Switch to Crafty-style killer ordering; fix mvNullmoveRefutations type-mixing bug and add a quiet-refutation killer backfill.

Killer tiers now try both of this ply's own killers before either
ply-2-back one, matching Crafty's ordering. Two earlier attempts at
this same swap were reverted for regressing; this pass lands on top of
NumLeftoverMovesToSelect (more SelectBestWithHistory budget to reach
these lower-tier slots) and a real bug fix below, and beats interleaved
order head-to-head on solves, node count, and first-move beta cutoff
across the three curated suites.

The bug: mvNullmoveRefutations's empty-killer-slot backfill could only
ever contain a capturing move (TryNullmovePruning only wrote it inside
the capture-refutation branch), but IS_SAME_MOVE's mask includes the
pCaptured bits, so that backfilled value could never match a real
quiet candidate -- the backfill was silently dead code. Fixed by
recording genuinely quiet null-move refutations into a new, separate
mvNullmoveQuietRefutations array (kept separate so it can't clobber the
capture history mvNullmoveRefutations still needs for the
Botvinnik-Markoff same-piece-two-squares extension check) and
backfilling the regular killer table from that instead. The
check-evasion killer table intentionally does *not* get this backfill:
a null-move refutation can never legitimately be an escaping-check
move (null moves can't deliver check), so backfilling there risks
IS_SAME_MOVE cross-context false positives instead of the old
guaranteed-inert no-op.

Measured at sd10 across ecm_ringers/ecm_confident_quick/ecm_hard_quick
against head_reference (commit d11e973): 115/191 solves (vs. 116
baseline), 924.36M total nodes (vs. 933.23M), first-move beta cutoff
within 0.1-0.9 points of baseline on all three suites -- and clearly
better than the same fix under interleaved order (113/191 solves,
963.10M nodes), which loses to head_reference on every metric.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EortUUkDVpsfrbqshBJYJg

1 files changed, 28 insertions, 6 deletions

diff --git a/src/searchsup.c b/src/searchsup.c
index e0d8251..54e1814 100644
--- a/src/searchsup.c
+++ b/src/searchsup.c
@@ -1167,7 +1167,7 @@ WeShouldTryNullmovePruning(SEARCHER_THREAD_CONTEXT *ctx,
         (FALSE == pi->fInCheck) &&
         (iBeta != +INFINITY) &&
         (FALSE == pi->fPvNode)) // <--- TODO: test this one please...
-    { 
+    {
         if (uNullDepth <= 6 * ONE_PLY)
         {
             u = uNullDepth / ONE_PLY;
@@ -1183,7 +1183,7 @@ WeShouldTryNullmovePruning(SEARCHER_THREAD_CONTEXT *ctx,
     }
     return FALSE;
 }
-        
+
 
 FLAG
 TryNullmovePruning(SEARCHER_THREAD_CONTEXT *ctx,
@@ -1203,10 +1203,10 @@ TryNullmovePruning(SEARCHER_THREAD_CONTEXT *ctx,
     SCORE iVerifyScore;
     MOVE mv;
     MOVE mvRef;
-       
+
     ASSERT(IS_VALID_SCORE(iAlpha) && IS_VALID_SCORE(iBeta));
     ASSERT(iAlpha < iBeta);
-    
+
     // TODO: more experiments with quick null
 
     // Ok, do it.
@@ -1265,7 +1265,7 @@ TryNullmovePruning(SEARCHER_THREAD_CONTEXT *ctx,
         ASSERT(SanityCheckMove(pos, mv));
         ASSERT(GET_COLOR(mv.pMoved) == pos->uToMove);
         ASSERT(ctx->uPly >= 2);
-        
+
         // If we make a nullmove that fails because we lose a
         // piece, remember that the piece in question is in danger.
         if ((mv.pCaptured) && !IS_PAWN(mv.pCaptured))
@@ -1291,7 +1291,29 @@ TryNullmovePruning(SEARCHER_THREAD_CONTEXT *ctx,
             }
             ctx->mvNullmoveRefutations[ctx->uPly] = mv;
         }
-        
+        else if (!IS_CAPTURE_OR_PROMOTION(mv))
+        {
+            // A quiet move refuting our nullmove is itself decent
+            // evidence that the move is strong here -- not enough to
+            // extend on (that's the capture-only Botvinnik-Markoff
+            // case above), but worth remembering as a fallback guess
+            // for _NewKillerMove's empty-slot backfill, which is only
+            // useful when this is actually a quiet move: a killer
+            // slot only ever gets matched against generated quiet
+            // candidates (IS_SAME_MOVE compares the pCaptured/
+            // pPromoted bits too), so storing a capture there (the
+            // old unconditional behavior) could never match anything
+            // and was silently dead.
+            //
+            // Deliberately a *separate* array from mvNullmoveRefutations,
+            // not a shared slot -- that one is read two plies later
+            // (ctx->uPly - 2, above) expecting a capture, to detect the
+            // same-piece-two-different-squares Botvinnik-Markoff pattern.
+            // Writing a quiet move into that same slot would silently
+            // clobber the capture history the extension depends on.
+            ctx->mvNullmoveQuietRefutations[ctx->uPly] = mv;
+        }
+
         // This is an idea from Tord Romstad on ccc: if we are below a
         // history reduced node and we try a nullmove and the nullmove
         // fails low and the move that refuted the nullmove involves