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### Re^4: Faster Luhn Check Digit Calculation?

by BrowserUk (Patriarch)
 on Dec 01, 2018 at 07:31 UTC ( #1226590=note: print w/replies, xml ) Need Help??

in reply to Re^3: Faster Luhn Check Digit Calculation?
in thread Faster Luhn Check Digit Calculation?

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;
}

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;

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
In the absence of evidence, opinion is indistinguishable from prejudice. Suck that fhit

Replies are listed 'Best First'.
Re^5: Faster Luhn Check Digit Calculation?
by kschwab (Vicar) on Dec 01, 2018 at 15:24 UTC
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;
}
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;
}

I doubt there is much fat left in there :)

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
In the absence of evidence, opinion is indistinguishable from prejudice. Suck that fhit

Looks fat to me ( just kidding :)

Is this any faster? Or does the compiler optimize away the differences?

```int l[] = {
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0,
0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

int
c_luhn2( char *s ) {
return ((
l[ (int)s[ 0] ]
+    s[ 1]
+ l[ (int)s[ 2] ]
+    s[ 3]
+ l[ (int)s[ 4] ]
+    s[ 5]
+ l[ (int)s[ 6] ]
+    s[ 7]
+ l[ (int)s[ 8] ]
+    s[ 9]
+ l[ (int)s[10] ]
+    s[11]
+ l[ (int)s[12] ]
+    s[13]
+ l[ (int)s[14] ] - 7 * '0' ) * 9) % 10;
}

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