// Friedman Test - Index of Coincidence

// This code was written by Tyler Akins and placed in the public domain.
// It would be nice if you left this header intact.  http://rumkin.com


// Calculates an index of coincidence for a given text against itself
// Returns the kappa-plaintext.  To get the normalized Friedman IC,
// divide the return number by the alphabet length.
function Friedman(text, alphabet)
{
   var n;  // Counts of each letter
   var i, IC, N;
   
   // Assign a default alphabet and turn it into upper case
   if (! alphabet || alphabet == '') {
      alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
   }
   alphabet = alphabet.toUpperCase();

   n = LetterFrequency(text.toUpperCase());
   
   // Calculate IC
   IC = 0;
   N = 0;
   for (i = 0; i < alphabet.length; i ++) {
      var count = alphabet.charAt(i);
      count = n[count];
      if (! count)
      {
         count = 0;
      }
      IC += count * (count - 1);
      N += count;
   }
   IC /= N * (N - 1);
   
   return IC;
}


document.Friedman_Loaded = 1;