feat(domain): add native-feed detector (Atom/RSS/JSON)

src/domain/native-feed.ts

@@ -0,0 +1,54 @@
+/**
+ * Pure detection of a newsletter's own syndication feed. No DOM, no I/O — it
+ * receives already-extracted <link> tuples (infra parses the HTML) and decides
+ * which ones are real feeds. This module owns the business knowledge: the strict
+ * set of recognized feed MIME types.
+ */
+import { NativeFeed } from "../types";
+
+// MIME type → feed kind. Strict: only the three canonical syndication types.
+// `application/json` is deliberately excluded — too broad, captures non-feeds.
+const MIME_TO_KIND: Record<string, NativeFeed["type"]> = {
+  "application/atom+xml": "atom",
+  "application/rss+xml": "rss",
+  "application/feed+json": "json",
+};
+
+// Drop MIME parameters ("; charset=…"), trim, lowercase.
+function normalizeMime(type: string): string {
+  return type.split(";")[0].trim().toLowerCase();
+}
+
+/** Map raw <link> tuples to recognized native feeds, deduped by URL. */
+export function detectNativeFeeds(
+  links: { href: string; type: string }[],
+): NativeFeed[] {
+  const out: NativeFeed[] = [];
+  const seen = new Set<string>();
+  for (const link of links) {
+    const kind = MIME_TO_KIND[normalizeMime(link.type)];
+    if (!kind) continue;
+    const url = link.href.trim();
+    if (!url || seen.has(url)) continue;
+    seen.add(url);
+    out.push({ url, type: kind });
+  }
+  return out;
+}
+
+/** Flatten per-sender native feeds into one list, deduped by URL (first wins). */
+export function unionNativeFeeds(
+  bySender: Record<string, NativeFeed[]> | undefined,
+): NativeFeed[] {
+  if (!bySender) return [];
+  const out: NativeFeed[] = [];
+  const seen = new Set<string>();
+  for (const feeds of Object.values(bySender)) {
+    for (const feed of feeds) {
+      if (seen.has(feed.url)) continue;
+      seen.add(feed.url);
+      out.push({ ...feed });
+    }
+  }
+  return out;
+}