From 1509b3e593258690cbe3eee761a54f105111fc72 Mon Sep 17 00:00:00 2001
From: Scott Gasch <scott@gasch.org>
Date: Fri, 28 Aug 2026 20:41:48 -0700
Subject: Rewire GetMoveFailHighPercentage: index by (cFrom, cTo, pMoved)
 instead of (cFrom, cTo, color), and expose sample size.

MOVE_TO_INDEX (used elsewhere for the counter-move table) folds any
two moves with the same from/to/color into one fail-high bucket --
e.g. a king shuffle and a queen sac to the same square/color shared
a slot. New MOVE_TO_FH_INDEX uses the low 20 bits of mv.uMove
(cFrom+cTo+pMoved), which already encodes color in pMoved's low bit,
so this is strictly more granular for free. Table grown 0x20000 ->
0x100000 entries to match. Also adds an optional ULONG *puAttempts
out-param so future callers can weight by sample confidence instead
of trusting a percentage computed from as few as one observation.

GetLMRReduction (proto-LMR, live at HEAD) is the only real consumer
right now; verified against head_reference at sd10 across
ecm_ringers/ecm_confident_quick/ecm_hard_quick: net +2 solves (88 vs
87 on confident_quick, 18 vs 17 on hard_quick), node counts flat
within noise (-1.5%/+1.4%/+0.5%).
---
 src/chess.h     |  2 +-
 src/dynamic.c   | 24 ++++++++++++++++++------
 src/searchsup.c |  2 +-
 3 files changed, 20 insertions(+), 8 deletions(-)

(limited to 'src')

diff --git a/src/chess.h b/src/chess.h
index 3a78b2b..9722414 100755
--- a/src/chess.h
+++ b/src/chess.h
@@ -2782,7 +2782,7 @@ TestBitboards(void);
 extern ULONG g_HistoryCounters[14][128];
 
 ULONG
-GetMoveFailHighPercentage(MOVE mv);
+GetMoveFailHighPercentage(MOVE mv, ULONG *puAttempts);
 
 void
 UpdateDynamicMoveOrdering(SEARCHER_THREAD_CONTEXT *ctx,
diff --git a/src/dynamic.c b/src/dynamic.c
index 33a673c..c6e86e3 100755
--- a/src/dynamic.c
+++ b/src/dynamic.c
@@ -44,7 +44,14 @@ extern double log(double);
 
 ULONG g_HistoryCounters[14][128];
 SCORE g_iLMRQuietReduction[MAX_PLY_PER_SEARCH + 1][LMR_TABLE_MAX_MOVES + 1];
-#define FH_STATS_TABLE_SIZE (0x20000)
+
+// Keyed by (cFrom, cTo, pMoved) -- the low 20 bits of mv.uMove -- rather
+// than MOVE_TO_INDEX's (cFrom, cTo, color) so that e.g. a king shuffle
+// and a queen sac to the same square/color aren't folded into the same
+// fail-high bucket. pMoved's low bit is already the color, so this
+// subsumes MOVE_TO_INDEX's color term for free.
+#define MOVE_TO_FH_INDEX(mv) ((mv).uMove & 0xFFFFF)
+#define FH_STATS_TABLE_SIZE (0x100000)
 
 typedef struct _FH_STATS
 {
@@ -197,7 +204,7 @@ Return value:
 
 **/
 {
-    ULONG u = MOVE_TO_INDEX(mv);
+    ULONG u = MOVE_TO_FH_INDEX(mv);
     ULONG v = g_FailHighs[u].uWholeThing;
 
     ASSERT(DYN_IS_LOCKED);
@@ -233,7 +240,7 @@ Return value:
 
 **/
 {
-    ULONG u = MOVE_TO_INDEX(mv);
+    ULONG u = MOVE_TO_FH_INDEX(mv);
 
     ASSERT(DYN_IS_LOCKED);
     if (g_FailHighs[u].u16Attempts == 0xFFFF)
@@ -827,8 +834,8 @@ Return value:
 
 
 
-ULONG 
-GetMoveFailHighPercentage(IN MOVE mv)
+ULONG
+GetMoveFailHighPercentage(IN MOVE mv, OUT ULONG *puAttempts)
 /**
 
 Routine description:
@@ -839,6 +846,10 @@ Routine description:
 Parameters:
 
     MOVE mv
+    ULONG *puAttempts : optional (may be NULL) -- receives the number of
+        observations the percentage is based on, so callers can weight
+        by sample confidence instead of trusting a percentage computed
+        from as few as one attempt.
 
 Return value:
 
@@ -846,11 +857,12 @@ Return value:
 
 **/
 {
-    ULONG u = MOVE_TO_INDEX(mv);
+    ULONG u = MOVE_TO_FH_INDEX(mv);
     ULONG n, d;
 
     n = g_FailHighs[u].u16FailHighs;
     d = g_FailHighs[u].u16Attempts;
+    if (puAttempts) *puAttempts = d;
     if (d == 0)
     {
         return(0);
diff --git a/src/searchsup.c b/src/searchsup.c
index b532d9a..5673bbe 100644
--- a/src/searchsup.c
+++ b/src/searchsup.c
@@ -227,7 +227,7 @@ Return value:
         ((ctx->uPly < 3) ||
          (!IS_SAME_MOVE(mv, ctx->mvKiller[ctx->uPly-3][0]) &&
           !IS_SAME_MOVE(mv, ctx->mvKiller[ctx->uPly-3][1]))) &&
-        (GetMoveFailHighPercentage(mv) <= 10))
+        (GetMoveFailHighPercentage(mv, NULL) <= 10))
     {
         ASSERT(!InCheck(&ctx->sPosition, ctx->sPosition.uToMove));
         return(-ONE_PLY);
-- 
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