Improved array Map/Reduce in PHP (wrote on the fly, need to be tested w/ real data)

mapreduce.php

<?php

class MapReduce
{
  /**
   * The original data
   *
   * @var array
   */
  protected $data;

  /**
   * The original data length
   *
   * @var integer
   */
  protected $dataLength;

  /**
   * The latest mapped data
   *
   * @var array
   */
  protected $mappedData;

  /**
   * The latest reduced data
   *
   * @var array
   */
  protected $reducedData;

  /**
   * Shorthand static method to chain a map/reduce in one shot
   *
   * @param array    $data           The original data
   * @param function $mapCallback    The map callback
   * @param function $reduceCallback The reduce callback
   *
   * @return array The final reduced data
   */
  public static function mapReduce($data, $mapCallback, $reduceCallback)
  {
    $mapReduce = new self($data);
    $mapReduce->map($mapCallback)->reduce($reduceCallback);
    return $mapReduce->getReducedData();
  }

  /**
   * Constructor
   *
   * @param array $data The original data
   */
  public function __construct($data = null)
  {
    if (!$data) {
      throw new \Exception("InvalidConstructArgs");
    }

    $this->setData($data);
  }

  /**
   * Get the originale data
   *
   * @return array
   */
  public function getData()
  {
    return $this->data;
  }

  /**
   * Set the original data to be manipulated through map and reduce operations (also calculate the data length)
   *
   * @param array $data An array of data that will be manipulated
   */
  public function setData($data = array())
  {
    $this->data       = $data;
    $this->dataLength = count($this->data);
  }

  /**
   * Returns the latest mapped data
   *
   * @return array
   */
  public function getMappedData()
  {
    return $this->mappedData ?: array();
  }

  /**
   * Returns the latest reduced data
   *
   * @return array
   */
  public function getReducedData()
  {
    return $this->mappedData ?: array();
  }

  /**
   * The map method
   *
   * @param function $mapCallback The callback function used during the map process
   *
   * @return \FriendSpray\Utils\MapReduce $this The current instance object, useful for chaining
   */
  public function map($mapCallback)
  {
    // our map array
    $map = array();

    // emit callback
    $emit = function($key, $value) use (&$map) {
      if (!array_key_exists($key, $map)) {
        $map[$key] = array();
      }
      array_push($map[$key], $value);
    };

    for ($i = 0; $i < $this->dataLength; $i++) {
      call_user_func($mapCallback, $this->data[$i], $emit);
    }

    // set mapped data
    $this->mappedData = $map;

    return $this;
  }

  /**
   * The reduce method
   *
   * @return \FriendSpray\Utils\MapReduce $this The current instance object, useful for chaining
   */
  public function reduce()
  {
    // Different signature usage
    $args = func_get_args();
    $reduceCallback = array_pop($args);
    $map = func_num_args() > 1 ? array_shift($args) : $this->mappedData;

    // our reduce array
    $reduce = array();

    foreach($map as $k => $v) {
      array_push($reduce, call_user_func($reduceCallback, $k, $v));
    }

    $this->reducedData = $reduce;

    return $this;
  }
}