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 <[email protected]>

1 files changed, 8 insertions, 3 deletions

diff --git a/src/split.c b/src/split.c
index ce9a752..b30cdb6 100755
--- a/src/split.c
+++ b/src/split.c
@@ -1125,8 +1125,12 @@ Return value:
                                                  iExtend))
             {
                 ASSERT(iExtend == 0);
-                iExtend = -ONE_PLY;
-                ctx->sPlyInfo[ctx->uPly].iExtensionAmount = -ONE_PLY;
+                iExtend = -(ONE_PLY + ExtraReduction(uDepth,
+                                                     (g_SplitInfo[u].uAlreadyDone +
+                                                      uMoveNum + 1),
+                                                     GetMoveFailHighPercentage(mv)));
+                ctx->sPlyInfo[ctx->uPly].iExtensionAmount = iExtend;
+                INC(ctx->sCounters.tree.u64HistoryPrunes);
             }
 
             //
@@ -1147,8 +1151,9 @@ Return value:
             //
             if ((iExtend < 0) && (iScore >= iBeta))
             {
-                uDepth += ONE_PLY;
+                uDepth -= iExtend; // undo the full reduction, whatever its magnitude
                 ctx->sPlyInfo[ctx->uPly].iExtensionAmount = 0;
+                INC(ctx->sCounters.tree.u64HistoryPruneReSearches);
                 iScore = -Search(ctx, -iBeta, -iAlpha, uDepth);
             }
             UnmakeMove(ctx, mv);