Shuffle detection #2064

Shuffle detection procedure :

Shuffling positions are detected if

    the last 36 moves are reversible (rule50_count() > 36),
    the position have been already in the TT,
    there is a still a pawn on the board (to avoid special endings like KBN vs K).

The position is then judged as a draw.

An extension is realized if we already made 14 successive reversible moves in PV to accelerate the detection of the eventual draw.

To go further : we can still improve the idea. The length of the tests need a lot of ressources.

    the limit of 36 is logic but must be checked again for special zugzwang positions,
    this limit can be decreased in special positions,
    the limit of 14 moves for extension has not been tuned.

STC
LLR: -2.94 (-2.94,2.94) [0.50,4.50]
Total: 32595 W: 7273 L: 7275 D: 18047 Elo +0.43
http://tests.stockfishchess.org/tests/view/5c90aa330ebc5925cfff1768

LTC
LLR: 2.95 (-2.94,2.94) [0.00,3.50]
Total: 51249 W: 8807 L: 8486 D: 33956 Elo +1.85
http://tests.stockfishchess.org/tests/view/5c90b2450ebc5925cfff1800

VLTC
LLR: 2.96 (-2.94,2.94) [0.00,3.50]
Total: 137974 W: 20503 L: 19983 D: 97488 Elo +1.05
http://tests.stockfishchess.org/tests/view/5c9243a90ebc5925cfff2a93

Bench: 3548313

src/search.cpp

@@ -602,6 +602,15 @@ namespace {
             : ttHit    ? tte->move() : MOVE_NONE;
     ttPv = (ttHit && tte->is_pv()) || (PvNode && depth > 4 * ONE_PLY);
 
+    // if position has been searched at higher depths and we are shuffling, return value_draw
+    if (pos.rule50_count() > 36
+        && ss->ply > 36
+        && depth < 3 * ONE_PLY
+        && ttHit
+        && tte->depth() > depth
+        && pos.count<PAWN>() > 0)
+        return VALUE_DRAW;
+
     // At non-PV nodes we check for an early TT cutoff
     if (  !PvNode
         && ttHit
@@ -920,6 +929,10 @@ moves_loop: // When in check, search starts from here
                && (pos.blockers_for_king(~us) & from_sq(move) || pos.see_ge(move)))
           extension = ONE_PLY;
 
+      // Shuffle extension
+      else if(pos.rule50_count() > 14 && ss->ply > 14 && depth < 3 * ONE_PLY && PvNode)
+          extension = ONE_PLY;
+
       // Castling extension
       else if (type_of(move) == CASTLING)
           extension = ONE_PLY;