(_ => {
    const nodemailer = require("nodemailer");

    const service = (process.env.MAIL_SERVICE || "").trim();
    const host = (process.env.MAIL_HOST || "").trim();
    const user = (process.env.MAIL_SERVICE_USER || "").trim();
    const password = (process.env.MAIL_SERVICE_PASSWORD || "");
    const secure = Boolean(parseInt(process.env.MAIL_HOST_SECURE || "0") || 0);
    const port = parseInt(process.env.MAIL_HOST_PORT || "25") || 25;

    const regex_host = /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/;

    module.exports = {
        cloud: createTransport({
            service: service,
            auth: {
                user: user,
                pass: password
            }
        }),
        host: createTransport({
            host: host,
            secure: secure,
            port: port,
            auth: {
                user: user,
                pass: password
            },
            tls: {
                rejectUnauthorized: false
            }
        })
    };

    function createTransport(config: any) {
        const is_service = (config.service || "").length > 0;
        const is_host = regex_host.test(config.host);

        let transport: any;

        if (
            ((config.service || "").length > 0 || regex_host.test(config.host)) &&
            config.auth.user.length > 0 &&
            config.auth.pass.length > 0
        ) {
            transport = nodemailer.createTransport(config);
        };

        if (is_service && transport == null) {
            console.error("請檢查 .env 中的 MAIL_SERVICE 的資料是否正確。");
        }
        else if (is_host && transport == null) {
            console.error("請檢查 .env 中的 MAIL_HOST 的資料是否正確。");
        };

        return transport;

    };
})();