class BufferStream

final class BufferStream implements StreamInterface {
    
    /** @var int */
    private $hwm;
    
    /** @var string */
    private $buffer = '';
    
    /**
     * @param int $hwm High water mark, representing the preferred maximum
     *                 buffer size. If the size of the buffer exceeds the high
     *                 water mark, then calls to write will continue to succeed
     *                 but will return 0 to inform writers to slow down
     *                 until the buffer has been drained by reading from it.
     */
    public function __construct(int $hwm = 16384) {
        $this->hwm = $hwm;
    }
    public function __toString() : string {
        return $this->getContents();
    }
    public function getContents() : string {
        $buffer = $this->buffer;
        $this->buffer = '';
        return $buffer;
    }
    public function close() : void {
        $this->buffer = '';
    }
    public function detach() {
        $this->close();
        return null;
    }
    public function getSize() : ?int {
        return strlen($this->buffer);
    }
    public function isReadable() : bool {
        return true;
    }
    public function isWritable() : bool {
        return true;
    }
    public function isSeekable() : bool {
        return false;
    }
    public function rewind() : void {
        $this->seek(0);
    }
    public function seek($offset, $whence = SEEK_SET) : void {
        throw new \RuntimeException('Cannot seek a BufferStream');
    }
    public function eof() : bool {
        return strlen($this->buffer) === 0;
    }
    public function tell() : int {
        throw new \RuntimeException('Cannot determine the position of a BufferStream');
    }
    
    /**
     * Reads data from the buffer.
     */
    public function read($length) : string {
        $currentLength = strlen($this->buffer);
        if ($length >= $currentLength) {
            // No need to slice the buffer because we don't have enough data.
            $result = $this->buffer;
            $this->buffer = '';
        }
        else {
            // Slice up the result to provide a subset of the buffer.
            $result = substr($this->buffer, 0, $length);
            $this->buffer = substr($this->buffer, $length);
        }
        return $result;
    }
    
    /**
     * Writes data to the buffer.
     */
    public function write($string) : int {
        $this->buffer .= $string;
        if (strlen($this->buffer) >= $this->hwm) {
            return 0;
        }
        return strlen($string);
    }
    
    /**
     * @return mixed
     */
    public function getMetadata($key = null) {
        if ($key === 'hwm') {
            return $this->hwm;
        }
        return $key ? null : [];
    }

}