Add a second level of follow-up moves

STC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 6438 W: 1229 L: 1077 D: 4132

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 4000 W: 605 L: 473 D: 2922

bench: 7378965

Resolves #636

src/movepick.cpp

@@ -67,22 +67,21 @@ namespace {
 /// search captures, promotions, and some checks) and how important good move
 /// ordering is at the current node.
 
-MovePicker::MovePicker(const Position& p, Move ttm, Depth d, const HistoryStats& h,
+MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Search::Stack* s)
-                       const CounterMoveStats& cmh, const CounterMoveStats& fmh,
+           : pos(p), ss(s), depth(d) {
-                       Move cm, Search::Stack* s)
-           : pos(p), history(h), counterMoveHistory(&cmh),
-             followupMoveHistory(&fmh), ss(s), countermove(cm), depth(d) {
 
   assert(d > DEPTH_ZERO);
 
+  Square prevSq = to_sq((ss-1)->currentMove);
+  countermove = pos.this_thread()->counterMoves[pos.piece_on(prevSq)][prevSq];
+
   stage = pos.checkers() ? EVASION : MAIN_SEARCH;
   ttMove = ttm && pos.pseudo_legal(ttm) ? ttm : MOVE_NONE;
   endMoves += (ttMove != MOVE_NONE);
 }
 
-MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
+MovePicker::MovePicker(const Position& p, Move ttm, Depth d, Square s)
-                       const HistoryStats& h, Square s)
+           : pos(p) {
-           : pos(p), history(h) {
 
   assert(d <= DEPTH_ZERO);
 
@@ -106,8 +105,8 @@ MovePicker::MovePicker(const Position& p, Move ttm, Depth d,
   endMoves += (ttMove != MOVE_NONE);
 }
 
-MovePicker::MovePicker(const Position& p, Move ttm, const HistoryStats& h, Value th)
+MovePicker::MovePicker(const Position& p, Move ttm, Value th)
-           : pos(p), history(h), threshold(th) {
+           : pos(p), threshold(th) {
 
   assert(!pos.checkers());
 
@@ -142,10 +141,17 @@ void MovePicker::score<CAPTURES>() {
 template<>
 void MovePicker::score<QUIETS>() {
 
+  const HistoryStats& history = pos.this_thread()->history;
+
+  const CounterMoveStats* cm = (ss-1)->counterMoves;
+  const CounterMoveStats* fm = (ss-2)->counterMoves;
+  const CounterMoveStats* f2 = (ss-4)->counterMoves;
+
   for (auto& m : *this)
-      m.value =  history[pos.moved_piece(m)][to_sq(m)]
+      m.value =      history[pos.moved_piece(m)][to_sq(m)]
-               + (*counterMoveHistory )[pos.moved_piece(m)][to_sq(m)]
+               + (cm ? (*cm)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
-               + (*followupMoveHistory)[pos.moved_piece(m)][to_sq(m)];
+               + (fm ? (*fm)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO)
+               + (f2 ? (*f2)[pos.moved_piece(m)][to_sq(m)] : VALUE_ZERO);
 }
 
 template<>
@@ -153,6 +159,7 @@ void MovePicker::score<EVASIONS>() {
   // Try winning and equal captures ordered by MVV/LVA, then non-captures ordered
   // by history value, then bad captures and quiet moves with a negative SEE ordered
   // by SEE value.
+  const HistoryStats& history = pos.this_thread()->history;
   Value see;
 
   for (auto& m : *this)

src/movepick.h

@@ -79,10 +79,9 @@ public:
   MovePicker(const MovePicker&) = delete;
   MovePicker& operator=(const MovePicker&) = delete;
 
-  MovePicker(const Position&, Move, const HistoryStats&, Value);
+  MovePicker(const Position&, Move, Value);
-  MovePicker(const Position&, Move, Depth, const HistoryStats&, Square);
+  MovePicker(const Position&, Move, Depth, Square);
-  MovePicker(const Position&, Move, Depth, const HistoryStats&,
+  MovePicker(const Position&, Move, Depth, Search::Stack*);
-             const CounterMoveStats&, const CounterMoveStats&, Move, Search::Stack*);
 
   Move next_move();
 
