RNG

Bomberman has a very complex random function. This may be necessary so as to avoid any noticeable patterns in the randomly generated level contents. 
PermutateRandomCtrlVars
        $D668  A5 54:       lda RandomCtrl1
        $D66A  2A:          rol a
        $D66B  2A:          rol a
        $D66C  49 41:       eor #$41
        $D66E  2A:          rol a   
        $D66F  2A:          rol a   
        $D670  49 93:       eor #$93
        $D672  65 55:       adc RandomCtrl2
        $D674  85 54:       sta RandomCtrl1
        $D676  2A:          rol a
        $D677  2A:          rol a
        $D678  49 12:       eor #$12
        $D67A  2A:          rol a   
        $D67B  2A:          rol a   
        $D67C  65 56:       adc RandomCtrl3
        $D67E  85 55:       sta RandomCtrl2
        $D680  65 54:       adc RandomCtrl1
        $D682  E6 56:       inc RandomCtrl3
        $D684  D0 09:       bne +               ; $D68F
        $D686  48:          pha 
        $D687  A5 57:        lda RandomCtrl4
        $D689  18:           clc 
        $D68A  69 1D:        adc #$1D
        $D68C  85 57:        sta RandomCtrl4
        $D68E  68:          pla
+       $D68F  45 57:       eor RandomCtrl4
        $D691  60:          rts
This function is called whenever randomness is needed, and often, the resulting value is even further permutated, for example, by executing "ror a" twice. The function is also called once every frame on the title screen.
Translated to C++, it becomes: 
unsigned char PermutateRandomVars(unsigned char random[3], bool carry)
{
    unsigned tmpval = carry << 8;    
    
    tmpval = ((random[0] << 4) | ((tmpval & 0x100) >> 5) | (random[0] >> 5)) ^ 0x87;         
    tmpval = (tmpval & 0xFF) + ((tmpval & 0x100) >> 8) + random[1];
    random[0] = tmpval;
    
    tmpval = ((random[0] << 4) | ((tmpval & 0x100) >> 5) | (random[0] >> 5)) ^ 0x48;
    random[0] = tmpval;

    tmpval = (tmpval & 0xFF) + ((tmpval & 0x100) >> 8) + random[2];
    random[1] = tmpval;

    tmpval = (tmpval & 0xFF) + ((tmpval & 0x100) >> 8) + random[0];
    
    if(!++random[2])
    {
        random[3] += 29;
    }
     
    return tmpval ^ random[3];
}
It does such a meticulous job that I think that it may be based on some standard cryptographical function such as CRC32... But I don't know.
GameResources/NES/Bomberman last edited by Randomno on 7/3/2023 1:41 PM
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