<?php
class Config {
	private const _ENVVAR_PREFIX = "TTRSS_";

	const T_BOOL = 1;
	const T_STRING = 2;
	const T_INT = 3;

	const SCHEMA_VERSION = 147;

	/** override default values, defined below in _DEFAULTS[], prefixing with _ENVVAR_PREFIX:
	 *
	 * DB_TYPE becomes:
	 *
	 * .env (docker environment):
	 *
	 * TTRSS_DB_TYPE=pgsql
	 *
	 * or config.php:
	 *
	 * putenv('TTRSS_DB_TYPE=pgsql');
	 *
	 * note lack of quotes and spaces before and after "=".
	 *
	*/

	/** database type: pgsql or mysql */
	const DB_TYPE = "DB_TYPE";

	/** database server hostname */
	const DB_HOST = "DB_HOST";

	/** database user */
	const DB_USER = "DB_USER";

	/** database name */
	const DB_NAME = "DB_NAME";

	/** database password */
	const DB_PASS = "DB_PASS";

	/** database server port */
	const DB_PORT = "DB_PORT";

	/** connection charset for MySQL. if you have a legacy database and/or experience
	 * garbage unicode characters with this option, try setting it to a blank string. */
	const MYSQL_CHARSET = "MYSQL_CHARSET";

	/** this is a fallback falue for the CLI SAPI, it should be set to a fully-qualified tt-rss URL */
	const SELF_URL_PATH = "SELF_URL_PATH";

	/** operate in single user mode, disables all functionality related to
	 * multiple users and authentication. enabling this assumes you have
	 * your tt-rss directory protected by other means (e.g. http auth). */
	const SINGLE_USER_MODE = "SINGLE_USER_MODE";

	/** enables fallback update mode where tt-rss tries to update feeds in
	 * background while tt-rss is open in your browser.
	 * if you don't have a lot of feeds and don't want to or can't run
	 * background processes while not running tt-rss, this method is generally
	 * viable to keep your feeds up to date. */
	const SIMPLE_UPDATE_MODE = "SIMPLE_UPDATE_MODE";

	/** use this PHP CLI executable to start various tasks */
	const PHP_EXECUTABLE = "PHP_EXECUTABLE";

	/** base directory for lockfiles (must be writable) */
	const LOCK_DIRECTORY = "LOCK_DIRECTORY";

	/** base directory for local cache (must be writable) */
	const CACHE_DIR = "CACHE_DIR";

	/** directory for feed favicons (directory must be writable) */
	const ICONS_DIR = "ICONS_DIR";

	/** URL for feed favicons */
	const ICONS_URL = "ICONS_URL";

	/** auto create users authenticated via external modules */
	const AUTH_AUTO_CREATE = "AUTH_AUTO_CREATE";

	/** auto log in users authenticated via external modules i.e. auth_remote */
	const AUTH_AUTO_LOGIN = "AUTH_AUTO_LOGIN";

	/** unconditinally purge all articles older than this amount, in days
	 * overrides user-controlled purge interval */
	const FORCE_ARTICLE_PURGE = "FORCE_ARTICLE_PURGE";

	/** default lifetime of a session (e.g. login) cookie. In seconds,
	 * 0 means cookie will be deleted when browser closes. */
	const SESSION_COOKIE_LIFETIME = "SESSION_COOKIE_LIFETIME";

	/** send email using this name */
	const SMTP_FROM_NAME = "SMTP_FROM_NAME";

	/** send email using this address */
	const SMTP_FROM_ADDRESS = "SMTP_FROM_ADDRESS";

	/** default subject for email digest */
	const DIGEST_SUBJECT = "DIGEST_SUBJECT";

	/** enable built-in update checker, both for core code and plugins (using git) */
	const CHECK_FOR_UPDATES = "CHECK_FOR_UPDATES";

	/** system plugins enabled for all users, comma separated list, no quotes
	 * keep at least one auth module in there (i.e. auth_internal) */
	const PLUGINS = "PLUGINS";

	/** available options: sql (default, event log), syslog, stdout (for debugging) */
	const LOG_DESTINATION = "LOG_DESTINATION";

