MappedIterator.php

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<?php
class MappedIterator extends FilterIterator {
    protected $vCallback;
    protected $aCallback;
    protected $cache = [];

    protected $rewound = false; // boolean; whether rewind() has been called

    public function __construct( $iter, $vCallback, array $options = [] ) {
        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 = $options['accept'] ?? null;
    }

    public function next() {
        $this->cache = [];
        parent::next();
    }

    public function rewind() {
        $this->rewound = true;
        $this->cache = [];
        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
        }
    }

    protected function init() {
        if ( !$this->rewound ) {
            $this->rewind();
        }
    }
}