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diff --git a/includes/libs/MappedIterator.php b/includes/libs/MappedIterator.php
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+++ b/includes/libs/MappedIterator.php
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+<?php
+/**
+ * Convenience class for generating iterators from iterators.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @author Aaron Schulz
+ */
+
+/**
+ * Convenience class for generating iterators from iterators.
+ *
+ * @since 1.21
+ */
+class MappedIterator extends FilterIterator {
+	/** @var callable */
+	protected $vCallback;
+	/** @var callable */
+	protected $aCallback;
+	/** @var array */
+	protected $cache = array();
+
+	protected $rewound = false; // boolean; whether rewind() has been called
+
+	/**
+	 * Build an new iterator from a base iterator by having the former wrap the
+	 * later, returning the result of "value" callback for each current() invocation.
+	 * The callback takes the result of current() on the base iterator as an argument.
+	 * The keys of the base iterator are reused verbatim.
+	 *
+	 * An "accept" callback can also be provided which will be called for each value in
+	 * the base iterator (post-callback) and will return true if that value should be
+	 * included in iteration of the MappedIterator (otherwise it will be filtered out).
+	 *
+	 * @param Iterator|Array $iter
+	 * @param callable $vCallback Value transformation callback
+	 * @param array $options Options map (includes "accept") (since 1.22)
+	 * @throws UnexpectedValueException
+	 */
+	public function __construct( $iter, $vCallback, array $options = array() ) {
+		if ( is_array( $iter ) ) {
+			$baseIterator = new ArrayIterator( $iter );
+		} elseif ( $iter instanceof Iterator ) {
+			$baseIterator = $iter;
+		} else {
+			throw new UnexpectedValueException( "Invalid base iterator provided." );
+		}
+		parent::__construct( $baseIterator );
+		$this->vCallback = $vCallback;
+		$this->aCallback = isset( $options['accept'] ) ? $options['accept'] : null;
+	}
+
+	public function next() {
+		$this->cache = array();
+		parent::next();
+	}
+
+	public function rewind() {
+		$this->rewound = true;
+		$this->cache = array();
+		parent::rewind();
+	}
+
+	public function accept() {
+		$value = call_user_func( $this->vCallback, $this->getInnerIterator()->current() );
+		$ok = ( $this->aCallback ) ? call_user_func( $this->aCallback, $value ) : true;
+		if ( $ok ) {
+			$this->cache['current'] = $value;
+		}
+
+		return $ok;
+	}
+
+	public function key() {
+		$this->init();
+
+		return parent::key();
+	}
+
+	public function valid() {
+		$this->init();
+
+		return parent::valid();
+	}
+
+	public function current() {
+		$this->init();
+		if ( parent::valid() ) {
+			return $this->cache['current'];
+		} else {
+			return null; // out of range
+		}
+	}
+
+	/**
+	 * Obviate the usual need for rewind() before using a FilterIterator in a manual loop
+	 */
+	protected function init() {
+		if ( !$this->rewound ) {
+			$this->rewind();
+		}
+	}
+}