	/** link this stylesheet on all pages (if it exists), should be placed in themes.local */
	const LOCAL_OVERRIDE_STYLESHEET = "LOCAL_OVERRIDE_STYLESHEET";

	/** same but this javascript file (you can use that for polyfills), should be placed in themes.local */
	const LOCAL_OVERRIDE_JS = "LOCAL_OVERRIDE_JS";

	/** in seconds, terminate update tasks that ran longer than this interval */
	const DAEMON_MAX_CHILD_RUNTIME = "DAEMON_MAX_CHILD_RUNTIME";

	/** max concurrent update jobs forking update daemon starts */
	const DAEMON_MAX_JOBS = "DAEMON_MAX_JOBS";

	/** How long to wait for response when requesting feed from a site (seconds)  */
	const FEED_FETCH_TIMEOUT = "FEED_FETCH_TIMEOUT";

	/** How long to wait for response when requesting uncached feed from a site (seconds)  */
	const FEED_FETCH_NO_CACHE_TIMEOUT = "FEED_FETCH_NO_CACHE_TIMEOUT";

	/** Default timeout when fetching files from remote sites */
	const FILE_FETCH_TIMEOUT = "FILE_FETCH_TIMEOUT";

	/** How long to wait for initial response from website when fetching remote files */
	const FILE_FETCH_CONNECT_TIMEOUT = "FILE_FETCH_CONNECT_TIMEOUT";

	/** stop updating feeds if user haven't logged in for X days */
	const DAEMON_UPDATE_LOGIN_LIMIT = "DAEMON_UPDATE_LOGIN_LIMIT";

	/** how many feeds to update in one batch */
	const DAEMON_FEED_LIMIT = "DAEMON_FEED_LIMIT";

	/** default sleep interval between feed updates (sec) */
	const DAEMON_SLEEP_INTERVAL = "DAEMON_SLEEP_INTERVAL";

	/** do not cache files larger than that (bytes) */
	const MAX_CACHE_FILE_SIZE = "MAX_CACHE_FILE_SIZE";

	/** do not download files larger than that (bytes) */
	const MAX_DOWNLOAD_FILE_SIZE = "MAX_DOWNLOAD_FILE_SIZE";

	/** max file size for downloaded favicons (bytes) */
	const MAX_FAVICON_FILE_SIZE = "MAX_FAVICON_FILE_SIZE";

	/** max age in days for various automatically cached (temporary) files */
	const CACHE_MAX_DAYS = "CACHE_MAX_DAYS";

	/** max interval between forced unconditional updates for servers
	 * not complying with http if-modified-since (seconds) */
	const MAX_CONDITIONAL_INTERVAL = "MAX_CONDITIONAL_INTERVAL";

	/** automatically disable updates for feeds which failed to
	 * update for this amount of days; 0 disables */
	const DAEMON_UNSUCCESSFUL_DAYS_LIMIT = "DAEMON_UNSUCCESSFUL_DAYS_LIMIT";

	/** log all sent emails in the event log */
	const LOG_SENT_MAIL = "LOG_SENT_MAIL";

	/** use HTTP proxy for requests */
	const HTTP_PROXY = "HTTP_PROXY";

	/** prevent users from changing passwords */
	const FORBID_PASSWORD_CHANGES = "FORBID_PASSWORD_CHANGES";

	/** default session cookie name */
	const SESSION_NAME = "SESSION_NAME";

	/** enable plugin update checker (using git) */
	const CHECK_FOR_PLUGIN_UPDATES = "CHECK_FOR_PLUGIN_UPDATES";

	/** allow installing first party plugins using plugin installer in prefs */
	const ENABLE_PLUGIN_INSTALLER = "ENABLE_PLUGIN_INSTALLER";

	/** minimum amount of seconds required between authentication attempts */
	const AUTH_MIN_INTERVAL = "AUTH_MIN_INTERVAL";

	/** http user agent (changing this is not recommended) */
	const HTTP_USER_AGENT = "HTTP_USER_AGENT";

	/** delay updates for this feed if received HTTP 429 (Too Many Requests) for this amount of seconds (base value, actual delay is base...base*2) */
	const HTTP_429_THROTTLE_INTERVAL = "HTTP_429_THROTTLE_INTERVAL";

