import type { SyncStorageAdapter } from '../types';
import { BROWSER_EVENT_STORAGE, STORAGE_ADAPTER_LOCAL } from '../consts';

function getLocalStorage(): Storage | null {
	if (typeof window === 'undefined') return null;
	return window.localStorage;
}

/**
 * `localStorage` adapter. Singleton — the underlying store is process-wide,
 * so multiple `EngineStorage` instances over `localAdapter` write to the
 * same backend.
 *
 * `onChange` subscribes to the `storage` event for cross-tab sync. The event
 * does **not** fire in the originating tab; intra-tab sync is handled by the
 * EntryBus inside `EngineStorage`.
 *
 * SSR-safe: every method is a no-op outside the browser. Reads return `null`
 * so consumers fall back to defaults.
 */
export const localAdapter: SyncStorageAdapter = {
	name: STORAGE_ADAPTER_LOCAL,
	getItem(key) {
		return getLocalStorage()?.getItem(key) ?? null;
	},
	setItem(key, value) {
		getLocalStorage()?.setItem(key, value);
	},
	removeItem(key) {
		getLocalStorage()?.removeItem(key);
	},
	onChange: (fn) => {
		if (typeof window === 'undefined') return () => {};
		let storage: Storage;
		try {
			const resolved = getLocalStorage();
			if (resolved === null) return () => {};
			storage = resolved;
		} catch {
			return () => {};
		}

		const listener = (event: StorageEvent): void => {
			// Filter out events from sessionStorage / clearAll (key === null).
			if (event.storageArea !== storage) return;
			if (event.key === null) return;
			fn(event.key, event.newValue);
		};
		window.addEventListener(BROWSER_EVENT_STORAGE, listener);
		return () => window.removeEventListener(BROWSER_EVENT_STORAGE, listener);
	}
};