[gnugo-devel] more reading defense moves

[Arend Bayer]

 - reading.c: new move for defend1(): try to set up snap back
 - reading.c: special_rescue4() generalized to hane_rescue_moves()

The snap-back case should be self-explaining.

A hane is in most cases a useful defense when both the defended string
and the hane itself have at least as many liberties as the neighbouring
hostile string.

So far this is only used by defend3. Also, it can only be used when
stackp <= backfill2_depth because otherwise the corresponding followup
attack moves (defending strings with two liberties) are not generated.

This gives two PASSes in reading.tst, saving 8% of reading nodes, and
a PASS at connection:95. Otherwise it gives a FAIL at nngs3:980. I don't
understand why this happens, but the change (now L11 and M8 are
amalgamted) makes sense.  Plus trevorb:840
(which was only passing due to luck) is now properly solved.
For nngs.tst, it also saved small percentage of reading nodes.

I'd think the potential for improvement by extended use of
hane_rescue_moves() is good.

Arend


Index: engine/reading.c
===================================================================
RCS file: /cvsroot/gnugo/gnugo/engine/reading.c,v
retrieving revision 1.84
diff -u -p -r1.84 reading.c
--- engine/reading.c    12 Nov 2002 16:34:05 -0000      1.84
+++ engine/reading.c    16 Nov 2002 19:52:30 -0000
@@ -146,8 +146,8 @@ static void special_rescue2_moves(int st
                                  struct reading_moves *moves);
 static int special_rescue3(int str, int libs[3], int *move,
                           int komaster, int kom_pos);
-static int special_rescue4(int str, int libs[3], int *move,
-                          int komaster, int kom_pos);
+static void hane_rescue_moves(int str, int libs[4],
+                             struct reading_moves *moves);
 static void special_rescue5_moves(int str, int libs[3],
                                  struct reading_moves *moves);
 static void special_rescue6_moves(int str, int libs[3],
@@ -1127,6 +1127,24 @@ defend1(int str, int *move, int komaster
   moves.num = 1;

   break_chain_moves(str, &moves);
+
+  /* Try to set up snap-back. */
+  if (stackp <= backfill_depth && countstones(str) == 1) {
+    int adjs[MAXCHAIN];
+    int adj = chainlinks2(str, adjs, 2);
+    int r;
+    int libs[2];
+    for (r = 0; r < adj; r++) {
+      findlib(adjs[r], 2, libs);
+      if (neighbor_of_string(libs[0], str)
+         && !is_self_atari(libs[1], color))
+       ADD_CANDIDATE_MOVE(libs[1], 0, moves);
+      if (neighbor_of_string(libs[1], str)
+         && !is_self_atari(libs[0], color))
+       ADD_CANDIDATE_MOVE(libs[0], 0, moves);
+    }
+  }
+
   order_moves(str, &moves, color, read_function_name, 0);

   for (k = 0; k < moves.num; k++) {
@@ -1471,6 +1489,9 @@ defend3(int str, int *move, int komaster
   propose_edge_moves(str, libs, liberties, &moves, color);
   edge_clamp_moves(str, &moves);

+  if (stackp <= backfill2_depth)
+    hane_rescue_moves(str, libs, &moves);
+
   order_moves(str, &moves, color, read_function_name, 0);

   for (k = 0; k < moves.num; k++) {
@@ -1571,13 +1592,7 @@ defend3(int str, int *move, int komaster
     CHECK_RESULT_UNREVERSED(savecode, savemove, dcode, xpos, move,
                            "special rescue3");
   }
-
-  if (stackp <= backfill_depth) {
-    int dcode = special_rescue4(str, libs, &xpos, komaster, kom_pos);
-    CHECK_RESULT_UNREVERSED(savecode, savemove, dcode, xpos, move,
-                           "special rescue4");
-  }
-
+
   if (level >= 10 && stackp <= backfill2_depth) {
     int dcode = superstring_breakchain(str, &xpos, komaster, kom_pos, 4);
     CHECK_RESULT_UNREVERSED(savecode, savemove, dcode, xpos, move,
@@ -1722,8 +1737,12 @@ defend4(int str, int *move, int komaster
   break_chain_moves(str, &moves);
   break_chain2_efficient_moves(str, &moves);

-  if (stackp <= backfill_depth)
+  if (stackp <= backfill_depth) {
     break_chain2_defense_moves(str, &moves);
+#if 0
+    hane_rescue_moves(str, libs, &moves);
+#endif
+  }

   order_moves(str, &moves, color, read_function_name, 0);

@@ -1960,22 +1979,24 @@ special_rescue3(int str, int libs[3], in
  *   ......     defensive move at '*'.
  *   ------
  *
- *   .*O   acd
- *   OX.   be.
+ *   .*   ac
+ *   OX   bd
+ *
+ * The only requirement is that d has at most as many liberties as b,
+ * and as the newly placed stone at c.
  */
-static int
-special_rescue4(int str, int libs[3], int *move, int komaster, int kom_pos)
+static void
+hane_rescue_moves(int str, int libs[4], struct reading_moves *moves)
 {
   int color = board[str];
   int other = OTHER_COLOR(color);
-  int apos, bpos, cpos, dpos, epos;
-  int savemove = 0;
-  int savecode = 0;
+  int apos, bpos, cpos, dpos;
+  int num_libs = countlib(str);
   int k, l, r;

-  ASSERT1(countlib(str) == 3, str);
+  ASSERT1(num_libs <= 4, str);

-  for (r = 0; r < 3; r++) {
+  for (r = 0; r < num_libs; r++) {
     /* Let (apos) be one of the three liberties. */
     apos = libs[r];
     /* Try to find the configuration above. */
@@ -1996,45 +2017,27 @@ special_rescue4(int str, int libs[3], in
        if (board[cpos] != EMPTY)
          continue;

-       dpos = cpos + normal;
-       if (board[dpos] != color)
-         continue;
-
-       epos = bpos + normal;
-       if (board[epos] != other)
+       dpos = bpos + normal;
+       if (board[dpos] != other)
          continue;

-       /* Configuration found. Now require that (dpos) has at least 3
-         * liberties and (epos) at most 3 liberties.
-        */
-
-       if (countlib(dpos) < 3)
-         continue;
-
-       if (countlib(epos) > 3)
-         continue;
+       /* Configuration found. Now check liberty constraint. */
+       {
+         int dlibs = countlib(dpos);
+         if (dlibs > num_libs
+             || dlibs > accuratelib(cpos, color, dlibs, NULL))
+           continue;
+       }

-       /* Try to play at (cpos). */
-       if (trymove(cpos, color, "special_rescue4", str, komaster, kom_pos)) {
-         int acode = do_attack(str, NULL, komaster, kom_pos);
-         if (acode != WIN) {
-           if (acode == 0) {
-             popgo();
-             *move = cpos;
-             return WIN;
-           }
-           UPDATE_SAVED_KO_RESULT(savecode, savemove, acode, cpos);
-         }
-         popgo();
+       if (0 && !in_list(cpos, moves->num, moves->pos)) {
+         gprintf("hane_rescue_move added for %1m at %1m\n", str, cpos);
+         dump_stack();
+         showboard(0);
        }
+       ADD_CANDIDATE_MOVE(cpos, 0, *moves);
       }
     }
   }
-
-  if (savecode != 0)
-    *move = savemove;
-
-  return savecode;
 }