Rename FromTo -> History (#963)

Previously, we had duplicated History:

- one with (piece,to) called History
- one with (from,to) called FromTo

Now that we have only one, rename it to History, which is the generally accepted
name in the chess programming litterature for this technique.

Also correct some comments that had not been updated since the introduction of CMH.

No functional change.

src/movepick.cpp

@@ -140,7 +140,7 @@ void MovePicker::score<CAPTURES>() {
 template<>
 void MovePicker::score<QUIETS>() {
 
-  const FromToStats& fromTo = pos.this_thread()->fromTo;
+  const HistoryStats& history = pos.this_thread()->history;
 
   const CounterMoveStats* cmh = (ss-1)->counterMoves;
   const CounterMoveStats* fmh = (ss-2)->counterMoves;
@@ -152,21 +152,21 @@ void MovePicker::score<QUIETS>() {
       m.value =  (cmh  ?  (*cmh)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
                + (fmh  ?  (*fmh)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
                + (fmh2 ? (*fmh2)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
-               + fromTo.get(c, m);
+               + history.get(c, m);
 }
 
 template<>
 void MovePicker::score<EVASIONS>() {
   // Try captures ordered by MVV/LVA, then non-captures ordered by stats heuristics
-  const FromToStats& fromTo = pos.this_thread()->fromTo;
+  const HistoryStats& history = pos.this_thread()->history;
   Color c = pos.side_to_move();
 
   for (auto& m : *this)
       if (pos.capture(m))
           m.value =  PieceValue[MG][pos.piece_on(to_sq(m))]
-                   - Value(type_of(pos.moved_piece(m))) + FromToStats::Max;
+                   - Value(type_of(pos.moved_piece(m))) + HistoryStats::Max;
       else
-          m.value = fromTo.get(c, m);
+          m.value = history.get(c, m);
 }
 
 

src/movepick.h

@@ -29,13 +29,37 @@
 #include "types.h"
 
 
-/// The Stats struct stores moves statistics. According to the template parameter
-/// the class can store History and Countermoves. History records how often
-/// different moves have been successful or unsuccessful during the current search
-/// and is used for reduction and move ordering decisions.
-/// Countermoves store the move that refute a previous one. Entries are stored
-/// using only the moving piece and destination square, hence two moves with
-/// different origin but same destination and piece will be considered identical.
+/// HistoryStats records how often quiet moves have been successful or unsuccessful
+/// during the current search, and is used for reduction and move ordering decisions.
+struct HistoryStats {
+
+  static const Value Max = Value(1 << 28);
+
+  Value get(Color c, Move m) const { return table[c][from_sq(m)][to_sq(m)]; }
+  void clear() { std::memset(table, 0, sizeof(table)); }
+  void update(Color c, Move m, Value v) {
+
+    if (abs(int(v)) >= 324)
+        return;
+
+    Square from = from_sq(m);
+    Square to = to_sq(m);
+
+    table[c][from][to] -= table[c][from][to] * abs(int(v)) / 324;
+    table[c][from][to] += int(v) * 32;
+  }
+
+private:
+  Value table[COLOR_NB][SQUARE_NB][SQUARE_NB];
+};
+
+
+/// A template struct, used to generate MoveStats and CounterMoveHistoryStats:
+/// MoveStats store the move that refute a previous one.
+/// CounterMoveHistoryStats is like HistoryStats, but with two consecutive moves.
+/// Entries are stored using only the moving piece and destination square, hence
+/// two moves with different origin but same destination and piece will be
+/// considered identical.
 template<typename T, bool CM = false>
 struct Stats {
   const T* operator[](Piece pc) const { return table[pc]; }
@@ -59,28 +83,6 @@ typedef Stats<Move> MoveStats;
 typedef Stats<Value, true> CounterMoveStats;
 typedef Stats<CounterMoveStats> CounterMoveHistoryStats;
 
-struct FromToStats {
-
-  static const Value Max = Value(1 << 28);
-
-  Value get(Color c, Move m) const { return table[c][from_sq(m)][to_sq(m)]; }
-  void clear() { std::memset(table, 0, sizeof(table)); }
-  void update(Color c, Move m, Value v) {
-
-    if (abs(int(v)) >= 324)
-        return;
-
-    Square from = from_sq(m);
-    Square to = to_sq(m);
-
-    table[c][from][to] -= table[c][from][to] * abs(int(v)) / 324;
-    table[c][from][to] += int(v) * 32;
-  }
-
-private:
-  Value table[COLOR_NB][SQUARE_NB][SQUARE_NB];
-};
-
 
 /// MovePicker class is used to pick one pseudo legal move at a time from the
 /// current position. The most important method is next_move(), which returns a

src/search.cpp

@@ -212,7 +212,7 @@ void Search::clear() {
   for (Thread* th : Threads)
   {
       th->counterMoves.clear();
-      th->fromTo.clear();
+      th->history.clear();
       th->counterMoveHistory.clear();
       th->resetCalls = true;
   }
@@ -985,7 +985,7 @@ moves_loop: // When in check search starts from here
               ss->history =  (cmh  ? (*cmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
                            + (fmh  ? (*fmh )[moved_piece][to_sq(move)] : VALUE_ZERO)
                            + (fmh2 ? (*fmh2)[moved_piece][to_sq(move)] : VALUE_ZERO)
-                           + thisThread->fromTo.get(~pos.side_to_move(), move)
+                           + thisThread->history.get(~pos.side_to_move(), move)
                            - 8000; // Correction factor
 
               // Decrease/increase reduction by comparing opponent's stat score
@@ -1415,7 +1415,7 @@ moves_loop: // When in check search starts from here
 
     Color c = pos.side_to_move();
     Thread* thisThread = pos.this_thread();
-    thisThread->fromTo.update(c, move, bonus);
+    thisThread->history.update(c, move, bonus);
     update_cm_stats(ss, pos.moved_piece(move), to_sq(move), bonus);
 
     if ((ss-1)->counterMoves)
@@ -1427,7 +1427,7 @@ moves_loop: // When in check search starts from here
     // Decrease all the other played quiet moves
     for (int i = 0; i < quietsCnt; ++i)
     {
-        thisThread->fromTo.update(c, quiets[i], -bonus);
+        thisThread->history.update(c, quiets[i], -bonus);
         update_cm_stats(ss, pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
     }
   }

src/thread.h

@@ -70,7 +70,7 @@ public:
   Depth completedDepth;
   std::atomic_bool resetCalls;
   MoveStats counterMoves;
-  FromToStats fromTo;
+  HistoryStats history;
   CounterMoveHistoryStats counterMoveHistory;
 };