// 当前正在收集依赖的 effect；null 表示非追踪态
let active: (() => void) | null = null;
// 当前 effect 已订阅到的「信号订阅集」集合：重跑前据此清旧订阅，注销时据此摘干净
let activeDeps: Set<Set<() => void>> | null = null;

/*** 响应式信号：无参调用读值并收集依赖，传参写值并触发订阅 ***/
export interface Signal<T> {
  (): T;
  (value: T): void;
}

/*** 创建信号：框架状态的最小单元 ***/
export function signal<T>(initial: T): Signal<T> {
  let value = initial;
  const subs = new Set<() => void>();
  const read = (...args: unknown[]) => {
    if (args.length === 0) {
      if (active) {
        subs.add(active); // 依赖收集
        activeDeps?.add(subs); // 同时记下「我订阅了哪个集」，注销时才能摘干净
      }
      return value;
    }
    const v = args[0] as T;
    if (Object.is(v, value)) return; // 值未变不触发
    value = v;
    for (const fn of [...subs]) fn(); // 拷贝后触发，防遍历中修改
  };
  return read as Signal<T>;
}

/*** 副作用：立即执行一次并追踪内部读取的信号，依赖变化自动重跑；返回注销函数 ***/
export function effect(fn: () => void): () => void {
  const deps = new Set<Set<() => void>>(); // 本 effect 订阅到的所有信号订阅集
  let stopped = false; // 注销标志：注销后即使还在某个订阅集里也不再执行
  const run = () => {
    if (stopped) return; // 卫语句：注销后不再跑
    for (const subs of deps) subs.delete(run); // 清上一轮订阅：条件分支换掉后的旧依赖不该再唤它
    deps.clear();
    const prev = active;
    const prevDeps = activeDeps;
    active = run;
    activeDeps = deps;
    try {
      fn();
    } finally {
      active = prev;
      activeDeps = prevDeps;
    }
  };
  run();
  return () => {
    stopped = true;
    for (const subs of deps) subs.delete(run); // 真摘订阅：否则卸载后的组件仍会被信号唤醒
    deps.clear();
  };
}

/*** 计算值：由信号派生的只读视图，同样参与上层依赖收集 ***/
export function computed<T>(fn: () => T): () => T {
  const out = signal<T>(undefined as T);
  effect(() => out(fn()));
  return () => out();
}