I doubt it'll beat the C version, but I think it should beat the Perl versions and the algorithm (which I think is correct but haven't verified) would readily convert to C:

sub luhn {
    my $total = 0;
    for my $i ( 0 .. length $_[0] ) {
        my $d = substr $_[0], $i, 1 );
        if( $i & 1 ) {
            $d *= 2;
            $s -=9 if $d > 10;
        }
        $total += $d;    
    }
    $total *= 9;
    return substr $total, -1
}

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In reply to Re: Faster Luhn Check Digit Calculation? by BrowserUk
in thread Faster Luhn Check Digit Calculation? by kschwab

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