Better document how history works

Both with added comment and changing the API to
reflect that only destination square and moved piece
is important for history.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/history.cpp

@@ -32,14 +32,13 @@
 //// Functions
 ////
 
+
 /// Constructor
 
-History::History() {
-  this->clear();
-}
+History::History() { clear(); }
 
 
-/// History::clear() clears the history tables.
+/// History::clear() clears the history tables
 
 void History::clear() {
   memset(history, 0, 2 * 8 * 64 * sizeof(int));
@@ -50,20 +49,21 @@ void History::clear() {
 
 /// History::success() registers a move as being successful. This is done
 /// whenever a non-capturing move causes a beta cutoff in the main search.
-/// The three parameters are the moving piece, the move itself, and the
-/// search depth.
+/// The three parameters are the moving piece, the destination square, and
+/// the search depth.
+
+void History::success(Piece p, Square to, Depth d) {
 
-void History::success(Piece p, Move m, Depth d) {
   assert(piece_is_ok(p));
-  assert(move_is_ok(m));
+  assert(square_is_ok(to));
 
-  history[p][move_to(m)] += int(d) * int(d);
-  successCount[p][move_to(m)]++;
+  history[p][to] += int(d) * int(d);
+  successCount[p][to]++;
 
-  // Prevent history overflow:
-  if(history[p][move_to(m)] >= HistoryMax)
-    for(int i = 0; i < 16; i++)
-      for(int j = 0; j < 64; j++)
+  // Prevent history overflow
+  if (history[p][to] >= HistoryMax)
+      for (int i = 0; i < 16; i++)
+          for (int j = 0; j < 64; j++)
               history[i][j] /= 2;
 }
 
@@ -72,31 +72,34 @@ void History::success(Piece p, Move m, Depth d) {
 /// called for each non-capturing move which failed to produce a beta cutoff
 /// at a node where a beta cutoff was finally found.
 
-void History::failure(Piece p, Move m) {
-  assert(piece_is_ok(p));
-  assert(move_is_ok(m));
+void History::failure(Piece p, Square to) {
 
-  failureCount[p][move_to(m)]++;
+  assert(piece_is_ok(p));
+  assert(square_is_ok(to));
+
+  failureCount[p][to]++;
 }
 
 
 /// History::move_ordering_score() returns an integer value used to order the
 /// non-capturing moves in the MovePicker class.
 
-int History::move_ordering_score(Piece p, Move m) const {
-  assert(piece_is_ok(p));
-  assert(move_is_ok(m));
+int History::move_ordering_score(Piece p, Square to) const {
 
-  return history[p][move_to(m)];
+  assert(piece_is_ok(p));
+  assert(square_is_ok(to));
+
+  return history[p][to];
 }
 
 
 /// History::ok_to_prune() decides whether a move has been sufficiently
 /// unsuccessful that it makes sense to prune it entirely.
 
-bool History::ok_to_prune(Piece p, Move m, Depth d) const {
-  assert(piece_is_ok(p));
-  assert(move_is_ok(m));
+bool History::ok_to_prune(Piece p, Square to, Depth d) const {
 
-  return (int(d) * successCount[p][move_to(m)] < failureCount[p][move_to(m)]);
+  assert(piece_is_ok(p));
+  assert(square_is_ok(to));
+
+  return (int(d) * successCount[p][to] < failureCount[p][to]);
 }

src/history.h

@@ -34,19 +34,22 @@
 //// Types
 ////
 
-/// The History class stores statistics about how often different moves have
-/// been successful or unsuccessful during the current search.  These
-/// statistics are used for reduction and move ordering decisions.
+/// The History class stores statistics about how often different moves
+/// have been successful or unsuccessful during the current search. These
+/// statistics are used for reduction and move ordering decisions. History
+/// entries are stored according only to moving piece and destination square,
+/// in particular two moves with different origin but same destination and
+/// same piece will be considered identical.
 
 class History {
 
 public:
   History();
   void clear();
-  void success(Piece p, Move m, Depth d);
-  void failure(Piece p, Move m);
-  int move_ordering_score(Piece p, Move m) const;
-  bool ok_to_prune(Piece p, Move m, Depth d) const;
+  void success(Piece p, Square to, Depth d);
+  void failure(Piece p, Square to);
+  int move_ordering_score(Piece p, Square to) const;
+  bool ok_to_prune(Piece p, Square to, Depth d) const;
 
 private:
   int history[16][64];  // [piece][square]

src/movepick.cpp

@@ -264,7 +264,7 @@ void MovePicker::score_noncaptures() {
       else if (m == killer2)
           hs = HistoryMax + 1;
       else
-          hs = H.move_ordering_score(pos.piece_on(move_from(m)), m);
+          hs = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
 
       // Ensure history is always preferred to pst
       if (hs > 0)
@@ -287,7 +287,7 @@ void MovePicker::score_evasions() {
           int seeScore = pos.see(m);
           moves[i].score = (seeScore >= 0)? seeScore + HistoryMax : seeScore;
       } else
-          moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), m);
+          moves[i].score = H.move_ordering_score(pos.piece_on(move_from(m)), move_to(m));
   }
 }
 

src/search.cpp

@@ -2332,7 +2332,7 @@ namespace {
       return false;
 
     // Case 4: Don't prune moves with good history.
-    if (!H.ok_to_prune(pos.piece_on(move_from(m)), m, d))
+    if (!H.ok_to_prune(pos.piece_on(mfrom), mto, d))
         return false;
 
     // Case 5: If the moving piece in the threatened move is a slider, don't
@@ -2379,13 +2379,13 @@ namespace {
   void update_history(const Position& pos, Move m, Depth depth,
                       Move movesSearched[], int moveCount) {
 
-    H.success(pos.piece_on(move_from(m)), m, depth);
+    H.success(pos.piece_on(move_from(m)), move_to(m), depth);
 
     for (int i = 0; i < moveCount - 1; i++)
     {
         assert(m != movesSearched[i]);
         if (ok_to_history(pos, movesSearched[i]))
-            H.failure(pos.piece_on(move_from(movesSearched[i])), movesSearched[i]);
+            H.failure(pos.piece_on(move_from(movesSearched[i])), move_to(movesSearched[i]));
     }
   }