Home * Organizations * ICGA * Investigations * Rybka Controversy * Pre-Fruit Rybka and Crafty * Equality of Even Score with 99999
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by Mark Watkins

Crafty Source

Here is the Crafty 19.15 code. This code was also mentioned in the ElChinito Case ^[1]:

  if ((TotalWhitePawns + TotalBlackPawns) == 0)
    do {
      int ms = EvaluateMate(tree);
 
      if (ms == 99999)
        break;
      score += ms;
#ifdef DEBUGEV
      printf("score[mater]=  %4d (%+d)\n", score,
          score - lastsc);
#endif
      if (score > DrawScore(1) && !(can_win & 1))
        score = score >> 2;
      if (score < DrawScore(1) && !(can_win & 2))
        score = score >> 2;
      return ((wtm) ? score : -score);
    } while (0);

Rybka Assembly

Here is the function call to something like EvaluateMate in a pre-Beta Rybka disassembly (Zach will likely provide details of how EvaluateMate works, and the closeness of the Rybka/Crafty match there).

0x00402b4b:     test   %ebx,%ebx              # test TotalWhitePawns + TotalBlackPawns
0x00402b59:     jne    0x402bca               # if nonzero, skip all this
0x00402b5b:     push   %esi
0x00402b5c:     call   0x404540               # function call: EvaluateMate
0x00402b61:     mov    0x8(%ebp),%ecx         # load "score" variable to ecx
0x00402b64:     add    $0x4,%esp
0x00402b67:     cmp    $0x1869f,%eax          # compare above return value to 99999
0x00402b6c:     je     0x402bcd               # if equal, then break
0x00402b6e:     add    %eax,%ecx              # score += ms
0x00402b70:     mov    0x6b8d7c,%eax          # load DrawScore(1)
0x00402b75:     cmp    %eax,%ecx              # compare score to DrawScore(1)
0x00402b77:     jle    0x402b84
0x00402b79:     testb  $0x1,-0x1c(%ebp)       # can_win & 1
0x00402b7d:     jne    0x402b82
0x00402b7f:     sar    $0x2,%ecx              # shift right by 2 if conditions met
0x00402b82:     cmp    %eax,%ecx              # compare score to DrawScore(1)
0x00402b84:     jge    0x402b8f
0x00402b86:     testb  $0x2,-0x1c(%ebp)       # can_win & 2
0x00402b8a:     jne    0x402b8f
0x00402b8c:     sar    $0x2,%ecx              # shift right by 2 if conditions met
0x00402b8f:     mov    0x10(%ebp),%eax        # load wtm
0x00402b92:     test   %eax,%eax              # determine by wtm whether score or -score
0x00402b94:     jne    0x402b98
0x00402b96:     neg    %ecx
0x00402b98:     mov    %ecx,%eax              # et cetera

EvaluateMate

Here are some relevant parts of that function call (Zach may provide more), enough to show that the return value is always even.

FUNCTION_CALL:
0x00404540:     mov    0x4(%esp),%ecx
[...]
0x004046b6:     lea    0x0(%ebp,%ebp,1),%eax     # add ebp to itself for the return value
0x004046ba:     pop    %ebp
0x004046bb:     pop    %ebx
0x004046bc:     ret
[...]
0x00404717:     lea    (%edi,%edi,1),%eax        # double
0x0040471a:     pop    %edi
0x0040471b:     pop    %esi
0x0040471c:     pop    %ebp
0x0040471d:     pop    %ebx
0x0040471e:     ret   
[...]
0x00404729:     lea    (%edi,%edi,1),%eax        # double
0x0040472c:     pop    %edi
0x0040472d:     pop    %esi
0x0040472e:     pop    %ebp
0x0040472f:     pop    %ebx
0x00404730:     ret   
0x00404731:     pop    %edi
0x00404732:     lea    (%esi,%esi,1),%eax        # double
0x00404735:     pop    %esi
0x00404736:     pop    %ebp
0x00404737:     pop    %ebx
0x00404738:     ret    

This is the only calling of 0x404540 in the code, and I've listed all the returns, with no jumps to instructions between a doubling and returning. In particular, the return value is always even, but then is compared for equality with 99999.

This code is largely relevant when mating KBN vs K.

References

1. ^ Re: Correction: List and ElChinito as Crafty clones, ... etc by Paul H., CCC, August 21, 2004

What links here?

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