MapReduce.php

<?php
/**
 * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
 * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
 *
 * Licensed under The MIT License
 * For full copyright and license information, please see the LICENSE.txt
 * Redistributions of files must retain the above copyright notice.
 *
 * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
 * @link          http://cakephp.org CakePHP(tm) Project
 * @license       MIT License (http://www.opensource.org/licenses/mit-license.php)
 */
namespace Cake\Collection\Iterator;

use ArrayIterator;
use IteratorAggregate;

/**
 * Implements a simplistic version of the popular Map-Reduce algorithm. Acts
 * like an iterator for the original passed data after each result has been
 * processed, thus offering a transparent wrapper for results coming from any
 * source.
 */
class MapReduce implements IteratorAggregate {

/**
 * Holds the shuffled results that were emitted from the map
 * phase
 *
 * @var array
 */
	protected $_intermediate = [];

/**
 * Holds the results as emitted during the reduce phase
 *
 * @var array
 */
	protected $_result = [];

/**
 * Whether the Map-Reduce routine has been executed already on the data
 *
 * @var boolean
 */
	protected $_executed = false;

/**
 * Holds the original data that needs to be processed
 *
 * @var \Traversable
 */
	protected $_data;

/**
 * A callable that will be executed for each record in the original data
 *
 * @var callable
 */
	protected $_mapper;

/**
 * A callable that will be executed for each intermediate record emitted during
 * the Map phase
 *
 * @var callable
 */
	protected $_reducer;

/**
 * Count of elements emitted during the Reduce phase
 *
 * @var string
 */
	protected $_counter = 0;

/**
 * Constructor
 *
 * ## Example:
 *
 * Separate all unique odd and even numbers in an array
 *
 * {{{
 *	$data = new \ArrayObject([1, 2, 3, 4, 5, 3]);
 *	$mapper = function ($value, $key, $mr) {
 *		$type = ($value % 2 === 0) ? 'even' : 'odd';
 *		$mr->emitIntermediate($value, $type);
 *	};
 *
 *	$reducer = function ($numbers, $type, $mr) {
 *		$mr->emit(array_unique($numbers), $type);
 *	};
 *	$results = new MapReduce($data, $mapper, $reducer);
 * }}}
 *
 * Previous example will generate the following result:
 *
 * {{{
 *	['odd' => [1, 3, 5], 'even' => [2, 4]]
 * }}}
 *
 * @param \Traversable $data the original data to be processed
 * @param callable $mapper the mapper callback. This function will receive 3 arguments.
 * The first one is the current value, second the current results key and third is
 * this class instance so you can call the result emitters.
 * @param callable $reducer the reducer callback. This function will receive 3 arguments.
 * The first one is the list of values inside a bucket, second one is the name
 * of the bucket that was created during the mapping phase and third one is an
 * instance of this class.
 */
	public function __construct(\Traversable $data, callable $mapper, callable $reducer = null) {
		$this->_data = $data;
		$this->_mapper = $mapper;
		$this->_reducer = $reducer;
	}

/**
 * Returns an iterator with the end result of running the Map and Reduce
 * phases on the original data
 *
 * @return \ArrayIterator
 */
	public function getIterator() {
		if (!$this->_executed) {
			$this->_execute();
		}
		return new ArrayIterator($this->_result);
	}

/**
 * Appends a new record to the bucket labelled with $key, usually as a result
 * of mapping a single record from the original data.
 *
 * @param mixed $value the record itself to store in the bucket
 * @param string $bucket the name of the bucket where to put the record
 * @return void
 */
	public function emitIntermediate($value, $bucket) {
		$this->_intermediate[$bucket][] = $value;
	}

/**
 * Appends a new record to the final list of results and optionally assign a key
 * for this record.
 *
 * @param mixed $value The value to be appended to the final list of results
 * @param string $key and optional key to assign to the value
 * @return void
 */
	public function emit($value, $key = null) {
		$this->_result[$key === null ? $this->_counter : $key] = $value;
		$this->_counter++;
	}

/**
 * Runs the actual Map-Reduce algorithm. This is iterate the original data
 * and call the mapper function for each , then for each intermediate
 * bucket created during the Map phase call the reduce function.
 *
 * @return void
 * @throws \LogicException if emitIntermediate was called but no reducer function
 * was provided
 */
	protected function _execute() {
		$mapper = $this->_mapper;
		foreach ($this->_data as $key => $value) {
			$mapper($value, $key, $this);
		}
		$this->_data = null;

		if (!empty($this->_intermediate) && empty($this->_reducer)) {
			throw new \LogicException('No reducer function was provided');
		}

		$reducer = $this->_reducer;
		foreach ($this->_intermediate as $key => $list) {
			$reducer($list, $key, $this);
		}
		$this->_intermediate = [];
		$this->_executed = true;
	}

}