#!/usr/local/bin/perl5 -w

################################################################
##
## Paul Garrett, garrett@math.umn.edu, 15 Sept 1997
##
## Script to compute Friedman's "Index of Coincidence" of a string
## with shifts-forward of itself, in order to guess the period
## of a periodic substitution cipher
##
## Usage: 
##
##       $0 <string>
##
## or
##
##       $0  <  <file>
## 
################################################################
##
## First, let's count the actual coincidences, shifted by ?
##
#################################################################

if ($in = shift) {
    chomp($in);
}
else {
    @in = <STDIN>;
    $in = join(' ',@in);
}

$in = lc($in);
$in =~ s/[^a-z]+//g; ## now it's all lowercase sans spaces

@in = split(//,$in);
$length = $#in;

foreach $shift (1..80) {
    $number_coincidences = 0;
    foreach $i (0..$length-$shift) {
	if ($in[$i] eq $in[$i + $shift]) {
	    $number_coincidences++;
	}
    }
    $out = int(10000 * $number_coincidences/($length-$shift+1))/100;
    print "Coincidences with shift ".$shift." = ".$out."\n"
	    if ($out > 4.7);
}
print "\n";
#####################################