(_ => {
    // const schedule = require("node-schedule");
    const mysql = require("mysql");

    const read_host = (process.env.DB_READ_HOST || "").trim();
    const read_port = parseInt(process.env.DB_READ_PORT || "3306") || 3306;
    const read_user = (process.env.DB_READ_USER || "").trim();
    const read_password = (process.env.DB_READ_PASSWORD || "");
    const read_database = (process.env.DB_READ_DATABASE || "").trim();
    const read_charset = (process.env.DB_READ_CHARSET || "utf8mb4").trim();
    const read_connection = parseInt(process.env.DB_READ_CONNECTION || "8") || 8;

    const write_host = (process.env.DB_WRITE_HOST || "").trim();
    const write_port = parseInt(process.env.DB_WRITE_PORT || "3306") || 3306;
    const write_user = (process.env.DB_WRITE_USER || "").trim();
    const write_password = (process.env.DB_WRITE_PASSWORD || "");
    const write_database = (process.env.DB_WRITE_DATABASE || "").trim();
    const write_charset = (process.env.DB_WRITE_CHARSET || "utf8mb4").trim();
    const write_connection = parseInt(process.env.DB_WRITE_CONNECTION || "4") || 4;

    const regex_host = /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/;
    const regex_database = /^[A-Za-z]{1}[\d\w_]{0,29}$/;

    // let read_pool: any;
    // let write_pool: any;

    module.exports = {
        read: createPool({
            is_read: true,
            host: read_host,
            port: read_port,
            user: read_user,
            password: read_password,
            database: read_database,
            charset: read_charset,
            connectionLimit: read_connection,
            useConnectionPooling: true
        }),
        write: createPool({
            is_write: true,
            host: write_host,
            port: write_port,
            user: write_user,
            password: write_password,
            database: write_database,
            charset: write_charset,
            connectionLimit: write_connection,
            useConnectionPooling: true
        }),
    };

    // const job = schedule.scheduleJob('*/30 * * * * *', function() {
    //     if (read_pool && write_pool) {
    //         const read_connection_now = read_pool._allConnections.length - read_pool._freeConnections.length;
    //         const write_connection_now = write_pool._allConnections.length - write_pool._freeConnections.length;
    //     }
    // });

    function createPool(config: any) {
        let pool: any;

        if (
            regex_host.test(config.host) &&
            regex_database.test(config.database) &&
            config.user.length > 0
        ) {
            pool = mysql.createPool(config);
        };

        // if (pool && config.is_read) {
        //     read_pool = pool;
        // }
        // else if (pool && config.is_write) {
        //     write_pool = pool;
        // };

        return (sql: any, options: any, callback: any) => {
            if (typeof options === "function") {
                callback = options;
                options = undefined;
            };

            if (pool == null) {
                callback(new Error("請檢查 .env 中的 DB_*_* 的資料是否正確。"), null, null);
                return;
            };

            pool.getConnection((err: Error, connection: any) => {
                if (err) {
                    callback(err, null, null);
                    return;
                };

                connection.query(sql, options, (err: Error, results: any, fields: any) => {
                    if (err) {
                        connection.rollback(() => {
                            callback(err, null, null);
                            connection.release();
                        });
                        return;
                    };

                    connection.commit(function (err: Error) {
                        if (err) {
                            connection.rollback(() => {
                                callback(err, null, null);
                                connection.release();
                            });
                            return;
                        };

                        callback(err, results, fields);
                        connection.release();
                    });
                });
            });
        };
    };
})();