	/** host running Jaeger collector to receive traces (disabled if empty) */
	const OPENTELEMETRY_ENDPOINT = "OPENTELEMETRY_ENDPOINT";

	/** Jaeger service name */
	const OPENTELEMETRY_SERVICE = "OPENTELEMETRY_SERVICE";

	/** default values for all global configuration options */
	private const _DEFAULTS = [
		Config::DB_TYPE => [ "pgsql", 									Config::T_STRING ],
		Config::DB_HOST => [ "db", 										Config::T_STRING ],
		Config::DB_USER => [ "",											Config::T_STRING ],
		Config::DB_NAME => [ "", 											Config::T_STRING ],
		Config::DB_PASS => [ "", 											Config::T_STRING ],
		Config::DB_PORT => [ "5432",										Config::T_STRING ],
		Config::MYSQL_CHARSET => [ "UTF8",								Config::T_STRING ],
		Config::SELF_URL_PATH => [ "https://example.com/tt-rss", Config::T_STRING ],
		Config::SINGLE_USER_MODE => [ "",								Config::T_BOOL ],
		Config::SIMPLE_UPDATE_MODE => [ "",								Config::T_BOOL ],
		Config::PHP_EXECUTABLE => [ "/usr/bin/php",					Config::T_STRING ],
		Config::LOCK_DIRECTORY => [ "lock",								Config::T_STRING ],
		Config::CACHE_DIR => [ "cache",									Config::T_STRING ],
		Config::ICONS_DIR => [ "feed-icons",							Config::T_STRING ],
		Config::ICONS_URL => [ "feed-icons",							Config::T_STRING ],
		Config::AUTH_AUTO_CREATE => [ "true",							Config::T_BOOL ],
		Config::AUTH_AUTO_LOGIN => [ "true",							Config::T_BOOL ],
		Config::FORCE_ARTICLE_PURGE => [ 0,								Config::T_INT ],
		Config::SESSION_COOKIE_LIFETIME => [ 86400,					Config::T_INT ],
		Config::SMTP_FROM_NAME => [ "Tiny Tiny RSS",					Config::T_STRING ],
		Config::SMTP_FROM_ADDRESS => [ "noreply@localhost",		Config::T_STRING ],
		Config::DIGEST_SUBJECT => [ "[tt-rss] New headlines for last 24 hours",
																					Config::T_STRING ],
		Config::CHECK_FOR_UPDATES => [ "true",							Config::T_BOOL ],
		Config::PLUGINS => [ "auth_internal",							Config::T_STRING ],
		Config::LOG_DESTINATION => [ Logger::LOG_DEST_SQL,			Config::T_STRING ],
		Config::LOCAL_OVERRIDE_STYLESHEET => [ "local-overrides.css",
																					Config::T_STRING ],
		Config::LOCAL_OVERRIDE_JS => [ "local-overrides.js",
																					Config::T_STRING ],
		Config::DAEMON_MAX_CHILD_RUNTIME => [ 1800,					Config::T_INT ],
		Config::DAEMON_MAX_JOBS => [ 2,									Config::T_INT ],
		Config::FEED_FETCH_TIMEOUT => [ 45,								Config::T_INT ],
		Config::FEED_FETCH_NO_CACHE_TIMEOUT => [ 15,					Config::T_INT ],
		Config::FILE_FETCH_TIMEOUT => [ 45,								Config::T_INT ],
		Config::FILE_FETCH_CONNECT_TIMEOUT => [ 15,					Config::T_INT ],
		Config::DAEMON_UPDATE_LOGIN_LIMIT => [ 30,					Config::T_INT ],
		Config::DAEMON_FEED_LIMIT => [ 500,								Config::T_INT ],
		Config::DAEMON_SLEEP_INTERVAL => [ 120,						Config::T_INT ],
		Config::MAX_CACHE_FILE_SIZE => [ 64*1024*1024,				Config::T_INT ],
		Config::MAX_DOWNLOAD_FILE_SIZE => [ 16*1024*1024,			Config::T_INT ],
		Config::MAX_FAVICON_FILE_SIZE => [ 1*1024*1024,				Config::T_INT ],
		Config::CACHE_MAX_DAYS => [ 7,									Config::T_INT ],
		Config::MAX_CONDITIONAL_INTERVAL => [ 3600*12,				Config::T_INT ],
		Config::DAEMON_UNSUCCESSFUL_DAYS_LIMIT => [ 30,				Config::T_INT ],
		Config::LOG_SENT_MAIL => [ "",									Config::T_BOOL ],
		Config::HTTP_PROXY => [ "",										Config::T_STRING ],
		Config::FORBID_PASSWORD_CHANGES => [ "",						Config::T_BOOL ],
		Config::SESSION_NAME => [ "ttrss_sid",							Config::T_STRING ],
		Config::CHECK_FOR_PLUGIN_UPDATES => [ "true",				Config::T_BOOL ],
		Config::ENABLE_PLUGIN_INSTALLER => [ "true",					Config::T_BOOL ],
		Config::AUTH_MIN_INTERVAL => [ 5,								Config::T_INT ],
		Config::HTTP_USER_AGENT => [ 'Tiny Tiny RSS/%s (https://tt-rss.org/)',
																					Config::T_STRING ],
		Config::HTTP_429_THROTTLE_INTERVAL => [ 3600,				Config::T_INT ],
		Config::OPENTELEMETRY_ENDPOINT => [ "",							Config::T_STRING ],
		Config::OPENTELEMETRY_SERVICE => [ "tt-rss",					Config::T_STRING ],
	];

