Some micro-ops yeild a further 40%:

sub luhn {
    use integer;
    my $s = $_[ 0 ];
    my $total = 0;
    for my $i ( 0 .. 14 ) {
        my $d = substr( $s, $i, 1 );
        unless( $i & 1 ) {
            $d *= 2;
            $d -= 9 if $d > 9;
        }
        $total += $d;
    }
    $total *= 9;
    return chop $total;
}
[download]

And an Inline::C implementation of the algorithm is 20x faster still:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_luhn', CLEAN_AFTER_BUILD =>0;

int c_luhn( char *s ) {
    int i, total = 0;

    for( i=0; i < 15; ++i ) {
        int d = s[ i ] - '0';
        if( !( i  & 1 ) ) {
            d *= 2;
            if( d > 9 ) d -= 9;
        }
        total += d;
    }
    total *= 9;
    return total % 10;
}

END_C
use Time::HiRes qw[ time ];

my @samples = qw[
    4011350000000008
    4011350000000016
    4011350000000024
    4011350000000032
    4011350000000040
    4011350000000057
    4011350000000065
    4011350000000073
    4011350000000081
    4011350000000099
];

sub luhn {
    use integer;
    my $s = $_[ 0 ];
    my $total = 0;
    for my $i ( 0 .. 14 ) {
        my $d = substr( $s, $i, 1 );
        unless( $i & 1 ) {
            $d *= 2;
            $d -= 9 if $d > 9;
        }
        $total += $d;
    }
    $total *= 9;
    return chop $total;
}

for ( @samples ) {
    print "$_: ", luhn( substr $_, 0, 15 );
}

my $start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = luhn( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;

for ( @samples ) {
    print "$_: ", c_luhn( $_ );
}

my $start = time;
for ( 401135000000000..401135000999999 ) {
    my $chk = c_luhn( $_ );
}
printf "Took %.9f seconds.\n", time() - $start;
[download]

That's funny, I was converting yours to Inline::C at the same time.

int cd2(char *number) {
    int i, d, total;
    total=0;
    for (i = 0; i<15; i++) {
        d=number[i]-48;
        if (!(i & 1)) {
            d *=2;
            if (d > 9) {d-=9;}
        }
        total+=d;
    }
    total *=9;
    return total % 10;
}
[download]

And, it is a little faster, appreciate it.

$perl ./script
Benchmark: timing 200 iterations of Inline::C (BrowserUK-His Conversion), Inline::C (BrowserUK-My Conversion), Inline::C (orig)...
Inline::C (BrowserUK-His Conversion):  2 wallclock secs ( 2.54 usr +  0.00 sys =  2.54 CPU) @ 78.74/s (n=200)
Inline::C (BrowserUK-My Conversion):  3 wallclock secs ( 2.51 usr +  0.00 sys =  2.51 CPU) @ 79.68/s (n=200)
Inline::C (orig):  3 wallclock secs ( 2.97 usr +  0.00 sys =  2.97 CPU) @ 67.34/s (n=200)

Unrolling the loop and using Anonymonk's lookup trick extracts another 5%:

int l[] = { 0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };
int c_luhn2( char *s ) {
    int total = 0;

    total += l[ s[ 0] - '0' ];
    total +=    s[ 1] - '0';
    total += l[ s[ 2] - '0' ];
    total +=    s[ 3] - '0';
    total += l[ s[ 4] - '0' ];
    total +=    s[ 5] - '0';
    total += l[ s[ 6] - '0' ];
    total +=    s[ 7] - '0';
    total += l[ s[ 8] - '0' ];
    total +=    s[ 9] - '0';
    total += l[ s[10] - '0' ];
    total +=    s[11] - '0';
    total += l[ s[12] - '0' ];
    total +=    s[13] - '0';
    total += l[ s[14] - '0' ];

    total *= 9;
    return total % 10;
}
[download]

I doubt there is much fat left in there :)

---

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"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.

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