import { Injectable } from '@angular/core';

// Cache expiration time. (In a real shop this should not be the beast approach due to stock changes)
const CACHE_TTL_MS = 60 * 60 * 1000;

// Prefix for cache keys to avoid collisions with other localStorage entries.
const CACHE_PREFIX = 'itx-cache:';

interface CacheEntry<T> {
  timestamp: number;
  data: T;
}

// Generic cache service
@Injectable({ providedIn: 'root' })
export class CacheService {
  // returns cached item if it exists and is not expired, otherwise fetches it using the provided fetcher function.
  async getOrFetch<T>(key: string, fetcher: () => Promise<T>): Promise<T> {
    const cached = this.read<T>(key);
    if (cached && Date.now() - cached.timestamp < CACHE_TTL_MS) {
      return cached.data;
    }

    const data = await fetcher();
    this.write(key, data);
    return data;
  }

  // Clears all cache entries managed by this service.
  clear(): void {
    Object.keys(localStorage)
      .filter((key) => key.startsWith(CACHE_PREFIX))
      .forEach((key) => localStorage.removeItem(key));
  }

  private read<T>(key: string): CacheEntry<T> | null {
    const raw = localStorage.getItem(CACHE_PREFIX + key);
    if (!raw) {
      return null;
    }
    try {
      return JSON.parse(raw) as CacheEntry<T>;
    } catch {
      // corrupted data
      return null;
    }
  }

  private write<T>(key: string, data: T): void {
    const entry: CacheEntry<T> = { timestamp: Date.now(), data };
    localStorage.setItem(CACHE_PREFIX + key, JSON.stringify(entry));
  }
}