	/** @var Config|null */
	private static $instance;

	/** @var array<string, array<bool|int|string>> */
	private $params = [];

	/** @var array<string, mixed> */
	private $version = [];

	/** @var Db_Migrations|null $migrations */
	private $migrations;

	public static function get_instance() : Config {
		if (self::$instance == null)
			self::$instance = new self();

		return self::$instance;
	}

	private function __clone() {
		//
	}

	function __construct() {
		$ref = new ReflectionClass(get_class($this));

		foreach ($ref->getConstants() as $const => $cvalue) {
			if (isset(self::_DEFAULTS[$const])) {
				$override = getenv(self::_ENVVAR_PREFIX . $const);

				list ($defval, $deftype) = self::_DEFAULTS[$const];

				$this->params[$cvalue] = [ self::cast_to($override !== false ? $override : $defval, $deftype), $deftype ];
			}
		}
	}

	/** determine tt-rss version (using git)
	 *
	 * package maintainers who don't use git: if version_static.txt exists in tt-rss root
	 * directory, its contents are displayed instead of git commit-based version, this could be generated
	 * based on source git tree commit used when creating the package
	 * @return array<string, mixed>|string
	 */
	static function get_version(bool $as_string = true) {
		return self::get_instance()->_get_version($as_string);
	}

	// returns version showing (if possible) full timestamp of commit id
	static function get_version_html() : string {
		$version = self::get_version(false);

		return sprintf("<span title=\"%s\n%s\n%s\">%s</span>",
			date("Y-m-d H:i:s", ($version['timestamp'] ?? 0)),
				$version['commit'] ?? '',
				$version['branch'] ?? '',
				$version['version']);
	}

	/**
	 * @return array<string, mixed>|string
	 */
	private function _get_version(bool $as_string = true) {
		$root_dir = dirname(__DIR__);

		if (empty($this->version)) {
			$this->version["status"] = -1;

			if (getenv("CI_COMMIT_SHORT_SHA") && getenv("CI_COMMIT_TIMESTAMP")) {

				$this->version["branch"] = getenv("CI_COMMIT_BRANCH");
				$this->version["timestamp"] = strtotime(getenv("CI_COMMIT_TIMESTAMP"));
				$this->version["version"] = sprintf("%s-%s", date("y.m", $this->version["timestamp"]), getenv("CI_COMMIT_SHORT_SHA"));
				$this->version["commit"] = getenv("CI_COMMIT_SHORT_SHA");
				$this->version["status"] = 0;

