From ba803fe406ee2470aa461578f530d292380ec241 Mon Sep 17 00:00:00 2001
From: Scott Gasch <scott@gasch.org>
Date: Tue, 25 Aug 2026 14:03:46 -0700
Subject: Add LMR with PV-adjacency guard (v7), verified against saved binary

Late Move Reductions using a depth x movecount table (Ethereal-style
formula), gated off PV nodes and the ply directly below a PV node
(PLY_INFO.fIsPVNode), with magnitude-aware re-search on fail-high.
Verified node-for-node identical to the previously tested-good v7
binary on a canary position (sd 10) after reconstructing from a ZFS
snapshot of search.c/root.c/split.c taken just before that binary was
built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 src/searchsup.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 60 insertions(+), 2 deletions(-)

(limited to 'src/searchsup.c')

diff --git a/src/searchsup.c b/src/searchsup.c
index 9e842d4..7af1a18 100644
--- a/src/searchsup.c
+++ b/src/searchsup.c
@@ -18,6 +18,7 @@ Revision History:
 
 **/
 
+#include <math.h>
 #include "chess.h"
 
 extern SCORE g_iRootScore[2];
@@ -224,7 +225,8 @@ Return value:
     ASSERT((uMoveNum > 0) || (uLegalMoves == 0));
     if ((uRemainingDepth >= TWO_PLY) &&
         (iBeta == (iAlpha + 1)) &&
-        (uLegalMoves > 5) &&
+        (FALSE == ctx->sPlyInfo[ctx->uPly - 1].fIsPVNode) &&
+        (uLegalMoves > 3) &&
         (0 == iExtend) &&
 //        (iRoughEval + ComputeMoveScore(ctx, mv, uMoveNum - 1) + 200 < iAlpha) &&
         (!IS_ESCAPING_CHECK(mv)) &&
@@ -244,10 +246,66 @@ Return value:
 }
 
 
+#define LMR_TABLE_MAX_DEPTH  32
+#define LMR_TABLE_MAX_MOVES  63
+
+static INT g_iLMRTable[LMR_TABLE_MAX_DEPTH + 1][LMR_TABLE_MAX_MOVES + 1];
+
+void
+InitializeLMRTable(void)
+{
+    ULONG d, m;
+    double r;
+
+    g_iLMRTable[0][0] = 0;
+    for (d = 0; d <= LMR_TABLE_MAX_DEPTH; d++)
+    {
+        for (m = 0; m <= LMR_TABLE_MAX_MOVES; m++)
+        {
+            if ((d < 1) || (m < 1))
+            {
+                g_iLMRTable[d][m] = 0;
+                continue;
+            }
+            r = 0.7844 + (log((double)d) * log((double)m) / 2.4696);
+            if (r < 0.0) r = 0.0;
+            g_iLMRTable[d][m] = (INT)((r * ONE_PLY) + 0.5);
+        }
+    }
+}
+
+
+INT
+ExtraReduction(IN ULONG uRemainingDepth,
+              IN ULONG uMoveNum,
+              IN ULONG uFailHighPct)
+{
+    INT iExtra;
+    ULONG d = uRemainingDepth / ONE_PLY;
+    ULONG m = uMoveNum;
+
+    if (d > LMR_TABLE_MAX_DEPTH) d = LMR_TABLE_MAX_DEPTH;
+    if (m > LMR_TABLE_MAX_MOVES) m = LMR_TABLE_MAX_MOVES;
+    iExtra = g_iLMRTable[d][m];
+
+    if (uFailHighPct == 0)
+    {
+        iExtra += QUARTER_PLY;
+    }
+
+    if ((INT)uRemainingDepth - ONE_PLY - iExtra < TWO_PLY)
+    {
+        iExtra = MAX((INT)uRemainingDepth - ONE_PLY - TWO_PLY, 0);
+    }
+    ASSERT(iExtra >= 0);
+    return(iExtra);
+}
+
+
 SCORE
 ComputeMoveScore(IN SEARCHER_THREAD_CONTEXT *ctx,
                  IN MOVE mv,
-                 IN ULONG uMoveNum) 
+                 IN ULONG uMoveNum)
 {
     SCORE iMoveScore = (PIECE_VALUE(mv.pCaptured) +
                         PIECE_VALUE(mv.pPromoted));
-- 
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