<?php /** * Computes the entropy of a database column. */ class TableEntropy {   var $table       = "";   var $column      = "";   var $select      = "";       var $where       = "";     var $group_by    = "";           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 $ratio       = 0.0;       function setTable($table) {    $this->table = $table;  }  function setColumn($column) {    $this->column = $column;  }  function setSelect($sql) {    $this->select = $sql;  }  function setWhere($sql) {    $this->where = $sql;  }  function setGroupBy($sql) {    $this->group_by = $sql;  }    function getTokenFrequencies() {    global $db;                $sql  = " SELECT $this->column, count($this->column) as freq ";                       $sql .= " FROM ". $this->table;        if ($this->where != "")       $sql .= " WHERE ". $this->where;        if ($this->group_by != "")             $sql .= " GROUP BY ". $this->group_by;            else       $sql .= " GROUP BY ". $this->column;                $this->token_freqs = $db->getCol($sql, "freq");  }    function getEntropy() {    global $db;         $this->getTokenFrequencies();    $this->num_events  = array_sum($this->token_freqs);     $this->num_tokens  = count($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;          } } ?>