Search code documentation take II

[stockfish] / src / search.cpp

diff --git a/src/search.cpp b/src/search.cpp
index 670ab1511f5b51c085a4bad8df2e7a62b2a7b032..16f73d47bdee8ba3f5feedff584ba11aa5461900 100644 (file)
--- a/src/search.cpp
+++ b/src/search.cpp
@@ -1267,29 +1267,30 @@ namespace {
     if (depth < OnePly)
         return qsearch(pos, ss, beta-1, beta, Depth(0), ply, threadID);
 
-    // Initialize, and make an early exit in case of an aborted search,
-    // an instant draw, maximum ply reached, etc.
+    // Step 1. Initialize node and poll
+    // Polling can abort search.
     init_node(ss, ply, threadID);
 
-    // After init_node() that calls poll()
+    // Step 2. Check for aborted search and immediate draw
     if (AbortSearch || TM.thread_should_stop(threadID))
         return Value(0);
 
     if (pos.is_draw() || ply >= PLY_MAX - 1)
         return VALUE_DRAW;
 
-    // Mate distance pruning
+    // Step 3. Mate distance pruning
     if (value_mated_in(ply) >= beta)
         return beta;
 
     if (value_mate_in(ply + 1) < beta)
         return beta - 1;
 
+    // Step 4. Transposition table lookup
+
     // We don't want the score of a partial search to overwrite a previous full search
     // TT value, so we use a different position key in case of an excluded move exsists.
     Key posKey = excludedMove ? pos.get_exclusion_key() : pos.get_key();
 
-    // Transposition table lookup
     tte = TT.retrieve(posKey);
     ttMove = (tte ? tte->move() : MOVE_NONE);
 
@@ -1299,9 +1300,9 @@ namespace {
         return value_from_tt(tte->value(), ply);
     }
 
+    // Step 5. Evaluate the position statically
     isCheck = pos.is_check();
 
-    // Evaluate the position statically
     if (!isCheck)
     {
         if (tte && (tte->type() & VALUE_TYPE_EVAL))
@@ -1315,16 +1316,34 @@ namespace {
         update_gains(pos, ss[ply - 1].currentMove, ss[ply - 1].eval, ss[ply].eval);
     }
 
-    // Static null move pruning. We're betting that the opponent doesn't have
-    // a move that will reduce the score by more than FutilityMargins[int(depth)]
-    // if we do a null move.
+    // Step 6. Razoring
+    if (   !value_is_mate(beta)
+        && !isCheck
+        && depth < RazorDepth
+        && staticValue < beta - (0x200 + 16 * depth)
+        && ss[ply - 1].currentMove != MOVE_NULL
+        && ttMove == MOVE_NONE
+        && !pos.has_pawn_on_7th(pos.side_to_move()))
+    {
+        Value rbeta = beta - (0x200 + 16 * depth);
+        Value v = qsearch(pos, ss, rbeta-1, rbeta, Depth(0), ply, threadID);
+        if (v < rbeta)
+          return v; //FIXME: Logically should be: return (v + 0x200 + 16 * depth);
+    }
+
+    // Step 7. Static null move pruning
+    // We're betting that the opponent doesn't have a move that will reduce
+    // the score by more than fuility_margin(depth) if we do a null move.
     if (  !isCheck
         && allowNullmove
         && depth < RazorDepth
         && staticValue - futility_margin(depth, 0) >= beta)
         return staticValue - futility_margin(depth, 0);
 
-    // Null move search
+    // Step 8. Null move search with verification search
+    // When we jump directly to qsearch() we do a null move only if static value is
+    // at least beta. Otherwise we do a null move if static value is not more than
+    // NullMoveMargin under beta.
     if (    allowNullmove
         &&  depth > OnePly
         && !isCheck
@@ -1373,26 +1392,12 @@ namespace {
                 return beta - 1;
         }
     }
-    // Null move search not allowed, try razoring
-    else if (   !value_is_mate(beta)
-             && !isCheck
-             && depth < RazorDepth
-             && staticValue < beta - (NullMoveMargin + 16 * depth)
-             && ss[ply - 1].currentMove != MOVE_NULL
-             && ttMove == MOVE_NONE
-             && !pos.has_pawn_on_7th(pos.side_to_move()))
-    {
-        Value rbeta = beta - (NullMoveMargin + 16 * depth);
-        Value v = qsearch(pos, ss, rbeta-1, rbeta, Depth(0), ply, threadID);
-        if (v < rbeta)
-          return v;
-    }
 
-    // Go with internal iterative deepening if we don't have a TT move
+    // Step 9. Internal iterative deepening
     if (UseIIDAtNonPVNodes && ttMove == MOVE_NONE && depth >= 8*OnePly &&
         !isCheck && ss[ply].eval >= beta - IIDMargin)
     {
-        search(pos, ss, beta, Min(depth/2, depth-2*OnePly), ply, false, threadID);
+        search(pos, ss, beta, depth/2, ply, false, threadID);
         ttMove = ss[ply].pv[ply];
         tte = TT.retrieve(posKey);
     }