import { weekMonday } from '~/utils/toronto'

const TTL_MS = 24 * 60 * 60 * 1000

interface CacheEntry<T> {
  data: T
  ts: number
}

function storageKey(key: string) {
  return `cache:${key}`
}

export function useAppCache() {
  function set<T>(key: string, data: T): void {
    try {
      localStorage.setItem(storageKey(key), JSON.stringify({ data, ts: Date.now() } satisfies CacheEntry<T>))
    } catch { /* quota exceeded or unavailable */ }
  }

  /** Returns data only if fresh (< 24 h). Returns null if stale or absent. */
  function get<T>(key: string): T | null {
    try {
      const raw = localStorage.getItem(storageKey(key))
      if (!raw) return null
      const entry = JSON.parse(raw) as CacheEntry<T>
      if (Date.now() - entry.ts > TTL_MS) return null
      return entry.data
    } catch { return null }
  }

  /** Returns data regardless of age — for offline fallback when cache is stale. */
  function peek<T>(key: string): T | null {
    try {
      const raw = localStorage.getItem(storageKey(key))
      if (!raw) return null
      return (JSON.parse(raw) as CacheEntry<T>).data
    } catch { return null }
  }

  /** Returns age in ms, or null if absent. */
  function age(key: string): number | null {
    try {
      const raw = localStorage.getItem(storageKey(key))
      if (!raw) return null
      return Date.now() - (JSON.parse(raw) as CacheEntry<unknown>).ts
    } catch { return null }
  }

  function invalidate(key: string): void {
    localStorage.removeItem(storageKey(key))
  }

  /** Removes all cache entries whose key starts with prefix. */
  function invalidatePrefix(prefix: string): void {
    const full = storageKey(prefix)
    const toRemove: string[] = []
    for (let i = 0; i < localStorage.length; i++) {
      const k = localStorage.key(i)
      if (k?.startsWith(full)) toRemove.push(k)
    }
    toRemove.forEach(k => localStorage.removeItem(k))
  }

  /**
   * Cache key for schedule data (intervals or slots) for the ISO week containing
   * the given UTC ISO timestamp. Desktop and mobile both key to week-Monday.
   */
  function weekKey(utcIso: string): string {
    return weekMonday(utcIso.slice(0, 10))
  }

  return { set, get, peek, age, invalidate, invalidatePrefix, weekKey }
}