@@ -93,10 +92,7 @@ private:
   ExtMove* end() { return endMoves; }
 
   const Position& pos;
-  const HistoryStats& history;
+  const Search::Stack* ss;
-  const CounterMoveStats* counterMoveHistory;
-  const CounterMoveStats* followupMoveHistory;
-  Search::Stack* ss;
   Move countermove;
   Depth depth;
   Move ttMove;

src/search.cpp

@@ -380,12 +380,12 @@ void MainThread::search() {
 
 void Thread::search() {
 
-  Stack stack[MAX_PLY+4], *ss = stack+2; // To allow referencing (ss-2) and (ss+2)
+  Stack stack[MAX_PLY+7], *ss = stack+5; // To allow referencing (ss-5) and (ss+2)
   Value bestValue, alpha, beta, delta;
   Move easyMove = MOVE_NONE;
   MainThread* mainThread = (this == Threads.main() ? Threads.main() : nullptr);
 
-  std::memset(ss-2, 0, 5 * sizeof(Stack));
+  std::memset(ss-5, 0, 8 * sizeof(Stack));
 
   bestValue = delta = alpha = -VALUE_INFINITE;
   beta = VALUE_INFINITE;
@@ -657,6 +657,7 @@ namespace {
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
     ss->currentMove = (ss+1)->excludedMove = bestMove = MOVE_NONE;
+    ss->counterMoves = nullptr;
     (ss+1)->skipEarlyPruning = false;
     (ss+2)->killers[0] = (ss+2)->killers[1] = MOVE_NONE;
 
@@ -780,6 +781,7 @@ namespace {
         &&  pos.non_pawn_material(pos.side_to_move()))
     {
         ss->currentMove = MOVE_NULL;
+        ss->counterMoves = nullptr;
 
         assert(eval - beta >= 0);
 
@@ -828,13 +830,14 @@ namespace {
         assert((ss-1)->currentMove != MOVE_NONE);
         assert((ss-1)->currentMove != MOVE_NULL);
 
-        MovePicker mp(pos, ttMove, thisThread->history, PieceValue[MG][pos.captured_piece_type()]);
+        MovePicker mp(pos, ttMove, PieceValue[MG][pos.captured_piece_type()]);
         CheckInfo ci(pos);
 
         while ((move = mp.next_move()) != MOVE_NONE)
             if (pos.legal(move, ci.pinned))
             {
                 ss->currentMove = move;
+                ss->counterMoves = &CounterMoveHistory[pos.moved_piece(move)][to_sq(move)];
                 pos.do_move(move, st, pos.gives_check(move, ci));
                 value = -search<NonPV>(pos, ss+1, -rbeta, -rbeta+1, rdepth, !cutNode);
                 pos.undo_move(move);
@@ -860,12 +863,9 @@ namespace {
 moves_loop: // When in check search starts from here
 
     Square prevSq = to_sq((ss-1)->currentMove);
-    Square ownPrevSq = to_sq((ss-2)->currentMove);
-    Move cm = thisThread->counterMoves[pos.piece_on(prevSq)][prevSq];
     const CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
-    const CounterMoveStats& fmh = CounterMoveHistory[pos.piece_on(ownPrevSq)][ownPrevSq];
 
-    MovePicker mp(pos, ttMove, depth, thisThread->history, cmh, fmh, cm, ss);
+    MovePicker mp(pos, ttMove, depth, ss);
     CheckInfo ci(pos);
     value = bestValue; // Workaround a bogus 'uninitialized' warning under gcc
     improving =   ss->staticEval >= (ss-2)->staticEval
@@ -992,6 +992,7 @@ moves_loop: // When in check search starts from here
       }
 
       ss->currentMove = move;
+      ss->counterMoves = &CounterMoveHistory[pos.moved_piece(move)][to_sq(move)];
 
       // Step 14. Make the move
       pos.do_move(move, st, givesCheck);
@@ -1153,13 +1154,17 @@ moves_loop: // When in check search starts from here
              && !bestMove
              && !inCheck
              && !pos.captured_piece_type()
-             && is_ok((ss - 1)->currentMove)
+             && is_ok((ss-1)->currentMove))
-             && is_ok((ss - 2)->currentMove))
     {
         Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
-        Square prevPrevSq = to_sq((ss - 2)->currentMove);
+        if ((ss-2)->counterMoves)
-        CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
+            (ss-2)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
-        prevCmh.update(pos.piece_on(prevSq), prevSq, bonus);
+
+        if ((ss-3)->counterMoves)
+            (ss-3)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
+
+        if ((ss-5)->counterMoves)
+            (ss-5)->counterMoves->update(pos.piece_on(prevSq), prevSq, bonus);
     }
 
