Fast Way to Find Difference between Two Strings of Equal Length in Perl

Question

Given pairs of string like this.

    my $s1 = "ACTGGA";
    my $s2 = "AGTG-A";

   # Note the string can be longer than this.

I would like to find position and character in in $s1 where it differs with $s2. In this case the answer would be:

#String Position 0-based
# First col = Base in S1
# Second col = Base in S2
# Third col = Position in S1 where they differ
C G 1
G - 4

I can achieve that easily with substr(). But it is horribly slow. Typically I need to compare millions of such pairs.

Is there a fast way to achieve that?

Answer 1

Stringwise ^ is your friend:

use strict;
use warnings;
my $s1 = "ACTGGA";
my $s2 = "AGTG-A";

my $mask = $s1 ^ $s2;
while ($mask =~ /[^\0]/g) {
    print substr($s1,$-[0],1), ' ', substr($s2,$-[0],1), ' ', $-[0], "\n";
}

EXPLANATION:

The ^ (exclusive or) operator, when used on strings, returns a string composed of the result of an exclusive or on each bit of the numeric value of each character. Breaking down an example into equivalent code:

"AB" ^ "ab"
( "A" ^ "a" ) . ( "B" ^ "b" )
chr( ord("A") ^ ord("a") ) . chr( ord("B") ^ ord("b") )
chr( 65 ^ 97 ) . chr( 66 ^ 98 )
chr(32) . chr(32)
" " . " "
"  "

The useful feature of this here is that a nul character ("\0") occurs when and only when the two strings have the same character at a given position. So ^ can be used to efficiently compare every character of the two strings in one quick operation, and the result can be searched for non-nul characters (indicating a difference). The search can be repeated using the /g regex flag in scalar context, and the position of each character difference found using $-[0], which gives the offset of the beginning of the last successful match.