searched.
 
+  while (cur < endMoves)
+  {
+      if (T == BEST_SCORE)
+          std::swap(*cur, *std::max_element(cur, endMoves));
+
+      move = *cur++;
+
+      if (move != ttMove && filter())
+          return move;
+  }
+  return move = MOVE_NONE;
+}
+
+/// MovePicker::next_move() is the most important method of the MovePicker class. It
+/// returns a new pseudo legal move every time it is called, until there are no more
+/// moves left. It picks the move with the highest score from a list of generated moves.
 Move MovePicker::next_move(bool skipQuiets) {
 
-  Move move;
+top:
-
-begin_switch:
-
   switch (stage) {
 
-  case MAIN_SEARCH:
+  case MAIN_TT:
-  case EVASION:
+  case EVASION_TT:
-  case QSEARCH:
+  case QSEARCH_TT:
-  case PROBCUT:
+  case PROBCUT_TT:
       ++stage;
       return ttMove;
 
-  case CAPTURES_INIT:
+  case CAPTURE_INIT:
-  case PROBCUT_CAPTURES_INIT:
+  case PROBCUT_INIT:
-  case QCAPTURES_INIT:
+  case QCAPTURE_INIT:
       endBadCaptures = cur = moves;
       endMoves = generate<CAPTURES>(pos, cur);
       score<CAPTURES>();
       ++stage;
+      goto top;
 
-      // Rebranch at the top of the switch
+  case GOOD_CAPTURE:
-      goto begin_switch;
+      if (select_move<BEST_SCORE>([&](){ return  pos.see_ge(move, Value(-55 * (cur-1)->value / 1024)) ?
-
+                                                 // Move losing capture to endBadCaptures to be tried later
-  case GOOD_CAPTURES:
+                                                 true : (*endBadCaptures++ = move, false); }))
-      while (cur < endMoves)
+          return move;
-      {
-          move = pick_best(cur++, endMoves);
-          if (move != ttMove)
-          {
-              if (pos.see_ge(move, Value(-55 * (cur-1)->value / 1024)))
-                  return move;
-
-              // Losing capture, move it to the beginning of the array
-              *endBadCaptures++ = move;
-          }
-      }
-      ++stage;
 
       // If the countermove is the same as a killer, skip it
       if (   refutations[0] == refutations[2]
           || refutations[1] == refutations[2])
-         refutations[2] = MOVE_NONE;
+          refutations[2] = MOVE_NONE;
-
+      ++stage;
       /* fallthrough */
 
   case KILLER0:
@@ -192,7 +188,7 @@ begin_switch:
   case COUNTERMOVE:
       while (stage <= COUNTERMOVE)
       {
-          move = refutations[ stage++ - KILLER0 ];
+          move = refutations[ stage++ - KILLER0];
           if (    move != MOVE_NONE
               &&  move != ttMove
               &&  pos.pseudo_legal(move)
@@ -210,78 +206,55 @@ begin_switch:
       /* fallthrough */
 
   case QUIET:
-      if (!skipQuiets)
+      if (   !skipQuiets
-          while (cur < endMoves)
+          && select_move<NEXT>([&](){return    move != refutations[0]
-          {
+                                            && move != refutations[1]
-              move = *cur++;
+                                            && move != refutations[2];}))
-              if (   move != ttMove
+          return move;
-                  && move != refutations[0]
+
-                  && move != refutations[1]
+      // Point to beginning and end of bad captures
-                  && move != refutations[2])
+      cur = moves, endMoves = endBadCaptures;
-                  return move;
-          }
       ++stage;
-      cur = moves; // Point to beginning of bad captures
       /* fallthrough */
 
-  case BAD_CAPTURES:
+  case BAD_CAPTURE:
-      if (cur < endBadCaptures)
+      return select_move<NEXT>(Any);
-          return *cur++;
-      break;
 
-  case EVASIONS_INIT:
+  case EVASION_INIT:
       cur = moves;
       endMoves = generate<EVASIONS>(pos, cur);
       score<EVASIONS>();
       ++stage;
       /* fallthrough */
 
-  case ALL_EVASIONS:
+  case EVASION:
-      while (cur < endMoves)
+      return select_move<BEST_SCORE>(Any);
-      {
-          move = pick_best(cur++, endMoves);
-          if (move != ttMove)
-              return move;
-      }
-      break;
 
-  case PROBCUT_CAPTURES:
+  case PROBCUT:
-      while (cur < endMoves)
+      return select_move<BEST_SCORE>([&](){ return pos.see_ge(move, threshold); });
-      {
-          move = pick_best(cur++, endMoves);
-          if (   move != ttMove
-              && pos.see_ge(move, threshold))
-              return move;
-      }
-      break;
 
-  case QCAPTURES:
+  case QCAPTURE:
-      while (cur < endMoves)
+      if (select_move<BEST_SCORE>([&](){ return   depth > DEPTH_QS_RECAPTURES
-      {
+                                               || to_sq(move) == recaptureSquare; }))
-          move = pick_best(cur++, endMoves);
+          return move;
-          if (   move != ttMove
+
-              && (depth > DEPTH_QS_RECAPTURES || to_sq(move) == recaptureSquare))
+      // If we did not find any move and we do not try checks, we have finished
-              return move;
+      if (depth != DEPTH_QS_CHECKS)
-      }
+          return MOVE_NONE;
-      if (depth <= DEPTH_QS_NO_CHECKS)
+
-          break;
+      ++stage;
+      /* fallthrough */
+
+  case QCHECK_INIT:
       cur = moves;
       endMoves = generate<QUIET_CHECKS>(pos, cur);
       ++stage;
       /* fallthrough */
 
-  case QCHECKS:
+  case QCHECK:
-      while (cur < endMoves)
+      return select_move<NEXT>(Any);
-      {
-          move = *cur++;
-          if (move != ttMove)
-              return move;
-      }
-      break;
-
-  default:
-      assert(false);
   }
 
-  return MOVE_NONE;
+  assert(false);
+  return MOVE_NONE; // Silence warning
 }

src/movepick.h

@@ -49,7 +49,7 @@ public:
   void operator<<(int bonus) {
 
     assert(abs(bonus) <= D); // Ensure range is [-W * D, W * D]
-    assert(abs(W * D) < std::numeric_limits<T>::max()); // Ensure we don't overflow
+    assert(W * D < std::numeric_limits<T>::max()); // Ensure we don't overflow
 
     entry += bonus * W - entry * abs(bonus) / D;
 
@@ -110,6 +110,8 @@ typedef Stats<PieceToHistory, W32, NOT_USED, PIECE_NB, SQUARE_NB> ContinuationHi
 /// beta algorithm, MovePicker attempts to return the moves which are most likely
 /// to get a cut-off first.
 
+enum PickType { NEXT, BEST_SCORE };
+
 class MovePicker {
 public:
   MovePicker(const MovePicker&) = delete;
@@ -120,6 +122,7 @@ public:
   Move next_move(bool skipQuiets = false);
 
 private:
+  template<PickType T, typename Pred> Move select_move(Pred);
   template<GenType> void score();
   ExtMove* begin() { return cur; }
   ExtMove* end() { return endMoves; }
@@ -131,6 +134,7 @@ private:
   Move ttMove, refutations[3];
   ExtMove *cur, *endMoves, *endBadCaptures;
   int stage;
+  Move move;
   Square recaptureSquare;
   Value threshold;
   Depth depth;