     tte->save(posKey, value_to_tt(bestValue, ss->ply),
@@ -1280,7 +1285,7 @@ moves_loop: // When in check search starts from here
     // to search the moves. Because the depth is <= 0 here, only captures,
     // queen promotions and checks (only if depth >= DEPTH_QS_CHECKS) will
     // be generated.
-    MovePicker mp(pos, ttMove, depth, pos.this_thread()->history, to_sq((ss-1)->currentMove));
+    MovePicker mp(pos, ttMove, depth, to_sq((ss-1)->currentMove));
     CheckInfo ci(pos);
 
     // Loop through the moves until no moves remain or a beta cutoff occurs
@@ -1434,42 +1439,51 @@ moves_loop: // When in check search starts from here
     Value bonus = Value((depth / ONE_PLY) * (depth / ONE_PLY) + depth / ONE_PLY - 1);
 
     Square prevSq = to_sq((ss-1)->currentMove);
-    Square ownPrevSq = to_sq((ss-2)->currentMove);
+    CounterMoveStats* cmh  = (ss-1)->counterMoves;
-    CounterMoveStats& cmh = CounterMoveHistory[pos.piece_on(prevSq)][prevSq];
+    CounterMoveStats* fmh  = (ss-2)->counterMoves;
-    CounterMoveStats& fmh = CounterMoveHistory[pos.piece_on(ownPrevSq)][ownPrevSq];
+    CounterMoveStats* fmh2 = (ss-4)->counterMoves;
     Thread* thisThread = pos.this_thread();
 
     thisThread->history.update(pos.moved_piece(move), to_sq(move), bonus);
 
-    if (is_ok((ss-1)->currentMove))
+    if (cmh)
     {
         thisThread->counterMoves.update(pos.piece_on(prevSq), prevSq, move);
-        cmh.update(pos.moved_piece(move), to_sq(move), bonus);
+        cmh->update(pos.moved_piece(move), to_sq(move), bonus);
     }
 
-    if (is_ok((ss-2)->currentMove))
+    if (fmh)
-        fmh.update(pos.moved_piece(move), to_sq(move), bonus);
+        fmh->update(pos.moved_piece(move), to_sq(move), bonus);
+
+    if (fmh2)
+        fmh2->update(pos.moved_piece(move), to_sq(move), bonus);
 
     // Decrease all the other played quiet moves
     for (int i = 0; i < quietsCnt; ++i)
     {
         thisThread->history.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
 
-        if (is_ok((ss-1)->currentMove))
+        if (cmh)
-            cmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
+            cmh->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
 
-        if (is_ok((ss-2)->currentMove))
+        if (fmh)
-            fmh.update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
+            fmh->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
+
+        if (fmh2)
+            fmh2->update(pos.moved_piece(quiets[i]), to_sq(quiets[i]), -bonus);
     }
 
     // Extra penalty for a quiet TT move in previous ply when it gets refuted
-    if (   (ss-1)->moveCount == 1
+    if ((ss-1)->moveCount == 1 && !pos.captured_piece_type())
-        && !pos.captured_piece_type()
-        && is_ok((ss-2)->currentMove))
     {
-        Square prevPrevSq = to_sq((ss-2)->currentMove);
+        if ((ss-2)->counterMoves)
-        CounterMoveStats& prevCmh = CounterMoveHistory[pos.piece_on(prevPrevSq)][prevPrevSq];
+            (ss-2)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
-        prevCmh.update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
+
+        if ((ss-3)->counterMoves)
+            (ss-3)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
+
+        if ((ss-5)->counterMoves)
+            (ss-5)->counterMoves->update(pos.piece_on(prevSq), prevSq, -bonus - 2 * (depth + 1) / ONE_PLY);
     }
   }
 

src/search.h

@@ -28,6 +28,9 @@
 #include "position.h"
 #include "types.h"
 
+template<typename T, bool CM> struct Stats;
+typedef Stats<Value, true> CounterMoveStats;
+
 namespace Search {
 
 /// Stack struct keeps track of the information we need to remember from nodes
@@ -43,6 +46,7 @@ struct Stack {
   Value staticEval;
   bool skipEarlyPruning;
   int moveCount;
+  CounterMoveStats* counterMoves;
 };
 
 /// RootMove struct is used for moves at the root of the tree. For each root move