// Generic localStorage-backed setting — the shared plumbing a theme
// override and a reduced-motion override both need ("read a stored
// value, react to it changing, write a new one") rather than each
// caller reinventing its own key handling. Not settings-page-specific:
// any client-local preference reaches for `useLocalSetting` directly.
// Originated in swarm-ui's own `lib/settings-storage.ts`; moved here
// once the per-agent page needed the identical mechanism (mara: "i
// think the component should be shared" — see `SettingsMenu.tsx`'s
// file comment for the full story).
//
// `localStorage`'s own `storage` event only fires in *other* tabs, never
// the tab that made the write — so two components in the same tab both
// watching the same key (e.g. `SettingsMenu` writing, a top-level effect
// reading) need their own same-tab signal. The tiny module-level pub/sub
// below is that signal; it's deliberately not exported, callers only see
// the hook.
import { useEffect, useState } from "preact/hooks";

const listeners = new Map<string, Set<() => void>>();

function subscribe(key: string, onChange: () => void): () => void {
  let set = listeners.get(key);
  if (!set) {
    set = new Set();
    listeners.set(key, set);
  }
  set.add(onChange);
  return () => {
    set!.delete(onChange);
    if (set!.size === 0) listeners.delete(key);
  };
}

function notify(key: string): void {
  listeners.get(key)?.forEach((fn) => fn());
}

// Reads outside a component (e.g. an early top-level check) can call
// this directly; `useLocalSetting` builds on it rather than duplicating
// the try/catch.
export function readLocalSetting<T>(key: string, fallback: T): T {
  try {
    const raw = localStorage.getItem(key);
    return raw === null ? fallback : (JSON.parse(raw) as T);
  } catch {
    // Private-browsing storage bans, a corrupted value, quota errors —
    // all the same answer: behave as if nothing were stored.
    return fallback;
  }
}

function writeLocalSetting<T>(key: string, value: T): void {
  try {
    localStorage.setItem(key, JSON.stringify(value));
  } catch {
    // Best-effort: the setting just won't persist this time, not worth
    // surfacing an error for a client-local preference.
  }
  notify(key);
}

// `[value, setValue]`, same shape as `useState` — deliberately, so a
// caller that later needs to swap a plain `useState` for a persisted
// setting (or vice versa) changes one line, not its whole call site.
//
// `setValue` only writes + notifies; it doesn't also call this
// component's own `setValue` state setter directly. The subscription
// below already fires for a same-instance write (it's in the same
// `listeners` set as every other subscriber), so a self-write reaches
// this component's state through the identical "react to a change"
// path every other subscriber uses — one path, not two that have to
// agree.
export function useLocalSetting<T>(
  key: string,
  fallback: T,
): [T, (value: T) => void] {
  const [value, setValue] = useState<T>(() => readLocalSetting(key, fallback));

  useEffect(
    () => subscribe(key, () => setValue(readLocalSetting(key, fallback))),
    [key],
  );

  return [value, (next: T) => writeLocalSetting(key, next)];
}