/* @file event_bus.hpp
 * @brief Publish-subscribe event bus with shared_mutex for concurrent read access.
 * 发布-订阅事件总线，使用 shared_mutex 支持并发读访问。
 * Copyright (c) 2026 dstalk contributors. GPLv3.
 */

#pragma once

#include <functional>
#include <mutex>
#include <shared_mutex>
#include <unordered_map>
#include <vector>

namespace dstalk {

using EventHandler = std::function<void(int event_type, const void* data)>;

// 轻量级发布-订阅事件总线 / Lightweight pub-sub event bus.
// 读取者使用 shared_lock（emit），因此多个处理器可以并发分发；
// 写入者使用 unique_lock（subscribe / unsubscribe）。
// Readers use shared_lock (emit) so multiple handlers can be dispatched
// concurrently; writers use unique_lock (subscribe / unsubscribe).
class EventBus {
public:
    EventBus() = default;
    ~EventBus() = default;

    // 订阅事件，返回订阅ID / Subscribe to an event, returning a subscription id
    int subscribe(int event_type, EventHandler handler);

    // 取消订阅 / Unsubscribe by subscription id
    void unsubscribe(int subscription_id);

    // 发布事件 / Emit an event to all matching subscribers
    int emit(int event_type, const void* data);

private:
    struct Subscription {
        int id;
        int event_type;
        EventHandler handler;
    };

    mutable std::shared_mutex mutex_;             // 读写锁：emit 用 shared，subscribe/unsubscribe 用 unique / RW lock: shared for emit, unique for subscribe/unsubscribe
    std::vector<Subscription> subscriptions_;     // emit 时线性扫描；对少量订阅者足够 / Linear scan on emit; ok for small subscriber counts
    int next_id_ = 1;                             // 单调递增订阅 ID 计数器 / Monotonic subscription id counter
};

} // namespace dstalk