Re: [gnugo-devel] atari_atari & kikashi problems

[Gunnar Farneback]

Trevor wrote:
> This is a serious weakness that may be simple to fix.  Pointless (or 
> worse) kikashi before saving string:
>   trevord:280
>   trevord:760
>   trevorc:1300
>   trevorc:1100
>   trevorc:480
>   trevorc:540
>   trevorc:640
>   trevorc:760
> Normally GNU Go doesn't make pointless kikashi (i.e. moves with
> a followup, but no other value).  However, if the followup threatens
> to save a string by capturing a surrounding string, then the kikashi 
> gets overvalued - more than saving the group directly.

Many of these can be attributed to improper followup values and are
solved by the patch below. Suppose that a move at '*' is a threat
against a string 'a'. What the patch essentially does is to check
whether the defense of 'a' in turn is a threat against '*', assuming
that implies that '*' probably ends in gote and shouldn't have a
followup value.

/Gunnar

Index: engine/move_reasons.c
===================================================================
RCS file: /cvsroot/gnugo/gnugo/engine/move_reasons.c,v
retrieving revision 1.55
diff -u -r1.55 move_reasons.c
--- engine/move_reasons.c       21 Dec 2001 07:51:38 -0000      1.55
+++ engine/move_reasons.c       3 Jan 2002 17:12:32 -0000
@@ -2732,6 +2732,9 @@
        * attacking that worm. We make at most one adjustment
        * of each type.
        *
+       * No followup value is awarded if the defense move is a threat
+       * back on our move because we're likely to end in gote then.
+       *
        * FIXME: It might be possible that parts of the dragon
        *        can be cut in the process of capturing the (aa)
        *        worm. In that case, not the entire size of the 
@@ -2747,14 +2750,15 @@
        *        a structured manner.
        */
 
-      if (trymove(pos, color, "estimate_territorial_value",
-                  NO_MOVE, EMPTY, NO_MOVE)) {
+      if (trymove(pos, color, "estimate_territorial_value-A",
+                 NO_MOVE, EMPTY, NO_MOVE)) {
        int adjs[MAXCHAIN];
        float adjusted_value = 2 * worm[aa].effective_size;
        float adjustment_up = 0.0;
        float adjustment_down = 0.0;
        int s;
        int num_adj;
+       int defense_move;
 
        /* In rare cases it may happen that the trymove() above
          * actually removed the string at aa.
@@ -2764,6 +2768,27 @@
        else
          num_adj = chainlinks(aa, adjs);
 
+       /* No followup value if string can be defended with threat
+         * against our move.
+        *
+        * FIXME: This is somewhat halfhearted since only one defense
+        * move is tested.
+        */
+       if (board[aa] != EMPTY
+           && find_defense(aa, &defense_move) != 0
+           && defense_move != NO_MOVE) {
+         if (trymove(defense_move, OTHER_COLOR(color),
+                     "estimate_territorial_value-b", NO_MOVE,
+                     EMPTY, NO_MOVE)) {
+           if (board[pos] == EMPTY || attack(pos, NULL) != 0) {
+             popgo();
+             popgo();
+             break;
+           }
+           popgo();
+         }
+       }
+       
        for (s = 0; s < num_adj; s++) {
          int adj = adjs[s];
 
@@ -3038,7 +3063,7 @@
    */
   this_value = 0.0;
 
-  if (move[pos].move_safety == 1)
+  if (move[pos].move_safety == 1 && safe_move(pos, color) == WIN)
     saved_stones[pos] = INFLUENCE_SAVED_STONE;
   else
     saved_stones[pos] = INFLUENCE_CAPTURED_STONE;
Index: patterns/helpers.c
===================================================================
RCS file: /cvsroot/gnugo/gnugo/patterns/helpers.c,v
retrieving revision 1.20
diff -u -r1.20 helpers.c
--- patterns/helpers.c  15 Dec 2001 14:35:27 -0000      1.20
+++ patterns/helpers.c  3 Jan 2002 17:12:37 -0000
@@ -426,12 +426,23 @@
  * XXXXO.
  *
  * where X can get out of atari with profit by capturing three O stones.
+ *
+ * Another case where we don't award the followup value is when the
+ * opponent can defend with a threat against our move, e.g. in this
+ * position:
+ *
+ * .OOOXX.
+ * .OXXO.X
+ * ..*.X..
+ * ..XX...
+ * 
  */
 
 void
 threaten_to_capture_helper(int move, int str)
 {
   int adj, adjs[MAXCHAIN];
+  int defense_move;
   int k;
   
   adj = chainlinks2(str, adjs, 1);
@@ -439,8 +450,23 @@
     if (worm[adjs[k]].defend_codes[0] != 0
        && !does_defend(move, adjs[k]))
       return;
-    
+
+  if (!TRYMOVE(move, OTHER_COLOR(board[str])))
+    return;
+  if (find_defense(str, &defense_move) != 0
+      && defense_move != NO_MOVE
+      && TRYMOVE(defense_move, board[str])) {
+    if (board[move] == EMPTY || attack(move, NULL) != 0) {
+      popgo();
+      popgo();
+      return;
+    }
+    popgo();
+  }
+  popgo();
+  
   add_followup_value(move, 2.0 * worm[str].effective_size);
+  TRACE("...followup value %f\n", 2.0 * worm[str].effective_size);
 }