bp_document_get_download_range( int $file_size )

function bp_document_get_download_range( $file_size ) {
	$start          = 0;
	$download_range = array(
			'start'            => $start,
			'is_range_valid'   => false,
			'is_range_request' => false,
	);

	if ( ! $file_size ) {
		return $download_range;
	}

	$end                      = $file_size - 1;
	$download_range['length'] = $file_size;

	if ( isset( $_SERVER['HTTP_RANGE'] ) ) { // @codingStandardsIgnoreLine.
		$http_range                         = sanitize_text_field( wp_unslash( $_SERVER['HTTP_RANGE'] ) ); // WPCS: input var ok.
		$download_range['is_range_request'] = true;

		$c_start = $start;
		$c_end   = $end;
		// Extract the range string.
		list( , $range ) = explode( '=', $http_range, 2 );
		// Make sure the client hasn't sent us a multibyte range.
		if ( strpos( $range, ',' ) !== false ) {
			return $download_range;
		}

		/*
		 * If the range starts with an '-' we start from the beginning.
		 * If not, we forward the file pointer
		 * and make sure to get the end byte if specified.
		 */
		if ( '-' === $range[0] ) {
			// The n-number of the last bytes is requested.
			$c_start = $file_size - substr( $range, 1 );
		} else {
			$range   = explode( '-', $range );
			$c_start = ( isset( $range[0] ) && is_numeric( $range[0] ) ) ? (int) $range[0] : 0;
			$c_end   = ( isset( $range[1] ) && is_numeric( $range[1] ) ) ? (int) $range[1] : $file_size;
		}

		/*
		 * Check the range and make sure it's treated according to the specs: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html.
		 * End bytes can not be larger than $end.
		 */
		$c_end = ( $c_end > $end ) ? $end : $c_end;
		// Validate the requested range and return an error if it's not correct.
		if ( $c_start > $c_end || $c_start > $file_size - 1 || $c_end >= $file_size ) {
			return $download_range;
		}
		$start  = $c_start;
		$end    = $c_end;
		$length = $end - $start + 1;

		$download_range['start']          = $start;
		$download_range['length']         = $length;
		$download_range['is_range_valid'] = true;
	}
	return $download_range;
}