# https://rt.cpan.org/Public/Bug/Display.html?id=58572

--- Nilsimsa.pm.orig
+++ Nilsimsa.pm
@@ -37,6 +37,41 @@ wrapper around nilsimsa implementation i
 Pass in any text, any size, and get back a digest string composed 64
 hex chars.
 
+=item $nilsimsa->hex_distance($n1, $n2);
+
+Compare two nilsimsa digests n1 and n1 given as hex chars.
+The return value is a number in the range -128 .. 127, where any value over 24 
+(which is 3 sigma) is believed to indicate that the two texts are not generated
+independently.
+
+=cut
+# 773e2df0a02a319ec34a0b71d54029111da90838cbc20ecd3d2d4e18c25a3025
+# 47182cf0802a11dec24a3b75d5042d310ca90838c9d20ecc3d610e98560a3645
+# The nilsimsa of these two codes is 92 on a scale of -128 to +128. That means that
+# 36 bits are different and 220 bits the same. Any nilsimsa over 24 (which is 3
+# sigma) indicates that the two messages are probably not independently
+# generated.
+
+sub hex_distance
+{
+  shift @_ if ref $_[0];
+
+  #                0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
+  my @bit_diff = ( 0, 1, 1, 2, 1, 2, 2, 3, 1, 2,  2,  3,  2,  3,  3,  4);
+
+  my @n1 = split //, $_[0];
+  my @n2 = split //, $_[1];
+  die "length differs", if scalar(@n1) != scalar (@n2);
+  my $val = scalar(@n1) * 4;
+  for my $i (0..$#n1)
+    {
+      next if $n1[$i] eq $n2[$i];
+      my ($v1,$v2) = (hex($n1[$i]), hex($n2[$i]));
+      $val -= $bit_diff[$v1 ^ $v2];
+    }
+  return $val - scalar(@n1)*2;
+}
+
 =back
 
 =head1 SEE ALSO
--- t/01-hex_distance.t.orig
+++ t/01-hex_distance.t
@@ -0,0 +1,19 @@
+#!perl
+use strict;
+use warnings;
+use Test::More tests => 2;
+
+use Digest::Nilsimsa;
+
+my @n = qw(773e2df0a02a319ec34a0b71d54029111da90838cbc20ecd3d2d4e18c25a3025
+	   47182cf0802a11dec24a3b75d5042d310ca90838c9d20ecc3d610e98560a3645);
+
+# from http://ixazon.dynip.com/~cmeclax/nilsimsa.html:
+# The nilsimsa of these two codes is 92 on a scale of -128 to +128. That means
+# that 36 bits are different and 220 bits the same. Any nilsimsa over 24 (which
+# is 3 sigma) indicates that the two messages are probably not independently
+# generated.
+
+cmp_ok(Digest::Nilsimsa::hex_distance(@n),     '==', 92, 'hex_distance class method');
+cmp_ok(Digest::Nilsimsa::hex_distance({}, @n), '==', 92, 'hex_distance obj method');
+
--- MANIFEST.orig
+++ MANIFEST
@@ -7,6 +7,7 @@ README
 _nilsimsa.c
 nilsimsa.h
 t/00_load.t
+t/01-hex_distance.t
 t/a.txt
 t/b.txt
 t/d.txt