[gnugo-devel] Revision of inessentiality.

[Gunnar Farneback]

Near the end make_dragons(), critical worms are revised to inessential
under certain circumstances. This patch revises that code for reasons
which are best explained by the comments in the patch.

I don't know what this patch does to the regressions but I'd say it's
clearly correct and important in order to avoid abandoning monkey
jumps.

/Gunnar

Index: engine/dragon.c
===================================================================
RCS file: /cvsroot/gnugo/gnugo/engine/dragon.c,v
retrieving revision 1.87
diff -u -r1.87 dragon.c
--- engine/dragon.c     14 Oct 2002 15:41:07 -0000      1.87
+++ engine/dragon.c     15 Nov 2002 13:34:21 -0000
@@ -618,7 +618,45 @@
   /* Revise essentiality of critical worms. Specifically, a critical
    * worm which is adjacent to no enemy dragon with status
    * better than DEAD, is considered INESSENTIAL.
+   *
+   * A typical case of this is
+   *
+   * |.XXXX
+   * |.OO.X
+   * |X.O.X
+   * |.OO.X
+   * +-----
+   *
+   * However, to be able to deal with the position
+   *
+   * |.XXXX
+   * |.OOOO
+   * |..O.O
+   * |X.OOO
+   * |..O.O
+   * +-----
+   *
+   * we need to extend "adjacent" to "adjacent or shares a liberty",
+   * which is why we use extended_chainlinks() rather than
+   * chainlinks().
+   *
+   * Finally, if the position above is slightly modified to
+   *
+   * |.XXXXX
+   * |.OOOOO
+   * |...O.O
+   * |X..OOO
+   * |...O.O
+   * +------
+   *
+   * we have a position where the critical X stone doesn't share a
+   * liberty with any string at all. Thus the revised rule is:
+   *
+   * A critical worm which is adjacent to or share a liberty with at
+   * least one dead opponent dragon and no opponent dragon which is
+   * not dead, is considered inessential.
    */
+
   for (str = BOARDMIN; str < BOARDMAX; str++)
     if (ON_BOARD(str)) {
       if (is_worm_origin(str, str)
@@ -630,14 +668,14 @@
        int r;
        int essential = 0;
        
-       neighbors = chainlinks(str, adjs);
+       neighbors = extended_chainlinks(str, adjs, 0);
        for (r = 0; r < neighbors; r++)
          if (dragon[adjs[r]].status != DEAD) {
            essential = 1;
            break;
          }
 
-       if (!essential) {
+       if (!essential && neighbors > 0) {
          DEBUG(DEBUG_WORMS, "Worm %1m revised to be inessential.\n", str);
          worm[str].inessential = 1;
          propagate_worm(str);