import { Injectable } from '@angular/core';
import { from, Observable, of } from 'rxjs';
import { catchError, switchMap } from 'rxjs/operators';
import { AppConfigService } from './app-config.service';
import { AuthService } from './auth.service';

/** Shape of every entry stored in Cache Storage. */
export interface BrowserCacheEntry<T> {
  data: T;
  cachedAt: number; // epoch ms
}

/**
 * Generic browser-side Cache Storage wrapper.
 *
 * Each logical cache is identified by a **cacheName** (e.g. `'agm-jobs-list-v1'`).
 * Within that cache, individual entries are keyed by an arbitrary **key** string
 * (typically a serialised set of query params).
 *
 * Usage:
 * ```ts
 * // Read
 * this.browserCache.get<MyModel[]>('my-cache-v1', key, 60_000).subscribe(data => { ... });
 *
 * // Write
 * this.browserCache.put('my-cache-v1', key, data);
 *
 * // Invalidate
 * this.browserCache.invalidate('my-cache-v1');
 * ```
 *
 * All operations are no-ops when the Cache Storage API is unavailable
 * (e.g. in unit tests or older browsers).
 */
@Injectable({ providedIn: 'root' })
export class BrowserCacheService {

  private readonly supported = typeof caches !== 'undefined';
  private readonly fallbackMaxAgeMs = 60_000;

  constructor(
    private readonly appConfig: AppConfigService,
    private readonly authSvc: AuthService
  ) {}

  private ttlStorageKey(cacheName: string): string {
    const userId = this.authSvc.user?._id || 'anonymous';
    return `browser-cache-ttl:${userId}:${cacheName}`;
  }

  private get defaultMaxAgeMs(): number {
    return this.appConfig.settings?.browserListCacheTtlMs || this.fallbackMaxAgeMs;
  }

  getTtl(cacheName: string): number {
    const storedValue = localStorage.getItem(this.ttlStorageKey(cacheName));
    if (storedValue === null) {
      return this.defaultMaxAgeMs;
    }

    const parsedValue = Number(storedValue);
    return Number.isFinite(parsedValue) && parsedValue >= 0
      ? parsedValue
      : this.defaultMaxAgeMs;
  }

  setTtl(cacheName: string, ttlMs: number): number {
    const normalizedValue = Number.isFinite(ttlMs) && ttlMs >= 0
      ? Math.floor(ttlMs)
      : this.defaultMaxAgeMs;
    localStorage.setItem(this.ttlStorageKey(cacheName), String(normalizedValue));
    return normalizedValue;
  }

  /**
   * Build the pseudo-URL used as the cache key inside a named Cache bucket.
   * We prefix with a fixed path so it looks like a valid Request URL.
   */
  private entryUrl(cacheName: string, key: string): string {
    return `/browser-cache/${encodeURIComponent(cacheName)}?${key}`;
  }

  /**
   * Retrieve a cached value.
   *
   * @param cacheName  Name of the Cache Storage bucket (e.g. `'agm-jobs-list-v1'`).
   * @param key        Entry key — typically serialised query params.
   * @param maxAgeMs   Maximum age in milliseconds before the entry is treated as stale.
   *                   Defaults to `appConfig.browserListCacheTtlMs` or 60 000.
   * @returns          The cached value, or `null` when unavailable / stale / missing.
   */
  get<T>(cacheName: string, key: string, maxAgeMs?: number): Observable<T | null> {
    if (!this.supported) return of(null);

    const effectiveMaxAgeMs = maxAgeMs ?? this.getTtl(cacheName);

    return from(caches.open(cacheName)).pipe(
      switchMap(cache => from(cache.match(this.entryUrl(cacheName, key)))),
      switchMap(response => {
        if (!response) return of(null);
        return from(response.json() as Promise<BrowserCacheEntry<T>>);
      }),
      switchMap((entry: BrowserCacheEntry<T> | null) => {
        if (!entry) return of(null);
        if (Date.now() - entry.cachedAt > effectiveMaxAgeMs) return of(null);
        return of(entry.data);
      }),
      catchError(() => of(null))
    );
  }

  /**
   * Store a value in Cache Storage.
   * Fire-and-forget — errors are silently swallowed so they never block the caller.
   *
   * @param cacheName  Name of the Cache Storage bucket.
   * @param key        Entry key.
   * @param data       Value to store (must be JSON-serialisable).
   */
  put<T>(cacheName: string, key: string, data: T): void {
    if (!this.supported) return;

    const entry: BrowserCacheEntry<T> = { data, cachedAt: Date.now() };
    caches.open(cacheName)
      .then(cache => cache.put(
        this.entryUrl(cacheName, key),
        new Response(JSON.stringify(entry), { headers: { 'Content-Type': 'application/json' } })
      ))
      .catch(() => { /* silent */ });
  }

  /**
   * Delete an entire Cache Storage bucket, invalidating all its entries.
   * Typically called after a mutation (create / update / delete).
   *
   * @param cacheName  Name of the Cache Storage bucket to delete.
   */
  invalidate(cacheName: string): void {
    if (!this.supported) return;
    caches.delete(cacheName).catch(() => { /* silent */ });
  }
}