ai: Adjust White's Search Depth at 5th and 6th Moves to Resolve Issue #…

…744

This commit addresses issue #744 by modifying the artificial intelligence's
search depth for White's 5th and 6th moves from 5 to 6/7. The change is intended
to alter the outcome in the given sequence of moves.

Before the adjustment:

 1.    f4    b4
 2.    d6    d2
 3.    d3    b2
 4.    a4    f6
 5.    c4

Outcome: c4 loses in 63 steps.

 1.    f4    b4
 2.    d6    d2
 3.    d3    b2
 4.    a4    f6
 5.    c4    d5
 6.    d1

Outcome: d1 loses in 29 steps.

Change-Id: I299c14a90f5cd15f6aa227beea90079bd64c3d95

src/mills.cpp

@@ -481,7 +481,7 @@ Depth get_search_depth(const Position *pos)
             constexpr Depth placingDepthTable9[25] = {
                 +1, 1, +1, 1,  /* 0 ~ 3 */
                 +3, 3, +3, 15, /* 4 ~ 7 */
-                +5, 3, +5, 0,  /* 8 ~ 11 */
+                +6, 3, +7, 0,  /* 8 ~ 11 */
                 +0, 0, +0, 0,  /* 12 ~ 15 */
                 +0, 0, +0, 0,  /* 16 ~ 19 */
                 +0, 0, +0, 0,  /* 20 ~ 23 */