Fun and games - Data Structures and Problem Solving Using Java - page 434

1 final class Position
2 {
3 private int [ ][ ] board;
4
5 public Position( int [ ][ ] theBoard )
6 {
7 board = new int[ 3 ][ 3 ];
8 for( int i = 0; i < 3; i++ )
9 for( int j = 0; j < 3; j++ )
10 board[ i ][ j ] = theBoard[ i ][ j ];
11 }
12
13 public boolean equals( Object rhs )
14 {
15 if( ! (rhs instanceof Position ) )
16 return false;
17
18 Position other = (Position) rhs;
19
20 for( int i = 0; i < 3; i++ )
21 for( int j = 0; j < 3; j++ )
22 if( board[ i ][ j ] != ( (Position) rhs ).board[ i ][ j ] )
23 return false;
24 return true;
25 }
26
27 public int hashCode( )
28 {
29 int hashVal = 0;
30
31 for( int i = 0; i < 3; i++ )
32 for( int j = 0; j < 3; j++ )
33 hashVal = hashVal * 4 + board[ i ][ j ];
34
35 return hashVal;
36 }
37 }
figure 10.13
The Position class
Figures 10.15 and 10.16 show the new chooseMove . At line 8, we declare a
Position object, thisPosition . When the time comes it will be placed in the
transposition table. tableDepth tells us how deep in the search to allow posi-
tions to be placed in the transposition table. By experimenting we found that
depth 5 was optimal. Allowing positions at depth 6 to be saved hurt because