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21 years ago
bit by bit crc32 computation

<?php

function bitbybit_crc32($str,$first_call=false){

    //reflection in 32 bits of crc32 polynomial 0x04C11DB7
    $poly_reflected=0xEDB88320;

    //=0xFFFFFFFF; //keep track of register value after each call
    static $reg=0xFFFFFFFF;

    //initialize register on first call
    if($first_call) $reg=0xFFFFFFFF;
    
    $n=strlen($str);
    $zeros=$n<4 ? $n : 4;

    //xor first $zeros=min(4,strlen($str)) bytes into the register
    for($i=0;$i<$zeros;$i++)
        $reg^=ord($str{$i})<<$i*8;

    //now for the rest of the string
    for($i=4;$i<$n;$i++){
        $next_char=ord($str{$i});
        for($j=0;$j<8;$j++)
            $reg=(($reg>>1&0x7FFFFFFF)|($next_char>>$j&1)<<0x1F)
                ^($reg&1)*$poly_reflected;
    }

    //put in enough zeros at the end
    for($i=0;$i<$zeros*8;$i++)
        $reg=($reg>>1&0x7FFFFFFF)^($reg&1)*$poly_reflected;

    //xor the register with 0xFFFFFFFF
    return ~$reg;
}

$str="123456789"; //whatever
$blocksize=4; //whatever

for($i=0;$i<strlen($str);$i+=$blocksize) $crc=bitbybit_crc32(substr($str,$i,$blocksize),!$i);

?>
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