[iagno] ComputerPlayer: fix selection of some invalid moves

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 5375dd4ce7254a2c5b2063d1e4eae473242e160a
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Fri Sep 27 23:31:55 2013 -0500

    ComputerPlayer: fix selection of some invalid moves

 src/computer-player.vala |   13 +++++++++++--
 1 files changed, 11 insertions(+), 2 deletions(-)
---
diff --git a/src/computer-player.vala b/src/computer-player.vala
index bd50c14..da8369c 100644
--- a/src/computer-player.vala
+++ b/src/computer-player.vala
@@ -143,19 +143,28 @@ public class ComputerPlayer : Object
         foreach (var move in moves)
         {
             if (move.n_tiles == 0)
+            {
                 g.pass ();
+            }
             else if (g.place_tile (move.x, move.y) == 0)
             {
                 warning ("Computer marked move (depth %d, %d,%d, %d flips) as valid, but is invalid when 
checking", depth, move.x, move.y, move.n_tiles);
                 continue;
             }
 
+            /*
+             * We have to update the principle variant if the new alpha/beta is
+             * equal to the previous one, since we use (0, 0) as our default
+             * move, and a search could otherwise select it even if invalid
+             * when the search reaches the end of the game.
+             */
+
             /* If our move then maximise the result */
             if (p > 0)
             {
                 int next_x_move = 0, next_y_move = 0;
                 var a_new = search (g, strategy, depth - 1, a, b, -p, ref next_x_move, ref next_y_move);
-                if (a_new > a)
+                if (a_new >= a)
                 {
                     a = a_new;
                     move_x = move.x;
@@ -167,7 +176,7 @@ public class ComputerPlayer : Object
             {
                 int next_x_move = 0, next_y_move = 0;
                 var b_new = search (g, strategy, depth - 1, a, b, -p, ref next_x_move, ref next_y_move);
-                if (b_new < b)
+                if (b_new <= b)
                 {
                     b = b_new;
                     move_x = move.x;