Play EGTB move at root even when 'cursed'.

2 files changed, 25 insertions, 20 deletions

diff --git a/src/probe.c b/src/probe.c
index d08ef75..a6730d8 100644
--- a/src/probe.c
+++ b/src/probe.c
@@ -487,20 +487,19 @@ Return value:
     }
 
     //
-    // A blessed win / cursed loss / plain draw at the root: don't
-    // steer the search away from its own move choice, since these
-    // are all draws under the fifty move rule and normal search
-    // handles that correctly already.  Only override the root move
-    // when this is a clear win or a position we must play carefully
-    // to avoid losing (both cases: follow the DTZ suggestion).
+    // Always follow Fathom's root DTZ suggestion once the tablebase has
+    // a verdict, including plain draws and the cursed-win/blessed-loss
+    // buckets.  A cursed win's/blessed loss's DTZ move is still the one
+    // that makes maximum progress toward actually converting/surviving
+    // before the fifty-move counter forces a draw -- ordinary search,
+    // steered only by a heuristic eval with no notion of the tablebase's
+    // verdict, can and does wander off the single correct try in these
+    // cases (confirmed live: a blessed-loss position in blunders.pgn
+    // where unassisted search picked a move that lost outright). Plain
+    // TB_DRAW has many equally-drawing replies, so this is never worse
+    // than deferring to search, just occasionally no better.
     //
     uWdl = TB_GET_WDL(uResult);
-    if ((TB_DRAW == uWdl) ||
-        (TB_CURSED_WIN == uWdl) ||
-        (TB_BLESSED_LOSS == uWdl))
-    {
-        return(FALSE);
-    }
 
     cFrom = FROM64(TB_GET_FROM(uResult));
     cTo = FROM64(TB_GET_TO(uResult));
@@ -524,12 +523,18 @@ Return value:
             // towards 100, a reset becomes forced soon even though
             // mate itself is nowhere near), producing a bogus
             // "+MATE<n>"/"-MATE<n>" display untethered from reality.
-            // The draw/cursed-win/blessed-loss cases already returned
-            // FALSE above, so uWdl here is only ever TB_WIN or
-            // TB_LOSS -- report the same flat mate-magnitude score
-            // ProbeEGTB/RecognLookup use at interior nodes instead.
+            // Report the same flat mate-magnitude score ProbeEGTB/
+            // RecognLookup use at interior nodes for a clear win/loss;
+            // the drawish buckets (plain draw, cursed win, blessed
+            // loss) get the normal draw score instead, matching
+            // ProbeEGTB's own WDL->score mapping.
             *pmv = mv;
-            *piScore = (TB_WIN == uWdl) ? +NMATE : -NMATE;
+            switch (uWdl)
+            {
+                case TB_WIN:  *piScore = +NMATE; break;
+                case TB_LOSS: *piScore = -NMATE; break;
+                default:      *piScore = g_iDrawScore[pos->uToMove]; break;
+            }
             return(TRUE);
         }
     }
diff --git a/src/root.c b/src/root.c
index 8fcace7..e9e82d4 100644
--- a/src/root.c
+++ b/src/root.c
@@ -1411,13 +1411,13 @@ Return value:
     // See if we are sitting at a checkmate or stalemate position; no
     // sense searching if so.
     //
-    ret = _DetectPreMoveTerminalStates(ctx, 
-                                       fInCheck, 
+    ret = _DetectPreMoveTerminalStates(ctx,
+                                       fInCheck,
                                        &mv);
     if (ret.eResult != RESULT_IN_PROGRESS) {
         goto end;
     }
-    
+
     // Game still in progress but only one legal move available?
     if (mv.uMove != 0) {
         ctx->mvRootMove = mv;