[gnugo-devel] A speedup

[Portela Fernando]

A simple idea : do not try the wrong liberty in a ladder attack.

Regression breakage : none
Performance impact  : -6.8% reading nodes
                      -9.2% trymoves

I measured a 4% speed increase over the whole regression suite, but
better timings should be done by someone else.

/nando

- don't even try the wrong liberty in simple_ladder_attack()

Index: engine/reading.c
===================================================================
RCS file: /cvsroot/gnugo/gnugo/engine/reading.c,v
retrieving revision 1.110
diff -u -r1.110 reading.c
--- engine/reading.c    1 Mar 2003 12:12:33 -0000       1.110
+++ engine/reading.c    21 Apr 2003 18:39:50 -0000
@@ -5491,10 +5491,21 @@
   /* Get the two liberties of (str). */
   findlib(str, 2, libs);
 
-  for (k = 0; k < 2; k++)
-    ADD_CANDIDATE_MOVE(libs[k], 0, moves, "simple_ladder_attack");
-
-  order_moves(str, &moves, other, read_function_name, 0, NO_MOVE);
+  /* Kludge : If the defender can get enough liberties by playing
+   * one of these two, then we have no choice but to block there
+   * and consequently, it is unnecessary to try the other liberty.
+   */
+  if (approxlib(libs[0], color, MAXLIBS, NULL) > 3) {
+    ADD_CANDIDATE_MOVE(libs[0], 0, moves, "simple_ladder_attack");
+  }
+  else if (approxlib(libs[1], color, MAXLIBS, NULL) > 3) {
+    ADD_CANDIDATE_MOVE(libs[1], 0, moves, "simple_ladder_attack");
+  }
+  else {
+    for (k = 0; k < 2; k++)
+      ADD_CANDIDATE_MOVE(libs[k], 0, moves, "simple_ladder_attack");
+    order_moves(str, &moves, other, read_function_name, 0, NO_MOVE);
+  }
 
   for (k = 0; k < moves.num; k++) {
     int new_komaster;