<?php /*** Computes the entropy of an array of tokens (aka symbols).*/ class Entropy {     var $tokens      = array();   var $num_events  = 0;   var $token_freqs = array();   var $token_probs = array();     var $num_tokens  = 0;   var $bits        = 0.0;   var $maxent      = 0.0;     var $ratio       = 0.0;         function Entropy($tokens) {     $this->tokens      = $tokens;     $this->num_events  = count($this->tokens);     $this->token_freqs = $this->getTokenFrequencies();     $this->num_tokens  = count($this->token_freqs);     foreach ($this->token_freqs as $token => $freq) {       $this->token_probs[$token]  = $freq / $this->num_events;       $entropy += $this->token_probs[$token] * log($this->token_probs[$token], 2);       }     $this->bits   = -1.0 * $entropy;     $this->maxent = log($this->num_tokens, 2);         $this->ratio  = $this->bits / $this->maxent;  }     function getTokenFrequencies() {     for ($i = 0; $i < $this->num_events; $i++)      $token_freqs[$this->tokens[$i]]++;     return $token_freqs;   }   } ?>