<?php
namespace Relay\Rest;

use Relay\Files\Fs;

final class Stream
{
    /**
     * Streams a file as a download and ends the request. A Range header gets just that piece (206), which is how the
     * screens fetch a big file in short requests a server's time limit can't cut short, and how a browser resumes one.
     */
    public static function send(string $path, string $filename, string $range = ''): void
    {
        $size = (int) filesize($path);
        $piece = self::range($range, $size);
        nocache_headers();
        header('Accept-Ranges: bytes');
        while (ob_get_level() > 0) {
            ob_end_clean();
        }
        if ($piece === false) {
            status_header(416);
            header('Content-Range: bytes */' . $size);
            exit;
        }
        header('Content-Type: application/octet-stream');
        header('Content-Disposition: attachment; filename="' . str_replace(['"', "\r", "\n"], '', $filename) . '"');
        header('X-Content-Type-Options: nosniff');
        if ($piece === null) {
            header('Content-Length: ' . (string) $size);
            Fs::send($path);
            exit;
        }
        status_header(206);
        header('Content-Range: bytes ' . $piece[0] . '-' . $piece[1] . '/' . $size);
        header('Content-Length: ' . (string) ($piece[1] - $piece[0] + 1));
        Fs::send_range($path, $piece[0], $piece[1] - $piece[0] + 1);
        exit;
    }

    /**
     * The bytes a Range header asks for, as [first, last]: null for the whole file (no header, or one this doesn't
     * answer, such as several ranges), false when nothing of the file is in it.
     *
     * @return array{0: int, 1: int}|false|null
     */
    public static function range(string $header, int $size)
    {
        if (!preg_match('/^bytes=(\d*)-(\d*)$/', trim($header), $m) || ($m[1] === '' && $m[2] === '')) {
            return null;
        }
        if ($m[1] === '') { // the last N bytes
            $n = (int) $m[2];
            return $n === 0 || $size === 0 ? false : [max(0, $size - $n), $size - 1];
        }
        $first = (int) $m[1];
        if ($m[2] !== '' && (int) $m[2] < $first) {
            return null; // not a valid range, so the header is ignored
        }
        if ($first >= $size) {
            return false;
        }
        return [$first, $m[2] === '' ? $size - 1 : min((int) $m[2], $size - 1)];
    }

    /** A download's name: letters, digits, dots and dashes. (sanitize_file_name() would turn "example.co.za" into "example.co_.za".) */
    public static function file_name(string $name): string
    {
        return trim((string) preg_replace('/[^A-Za-z0-9._-]+/', '-', $name), '-.');
    }
}