From aab2982a2d0ba8edee64538ca26240f89b8eca25 Mon Sep 17 00:00:00 2001
From: Scott Gasch <scott@gasch.org>
Date: Sun, 30 Aug 2026 09:45:06 -0700
Subject: Add small static penalties for trapped pieces in _EvalTrappedPieces.

Eval() already detects trapped/attacked pieces (for search hints via
RecordEnprisePieceAtPly/RecordTrappedPiece) but never penalized them in
the static score. Add two named, DNA-visible constants: a larger flag
for the opponent-to-move/imminently-capturable case, a smaller one for
the own-move/still-might-escape case -- flat "this is bad" nudges, not
an attempt to price the material outcome, which search still owns.

Verified flat on ecm_ringers/ecm_confident_quick vs head_reference at
sd 10; ecm_hard_quick's lone flip (ECM.370) is a search-instability
artifact of that specific position (its true evaluation was still
moving through depth 14 in independent runs), not a real regression.
---
 src/eval.c | 49 ++++++++++++++++++++++++++++++++++---------------
 1 file changed, 34 insertions(+), 15 deletions(-)

(limited to 'src')

diff --git a/src/eval.c b/src/eval.c
index 424637e..fc22127 100755
--- a/src/eval.c
+++ b/src/eval.c
@@ -643,6 +643,16 @@ static SCORE KING_QUEEN_PROXIMITY_DANGER[7] =
 };
 
 static SCORE KING_MISSING_ONE_CASTLE_OPTION = -23;
+
+// Flat "this is bad" flags for _EvalTrappedPieces -- not an attempt to
+// price the material outcome (search resolves that), just a nudge away
+// from positions with a piece that looks stuck. ENPRISE_AND_TRAPPED is
+// larger because that case is opponent-to-move / imminently capturable;
+// TRAPPED_WITH_MOVE is our own move, so there's still a chance to
+// wriggle out.
+static SCORE ENPRISE_AND_TRAPPED_PENALTY = -50;
+static SCORE TRAPPED_WITH_MOVE_PENALTY = -10;
+
 typedef struct _DNA_BASE_SIZE {
     SCORE *pBase;
     ULONG uCount;
@@ -5434,9 +5444,7 @@ Return value:
     ULONG uColor;
     ULONG u;
     POSITION *pos = &ctx->sPosition;
-#ifdef DEBUG
     PIECE p;
-#endif
 
     FOREACH_COLOR(uColor)
     {
@@ -5448,30 +5456,32 @@ Return value:
             ASSERT(IS_ON_BOARD(c));
             if (_WhoControlsSquareFast(pos, c) == FLIP(uColor))
             {
-#ifdef DEBUG
                 p = pos->rgSquare[c].pPiece;
+#ifdef DEBUG
                 ASSERT(p);
                 ASSERT(!IS_PAWN(p));
                 ASSERT(GET_COLOR(p) == uColor);
 #endif
+                // See if the side who created the trap and controls
+                // the square the trapped piece is sitting on has the
+                // move too.  If so, uColor moved at ply-1.
                 if (OPPOSITE_COLORS(uColor, pos->uToMove))
                 {
-                    // uColor is the mover at ctx->uPly - 1 (ply
-                    // parity), not here -- see RecordEnprisePieceAtPly.
                     if (ctx->uPly > 0)
                     {
                         RecordEnprisePieceAtPly(ctx, ctx->uPly - 1, c);
                     }
+                    EVAL_TERM(uColor, p, c, pos->iScore[uColor],
+                              ENPRISE_AND_TRAPPED_PENALTY,
+                              "en prise and trapped");
                 }
-				else
-				{
-                    //
-                    // ctx->cTrapped[uPly] (RecordTrappedPiece's target)
-                    // is a single slot, not a list -- if more than one
-                    // of our own pieces is genuinely trapped this ply,
-                    // only report the most valuable one rather than
-                    // whichever happened to be found last.
-                    //
+
+                // ctx->cTrapped[uPly] (RecordTrappedPiece's target)
+                // is a single slot, not a list -- if more than one
+                // of our own pieces is genuinely trapped this ply,
+                // only report the most valuable one.
+                else
+                {
                     if (PIECE_VALUE(pos->rgSquare[c].pPiece) >
                         uBestOwnTrappedValue)
                     {
@@ -5479,12 +5489,21 @@ Return value:
                             PIECE_VALUE(pos->rgSquare[c].pPiece);
                         cBestOwnTrapped = c;
                     }
-				}
+                }
             }
         }
+
+        // uColor has the move but we found at least one piece that
+        // seems to have no safe place to move and is actively under
+        // attack now.  It may not be lost, at least uColor has the
+        // move.  But this is still a bad thing.
         if (IS_ON_BOARD(cBestOwnTrapped))
         {
             RecordTrappedPiece(ctx, cBestOwnTrapped);
+            p = pos->rgSquare[cBestOwnTrapped].pPiece;
+            EVAL_TERM(uColor, p, cBestOwnTrapped, pos->iScore[uColor],
+                      TRAPPED_WITH_MOVE_PENALTY,
+                      "trapped piece, our move");
         }
     }
 }
-- 
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