Pool Sampling

poolsample.php

<?php

class PoolSample
{
    private $_pool = array()

    /**
     * Constructor
     * 
     * @param    Array    The Pool to Sample - Zero Indexed Array
     * 
     * @return   void
     */ 
    public function __construct($pool)
    {
        $this->_pool = $pool;
    }

    /**
     * Sampling Function
     * 
     * @param    int    Amount of items to return
     * 
     * @return   Array  Random Sample of Pool
     */ 
    public function sample($sample_size)
    {
        $sample = array();

        foreach ($$this->_pool AS $k => $v)
        {
            if ($k < $sample_size)
            {
                $sample[] = $v;
            }
            elseif ($this->_random_float() < $sample_size / ($k + 1))
            {
                $replace = rand(0, count($sample) -1);
                $sample[$replace] = $v;
            }
        }

        return $sample;
    }

    /**
     * Get a random float between 0 and 1
     * 
     * @return   float    Random float between 0 and 1
     */ 
    private function _random_float()
    {
        return rand(0, getrandmax())/getrandmax();
    }
}