Add simple protections for search explosions due to extensions (#448)

Bench: 4897161 STC ELO | 1.69 +- 2.62 (95%) SPRT | 10.0+0.10s Threads=1 Hash=8MB LLR | 2.97 (-2.94, 2.94) [-2.50, 0.50] GAMES | N: 32520 W: 7945 L: 7787 D: 16788 LTC ELO | 0.07 +- 2.36 (95%) SPRT | 60.0+0.60s Threads=1 Hash=64MB LLR | 1.12 (-2.94, 2.94) [-2.50, 0.50] GAMES | N: 37456 W: 8476 L: 8468 D: 20512
jhonnold · Jan 16, 2023 · 6c17e81 · 6c17e81
1 parent f2aaf92
commit 6c17e81
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Showing 2 changed files with 35 additions and 33 deletions.
diff --git a/src/makefile b/src/makefile
@@ -4,7 +4,7 @@
 EXE      = berserk
 SRC      = *.c pyrrhic/tbprobe.c
 CC       = gcc
-VERSION  = 20230116
+VERSION  = 20230116b
 MAIN_NETWORK = networks/berserk-bf1200bb0547.nn
 EVALFILE = $(MAIN_NETWORK)
 DEFS     = -DVERSION=\"$(VERSION)\" -DEVALFILE=\"$(EVALFILE)\" -DNDEBUG

diff --git a/src/search.c b/src/search.c
@@ -548,40 +548,42 @@ int Negamax(int alpha, int beta, int depth, int cutnode, ThreadData* thread, PV*
     // other moves at a shallow depth on a nullwindow that is somewhere below
     // the tt evaluation implemented using "skip move" recursion like in SF
     // (allows for reductions when doing singular search)
-    if (!isRoot && depth >= 7 && tt && move == hashMove && TTDepth(tt) >= depth - 3 && (tt->flags & TT_LOWER) &&
-        abs(ttScore) < WINNING_ENDGAME) {
-      int sBeta  = max(ttScore - 3 * depth / 2, -CHECKMATE);
-      int sDepth = depth / 2 - 1;
-
-      ss->skip = move;
-      score    = Negamax(sBeta - 1, sBeta, sDepth, cutnode, thread, pv, ss);
-      ss->skip = NULL_MOVE;
-
-      // no score failed above sBeta, so this is singular
-      if (score < sBeta) {
-        singularExtension = 1;
-
-        if (!isPV && score < sBeta - 35 && ss->de <= 6) {
-          extension = 2;
-          ss->de    = (ss - 1)->de + 1;
-        } else {
-          extension = 1;
-        }
-      } else if (sBeta >= beta)
-        return sBeta;
-      else if (ttScore >= beta)
-        extension = -1;
-    }
+    if (ss->ply < thread->depth * 2) {
+      if (!isRoot && depth >= 7 && tt && move == hashMove && TTDepth(tt) >= depth - 3 && (tt->flags & TT_LOWER) &&
+          abs(ttScore) < WINNING_ENDGAME) {
+        int sBeta  = max(ttScore - 3 * depth / 2, -CHECKMATE);
+        int sDepth = depth / 2 - 1;
+
+        ss->skip = move;
+        score    = Negamax(sBeta - 1, sBeta, sDepth, cutnode, thread, pv, ss);
+        ss->skip = NULL_MOVE;
+
+        // no score failed above sBeta, so this is singular
+        if (score < sBeta) {
+          singularExtension = 1;
+
+          if (!isPV && score < sBeta - 35 && ss->de <= 6) {
+            extension = 2;
+            ss->de    = (ss - 1)->de + 1;
+          } else {
+            extension = 1;
+          }
+        } else if (sBeta >= beta)
+          return sBeta;
+        else if (ttScore >= beta)
+          extension = -1;
+      }
 
-    // history extension - if the tt move has a really good history score,
-    // extend. thank you to Connor, author of Seer for this idea
-    else if (!isRoot && depth >= 7 && tt && move == hashMove && history >= 98304)
-      extension = 1;
+      // history extension - if the tt move has a really good history score,
+      // extend. thank you to Connor, author of Seer for this idea
+      else if (!isRoot && depth >= 7 && tt && move == hashMove && history >= 98304 && abs(ttScore) < WINNING_ENDGAME)
+        extension = 1;
 
-    // re-capture extension - looks for a follow up capture on the same square
-    // as the previous capture
-    else if (!isRoot && isPV && IsRecapture(ss, move))
-      extension = 1;
+      // re-capture extension - looks for a follow up capture on the same square
+      // as the previous capture
+      else if (!isRoot && isPV && IsRecapture(ss, move))
+        extension = 1;
+    }
 
     ss->move = move;
     ss->ch   = &thread->ch[IsCap(move)][Moving(move)][To(move)];