			} else if (PHP_OS === "Darwin") {
				$this->version["version"] = "UNKNOWN (Unsupported, Darwin)";
			} else if (file_exists("$root_dir/version_static.txt")) {
				$this->version["version"] = trim(file_get_contents("$root_dir/version_static.txt")) . " (Unsupported)";
			} else if (ini_get("open_basedir")) {
				$this->version["version"] = "UNKNOWN (Unsupported, open_basedir)";
			} else if (is_dir("$root_dir/.git")) {
				$this->version = self::get_version_from_git($root_dir);

				if ($this->version["status"] != 0) {
					user_error("Unable to determine version: " . $this->version["version"], E_USER_WARNING);

					$this->version["version"] = "UNKNOWN (Unsupported, Git error)";
				} else if (!getenv("SCRIPT_ROOT") || !file_exists("/.dockerenv")) {
					$this->version["version"] .= " (Unsupported)";
				}

			} else {
				$this->version["version"] = "UNKNOWN (Unsupported)";
			}
		}

		return $as_string ? $this->version["version"] : $this->version;
	}

	/**
	 * @return array<string, int|string>
	 */
	static function get_version_from_git(string $dir): array {
		$descriptorspec = [
			1 => ["pipe", "w"], // STDOUT
			2 => ["pipe", "w"], // STDERR
		];

		$rv = [
			"status" => -1,
			"version" => "",
			"branch" => "",
			"commit" => "",
			"timestamp" => 0,
		];

		$proc = proc_open("git --no-pager log --pretty=\"version-%ct-%h\" -n1 HEAD",
						$descriptorspec, $pipes, $dir);

		if (is_resource($proc)) {
			$stdout = trim(stream_get_contents($pipes[1]));
			$stderr = trim(stream_get_contents($pipes[2]));
			$status = proc_close($proc);

			$rv["status"] = $status;

			list($check, $timestamp, $commit) = explode("-", $stdout);

			if ($check == "version") {

				$rv["version"] = sprintf("%s-%s", date("y.m", (int)$timestamp), $commit);
				$rv["commit"] = $commit;
				$rv["timestamp"] = $timestamp;

				// proc_close() may return -1 even if command completed successfully
				// so if it looks like we got valid data, we ignore it

				if ($rv["status"] == -1)
					$rv["status"] = 0;

			} else {
				$rv["version"] = T_sprintf("Git error [RC=%d]: %s", $status, $stderr);
			}
		}

		return $rv;
	}

	static function get_migrations() : Db_Migrations {
		return self::get_instance()->_get_migrations();
	}

	private function _get_migrations() : Db_Migrations {
		if (empty($this->migrations)) {
			$this->migrations = new Db_Migrations();
			$this->migrations->initialize(dirname(__DIR__) . "/sql", "ttrss_version", true, self::SCHEMA_VERSION);
		}

		return $this->migrations;
	}

	static function is_migration_needed() : bool {
		return self::get_migrations()->is_migration_needed();
	}

	static function get_schema_version() : int {
		return self::get_migrations()->get_version();
	}

	/**
	 * @return bool|int|string
	 */
	static function cast_to(string $value, int $type_hint) {
		switch ($type_hint) {
			case self::T_BOOL:
				return sql_bool_to_bool($value);
			case self::T_INT:
				return (int) $value;
			default:
				return $value;
		}
	}

	/**
	 * @return bool|int|string
	 */
	private function _get(string $param) {
		list ($value, $type_hint) = $this->params[$param];

		return $this->cast_to($value, $type_hint);
	}

	private function _add(string $param, string $default, int $type_hint): void {
		$override = getenv(self::_ENVVAR_PREFIX . $param);

		$this->params[$param] = [ self::cast_to($override !== false ? $override : $default, $type_hint), $type_hint ];
	}

	static function add(string $param, string $default, int $type_hint = Config::T_STRING): void {
		$instance = self::get_instance();

		$instance->_add($param, $default, $type_hint);
	}

	/**
	 * @return bool|int|string
	 */
	static function get(string $param) {
		$instance = self::get_instance();

		return $instance->_get($param);
	}

	static function is_server_https() : bool {
		return (!empty($_SERVER['HTTPS']) && ($_SERVER['HTTPS'] != 'off')) ||
			(!empty($_SERVER['HTTP_X_FORWARDED_PROTO']) && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https');
	}

