PHP map reduce (I hate myself)

mapreduce.php

<?php

$partition = function ($mapper, array $data, $nb_batches = 2) {
	$length = count($data);
	$nb_chunk = ceil($length / $nb_batches);

	$chunks = array_chunk($data, $nb_chunk);
	$batches = array();

	foreach ($chunks as $chunk) {
		$group = array();

		array_map(function($to_map) use ($mapper, &$group) {
			list($k, $v) = $mapper($to_map);
			if (!isset($group[$k])) {
				$group[$k] = array();
			}

			$group[$k][] = $v;
		}, $chunk);

		$batches[] = $group;
	}

	return $batches;
};

$reduce_batches = function($reducer, $batches) {
	$i = 0;
	$new_batches = array();
	foreach ($batches as $batch) {
		$new_batch = array();
		foreach ($batch as $k => $v) {
			$new_batch[$k] = array($reducer($k, $v));
		}

		if ($i++ % 2 != 0) {
			$last_batch = count($new_batches) - 1;
			$new_batches[$last_batch] = array_merge_recursive($new_batches[$last_batch], $new_batch);
		} else {
			$new_batches[] = $new_batch;
		}
	}

	return $new_batches;
};

$map_reduce = function(array $data, $mapper, $reducer, $nb_batches = 2) use ($partition, $reduce_batches) {
	$batches = $partition($mapper, $data, $nb_batches);

	$result = array();

	do {
		$batches = $reduce_batches($reducer, $batches);
	} while(count($batches) > 1);
	$batches = $reduce_batches($reducer, $batches);

	return $batches[0];
};

num_char.php

<?php

/**
 * USAGE: php num_char.php your string here
 */

require_once 'mapreduce.php';

array_shift($argv);
$chars = str_split(implode(' ', $argv));

$map = function($char) {
	return array($char, 1);
};

$reduce = function($key, $values) {
	return array_sum($values);
};

var_dump($map_reduce($chars, $map, $reduce));