import { createHash } from 'node:crypto';

/**
 * The "Smart caching" Module (CONTEXT.md). Names the cache-related helpers
 * `fetchFeed` weaves together: HTTP conditional headers (layer 1) and MD5
 * content-hash short-circuit (layer 2). Layer 3 (intra-payload dedupe) lives
 * in `article-dedup` because it concerns Article identity, not feed freshness.
 *
 * Pure: no I/O. The orchestrator owns the HTTP request and the database.
 */

/** Identity of the cache validators stored on a Feed row. */
export interface FeedValidators {
  etag: string | null;
  lastModified: string | null;
}

/**
 * Conditional request headers to add when re-fetching a Feed. Returns an
 * empty object if `force` is set or if the Feed has no validators stored.
 */
export function buildConditionalHeaders(feed: FeedValidators, force: boolean): Record<string, string> {
  if (force) return {};
  const headers: Record<string, string> = {};
  if (feed.etag) headers['If-None-Match'] = feed.etag;
  if (feed.lastModified) headers['If-Modified-Since'] = feed.lastModified;
  return headers;
}

/** MD5 of the raw XML payload, used as a content-change detector. */
export function contentHash(content: string): string {
  return createHash('md5').update(content).digest('hex');
}

/**
 * True when the new XML hashes to the same value previously stored in the
 * `fetch_cache` row — i.e. the server returned 200 but the body didn't change.
 * Always false when there is no prior cached hash.
 */
export function isXmlUnchanged(xml: string, cachedHash: string | null | undefined): boolean {
  if (!cachedHash) return false;
  return contentHash(xml) === cachedHash;
}