	/** returns fully-qualified external URL to tt-rss (no trailing slash)
	 * SELF_URL_PATH configuration variable is used as a fallback for the CLI SAPI
	 * */
	static function get_self_url() : string {
		if (php_sapi_name() == "cli") {
			return self::get(Config::SELF_URL_PATH);
		} else {
			$proto = self::is_server_https() ? 'https' : 'http';

			$self_url_path = $proto . '://' . $_SERVER["HTTP_HOST"] . parse_url($_SERVER["REQUEST_URI"], PHP_URL_PATH);
			$self_url_path = preg_replace("/\w+\.php(\?.*$)?$/", "", $self_url_path);

			if (substr($self_url_path, -1) === "/") {
				return substr($self_url_path, 0, -1);
			} else {
				return $self_url_path;
			}
		}
	}
	/* sanity check stuff */

	/** checks for mysql tables not using InnoDB (tt-rss is incompatible with MyISAM)
	 * @return array<int, array<string, string>> A list of entries identifying tt-rss tables with bad config
	 */
	private static function check_mysql_tables() {
		$pdo = Db::pdo();

		$sth = $pdo->prepare("SELECT engine, table_name FROM information_schema.tables WHERE
				table_schema = ? AND table_name LIKE 'ttrss_%' AND engine != 'InnoDB'");
		$sth->execute([self::get(Config::DB_NAME)]);

		$bad_tables = [];

		while ($line = $sth->fetch()) {
			array_push($bad_tables, $line);
		}

		return $bad_tables;
	}

	static function sanity_check(): void {

		/*
			we don't actually need the DB object right now but some checks below might use ORM which won't be initialized
			because it is set up in the Db constructor, which is why it's a good idea to invoke it as early as possible

			it is a bit of a hack, maybe ORM should be initialized somewhere else (functions.php?)
		*/

		$pdo = Db::pdo();

		$errors = [];

		if (strpos(self::get(Config::PLUGINS), "auth_") === false) {
			array_push($errors, "Please enable at least one authentication module via PLUGINS");
		}

		/* we assume our dependencies are sane under docker, so some sanity checks are skipped.
			this also allows tt-rss process to run under root if requested (I'm using this for development
			under podman because of uidmapping issues with rootless containers, don't use in production -fox) */
		if (!getenv("container")) {
			if (function_exists('posix_getuid') && posix_getuid() == 0) {
				array_push($errors, "Please don't run this script as root.");
			}

			if (version_compare(PHP_VERSION, '7.4.0', '<')) {
				array_push($errors, "PHP version 7.4.0 or newer required. You're using " . PHP_VERSION . ".");
			}

			if (!class_exists("UConverter")) {
				array_push($errors, "PHP UConverter class is missing, it's provided by the Internationalization (intl) module.");
			}

			if (!function_exists("curl_init") && !ini_get("allow_url_fopen")) {
				array_push($errors, "PHP configuration option allow_url_fopen is disabled, and CURL functions are not present. Either enable allow_url_fopen or install PHP extension for CURL.");
			}

			if (!function_exists("json_encode")) {
				array_push($errors, "PHP support for JSON is required, but was not found.");
			}

			if (!function_exists("flock")) {
				array_push($errors, "PHP support for flock() function is required.");
			}

			if (!class_exists("PDO")) {
				array_push($errors, "PHP support for PDO is required but was not found.");
			}

			if (!function_exists("mb_strlen")) {
				array_push($errors, "PHP support for mbstring functions is required but was not found.");
			}

			if (!function_exists("hash")) {
				array_push($errors, "PHP support for hash() function is required but was not found.");
			}

			if (ini_get("safe_mode")) {
				array_push($errors, "PHP safe mode setting is obsolete and not supported by tt-rss.");
			}

			if (!function_exists("mime_content_type")) {
				array_push($errors, "PHP function mime_content_type() is missing, try enabling fileinfo module.");
			}

			if (!class_exists("DOMDocument")) {
				array_push($errors, "PHP support for DOMDocument is required, but was not found.");
			}
		}

		if (!is_writable(self::get(Config::CACHE_DIR) . "/images")) {
			array_push($errors, "Image cache is not writable (chmod -R 777 ".self::get(Config::CACHE_DIR)."/images)");
		}

