/**
 * MqttClient —— 浏览器/小程序 MQTT over WebSocket 重连封装
 *
 * 稳定性策略（对应路线 B2「即时通信稳定性」）：
 *  1. 指数退避重连：基数 1s，上限 30s，成功一次后重置。
 *  2. 会话恢复：clean=false 时重连自动补投离线消息（由 Broker 保证）。
 *  3. 遗嘱（LWT）：连接异常断开时 Broker 代发 will，上层可据此标记「离线」。
 *  4. 心跳兜底：onclose 即触发重连，不依赖 broker 的 keepalive 超时。
 *
 * 用法：
 *   const c = new MqttClient('wss://your.host/mqtt', {
 *     clientId: 'web-' + uid, username, password, clean: false,
 *     will: { topic: 'presence/' + uid, payload: 'offline', qos: 1, retain: true }
 *   });
 *   c.on('connect', () => c.subscribe('topic/a'));
 *   c.on('message', (t, p) => console.log(t, p));
 */
(function (global) {
    'use strict';

    const BACKOFF_BASE = 1000;   // 1s
    const BACKOFF_MAX  = 30000;  // 30s
    const BACKOFF_FACTOR = 1.8;

    function MqttClient(url, opts) {
        this.url = url;
        this.opts = opts || {};
        this.clientId = this.opts.clientId || ('mqttjs_' + Math.random().toString(16).slice(2, 10));
        this._listeners = {};
        this._ws = null;
        this._connected = false;
        this._closedByUser = false;
        this._backoff = BACKOFF_BASE;
        this._retryTimer = null;
        this._subscriptions = {}; // topic -> qos，重连后自动重订
    }

    MqttClient.prototype.on = function (evt, fn) {
        (this._listeners[evt] = this._listeners[evt] || []).push(fn);
        return this;
    };

    MqttClient.prototype._emit = function (evt, a, b) {
        (this._listeners[evt] || []).forEach((fn) => fn(a, b));
    };

    MqttClient.prototype.connect = function () {
        this._closedByUser = false;
        this._open();
        return this;
    };

    MqttClient.prototype._open = function () {
        let ws;
        try {
            ws = new WebSocket(this.url, ['mqtt']);
        } catch (e) {
            return this._scheduleReconnect();
        }
        this._ws = ws;
        ws.binaryType = 'arraybuffer';

        ws.onopen = () => {
            // 发送 MQTT CONNECT（极简 CONNECT 报文，支持 will）
            const buf = this._buildConnect();
            ws.send(buf);
        };
        ws.onmessage = (ev) => {
            const pkt = this._parseConnAck(ev.data);
            if (pkt && pkt.sessionPresent) {
                this._emit('resume'); // 会话恢复，Broker 会在后台补投离线消息
            }
            this._connected = true;
            this._backoff = BACKOFF_BASE;
            this._emit('connect');
            Object.keys(this._subscriptions).forEach((t) => this.subscribe(t, this._subscriptions[t]));
        };
        ws.onclose = () => {
            if (this._connected) this._emit('disconnect');
            this._connected = false;
            if (!this._closedByUser) this._scheduleReconnect();
        };
        ws.onerror = () => { try { ws.close(); } catch (e) {} };
    };

    MqttClient.prototype._scheduleReconnect = function () {
        if (this._closedByUser) return;
        this._emit('reconnecting', this._backoff);
        clearTimeout(this._retryTimer);
        this._retryTimer = setTimeout(() => {
            this._backoff = Math.min(BACKOFF_MAX, Math.floor(this._backoff * BACKOFF_FACTOR));
            this._open();
        }, this._backoff);
    };

    MqttClient.prototype.subscribe = function (topic, qos) {
        this._subscriptions[topic] = qos || 0;
        if (!this._connected || !this._ws) return;
        this._ws.send(this._buildSubscribe(topic, qos || 0));
    };

    MqttClient.prototype.publish = function (topic, payload, qos, retain) {
        if (!this._connected || !this._ws) return false;
        this._ws.send(this._buildPublish(topic, payload, qos || 0, retain ? 1 : 0));
        return true;
    };

    MqttClient.prototype.end = function () {
        this._closedByUser = true;
        clearTimeout(this._retryTimer);
        if (this._ws) { try { this._ws.close(); } catch (e) {} }
    };

    /* ---------- 以下为最小 MQTT 报文构造（仅覆盖 CONNECT/SUBSCRIBE/PUBLISH） ---------- */
    MqttClient.prototype._encStr = function (s) {
        const b = new TextEncoder().encode(s);
        return new Uint8Array([b.length >> 8, b.length & 0xff, ...b]);
    };
    MqttClient.prototype._buildConnect = function () {
        const o = this.opts;
        const cid = this._encStr(this.clientId);
        let payload = cid;
        if (o.username) payload = this._join(payload, this._encStr(o.username), this._encStr(o.password || ''));
        if (o.will) {
            payload = this._join(payload, this._encStr(o.will.topic), this._encStr(o.will.payload || ''));
        }
        let varHdr = new Uint8Array([4, 0x02 /*clean*/, 0x3c /*keepalive 60*/]);
        if (o.clean === false) varHdr[1] = 0;
        let flags = 0;
        if (o.username) flags |= 0x80;
        if (o.password) flags |= 0x40;
        if (o.will) flags |= 0x04 | ((o.will.qos || 0) << 3) | ((o.will.retain ? 1 : 0) << 5);
        varHdr[1] |= flags;
        const body = this._join(varHdr, payload);
        return this._wrap(1, body);
    };
    MqttClient.prototype._buildSubscribe = function (topic, qos) {
        const pid = new Uint8Array([0x00, 0x01]);
        const t = this._join(this._encStr(topic), new Uint8Array([qos]));
        return this._wrap(8, this._join(pid, t));
    };
    MqttClient.prototype._buildPublish = function (topic, payload, qos, retain) {
        const t = this._encStr(topic);
        const p = typeof payload === 'string' ? new TextEncoder().encode(payload) : new Uint8Array(payload);
        return this._wrap(3 | (qos << 1) | (retain << 0), this._join(t, p));
    };
    MqttClient.prototype._wrap = function (cmd, body) {
        const len = body.length;
        const rem = [len & 0x7f];
        if (len > 127) rem.unshift((len >> 7) & 0x7f | 0x80);
        return new Uint8Array([cmd << 4, ...rem, ...body]);
    };
    MqttClient.prototype._join = function (...arrs) {
        let n = 0; arrs.forEach((a) => n += a.length);
        const out = new Uint8Array(n);
        let p = 0; arrs.forEach((a) => { out.set(a, p); p += a.length; });
        return out;
    };
    MqttClient.prototype._parseConnAck = function (data) {
        try {
            const buf = new Uint8Array(data);
            return { sessionPresent: (buf[2] & 0x01) === 1, code: buf[3] };
        } catch (e) { return null; }
    };

    global.MqttClient = MqttClient;
    if (typeof module !== 'undefined') module.exports = MqttClient;
})(typeof window !== 'undefined' ? window : this);