class LimitStream

final class LimitStream implements StreamInterface {
    use StreamDecoratorTrait;
    
    /** @var int Offset to start reading from */
    private $offset;
    
    /** @var int Limit the number of bytes that can be read */
    private $limit;
    
    /** @var StreamInterface */
    private $stream;
    
    /**
     * @param StreamInterface $stream Stream to wrap
     * @param int             $limit  Total number of bytes to allow to be read
     *                                from the stream. Pass -1 for no limit.
     * @param int             $offset Position to seek to before reading (only
     *                                works on seekable streams).
     */
    public function __construct(StreamInterface $stream, int $limit = -1, int $offset = 0) {
        $this->stream = $stream;
        $this->setLimit($limit);
        $this->setOffset($offset);
    }
    public function eof() : bool {
        // Always return true if the underlying stream is EOF
        if ($this->stream
            ->eof()) {
            return true;
        }
        // No limit and the underlying stream is not at EOF
        if ($this->limit === -1) {
            return false;
        }
        return $this->stream
            ->tell() >= $this->offset + $this->limit;
    }
    
    /**
     * Returns the size of the limited subset of data
     */
    public function getSize() : ?int {
        if (null === ($length = $this->stream
            ->getSize())) {
            return null;
        }
        elseif ($this->limit === -1) {
            return $length - $this->offset;
        }
        else {
            return min($this->limit, $length - $this->offset);
        }
    }
    
    /**
     * Allow for a bounded seek on the read limited stream
     */
    public function seek($offset, $whence = SEEK_SET) : void {
        if ($whence !== SEEK_SET || $offset < 0) {
            throw new \RuntimeException(sprintf('Cannot seek to offset %s with whence %s', $offset, $whence));
        }
        $offset += $this->offset;
        if ($this->limit !== -1) {
            if ($offset > $this->offset + $this->limit) {
                $offset = $this->offset + $this->limit;
            }
        }
        $this->stream
            ->seek($offset);
    }
    
    /**
     * Give a relative tell()
     */
    public function tell() : int {
        return $this->stream
            ->tell() - $this->offset;
    }
    
    /**
     * Set the offset to start limiting from
     *
     * @param int $offset Offset to seek to and begin byte limiting from
     *
     * @throws \RuntimeException if the stream cannot be seeked.
     */
    public function setOffset(int $offset) : void {
        $current = $this->stream
            ->tell();
        if ($current !== $offset) {
            // If the stream cannot seek to the offset position, then read to it
            if ($this->stream
                ->isSeekable()) {
                $this->stream
                    ->seek($offset);
            }
            elseif ($current > $offset) {
                throw new \RuntimeException("Could not seek to stream offset {$offset}");
            }
            else {
                $this->stream
                    ->read($offset - $current);
            }
        }
        $this->offset = $offset;
    }
    
    /**
     * Set the limit of bytes that the decorator allows to be read from the
     * stream.
     *
     * @param int $limit Number of bytes to allow to be read from the stream.
     *                   Use -1 for no limit.
     */
    public function setLimit(int $limit) : void {
        $this->limit = $limit;
    }
    public function read($length) : string {
        if ($this->limit === -1) {
            return $this->stream
                ->read($length);
        }
        // Check if the current position is less than the total allowed
        // bytes + original offset
        $remaining = $this->offset + $this->limit - $this->stream
            ->tell();
        if ($remaining > 0) {
            // Only return the amount of requested data, ensuring that the byte
            // limit is not exceeded
            return $this->stream
                ->read(min($remaining, $length));
        }
        return '';
    }

}