		if (!is_writable(self::get(Config::CACHE_DIR) . "/upload")) {
			array_push($errors, "Upload cache is not writable (chmod -R 777 ".self::get(Config::CACHE_DIR)."/upload)");
		}

		if (!is_writable(self::get(Config::CACHE_DIR) . "/export")) {
			array_push($errors, "Data export cache is not writable (chmod -R 777 ".self::get(Config::CACHE_DIR)."/export)");
		}

		if (!is_writable(self::get(Config::ICONS_DIR))) {
			array_push($errors, "ICONS_DIR defined in config.php is not writable (chmod -R 777 ".self::get(Config::ICONS_DIR).").\n");
		}

		if (!is_writable(self::get(Config::LOCK_DIRECTORY))) {
			array_push($errors, "LOCK_DIRECTORY is not writable (chmod -R 777 ".self::get(Config::LOCK_DIRECTORY).").\n");
		}

		// ttrss_users won't be there on initial startup (before migrations are done)
		if (!Config::is_migration_needed() && self::get(Config::SINGLE_USER_MODE)) {
			if (UserHelper::get_login_by_id(1) != "admin") {
				array_push($errors, "SINGLE_USER_MODE is enabled but default admin account (ID: 1) is not found.");
			}
		}

		// skip check for CLI scripts so that we could install database schema if it is missing.
		if (php_sapi_name() != "cli") {
			if (self::get_schema_version() < 0) {
				array_push($errors, "Base database schema is missing. Either load it manually or perform a migration (<code>update.php --update-schema</code>)");
			}
		}

		if (self::get(Config::DB_TYPE) == "mysql") {
			$bad_tables = self::check_mysql_tables();

			if (count($bad_tables) > 0) {
				$bad_tables_fmt = [];

				foreach ($bad_tables as $bt) {
					array_push($bad_tables_fmt, sprintf("%s (%s)", $bt['table_name'], $bt['engine']));
				}

				$msg = "<p>The following tables use an unsupported MySQL engine: <b>" .
					implode(", ", $bad_tables_fmt) . "</b>.</p>";

				$msg .= "<p>The only supported engine on MySQL is InnoDB. MyISAM lacks functionality to run
					tt-rss.
					Please backup your data (via OPML) and re-import the schema before continuing.</p>
					<p><b>WARNING: importing the schema would mean LOSS OF ALL YOUR DATA.</b></p>";


				array_push($errors, $msg);
			}
		}

		if (count($errors) > 0 && php_sapi_name() != "cli") { ?>
			<!DOCTYPE html>
			<html>
				<head>
					<title>Startup failed</title>
					<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
					<link rel="stylesheet" type="text/css" href="themes/light.css">
				</head>
				<body class="sanity_failed flat ttrss_utility">
					<div class="content">
						<h1>Startup failed</h1>

						<p>Please fix errors indicated by the following messages:</p>

						<?php foreach ($errors as $error) { echo self::format_error($error); } ?>

						<p>You might want to check tt-rss <a target="_blank" href="https://tt-rss.org/wiki.php">wiki</a> or the
							<a target="_blank" href="https://community.tt-rss.org/">forums</a> for more information. Please search the forums before creating new topic
							for your question.</p>
					</div>
				</body>
			</html>

		<?php
			die;
		} else if (count($errors) > 0) {
			echo "Please fix errors indicated by the following messages:\n\n";

			foreach ($errors as $error) {
				echo " * " . strip_tags($error)."\n";
			}

			echo "\nYou might want to check tt-rss wiki or the forums for more information.\n";
			echo "Please search the forums before creating new topic for your question.\n";

			exit(1);
		}
	}

	private static function format_error(string $msg): string {
		return "<div class=\"alert alert-danger\">$msg</div>";
	}

	static function get_override_links(): string {
		$rv = "";

		$local_css = get_theme_path(self::get(self::LOCAL_OVERRIDE_STYLESHEET));
		if ($local_css) $rv .= stylesheet_tag($local_css);

		$local_js = get_theme_path(self::get(self::LOCAL_OVERRIDE_JS));
		if ($local_js) $rv .= javascript_tag($local_js);

		return $rv;
	}

	static function get_user_agent(): string {
		return sprintf(self::get(self::HTTP_USER_AGENT), self::get_version());
	}
}