feat(feed): auto-discover feed URLs from HTML pages

When an HTML page is provided instead of a direct feed URL, parse
<link rel="alternate"> tags to find RSS/Atom feeds. Atom is preferred
over RSS when both are present.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

1 files changed, 94 insertions, 0 deletions

diff --git a/backend/feed/discover.go b/backend/feed/discover.go
new file mode 100644
index 0000000..962cbcd
--- /dev/null
+++ b/backend/feed/discover.go
@@ -0,0 +1,94 @@
+package feed
+
+import (
+	"io"
+	"net/url"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+type feedLink struct {
+	URL  string
+	Type string // "atom" or "rss"
+}
+
+// discoverFeeds parses an HTML document and extracts feed URLs from
+// <link rel="alternate"> tags. It resolves relative URLs against baseURL.
+func discoverFeeds(body io.Reader, baseURL *url.URL) []feedLink {
+	var links []feedLink
+	z := html.NewTokenizer(body)
+	for {
+		tt := z.Next()
+		switch tt {
+		case html.ErrorToken:
+			return links
+		case html.StartTagToken, html.SelfClosingTagToken:
+			tn, _ := z.TagName()
+			tagName := string(tn)
+
+			if tagName == "body" {
+				return links
+			}
+			if tagName != "link" {
+				continue
+			}
+
+			attrs := tokenAttrs(z)
+			rel := strings.ToLower(attrs["rel"])
+			typ := strings.ToLower(attrs["type"])
+			href := attrs["href"]
+
+			if rel != "alternate" || href == "" {
+				continue
+			}
+
+			var feedType string
+			switch typ {
+			case "application/atom+xml":
+				feedType = "atom"
+			case "application/rss+xml":
+				feedType = "rss"
+			default:
+				continue
+			}
+
+			ref, err := url.Parse(href)
+			if err != nil {
+				continue
+			}
+			resolved := baseURL.ResolveReference(ref).String()
+			links = append(links, feedLink{URL: resolved, Type: feedType})
+		}
+	}
+}
+
+// selectFeed picks the best feed URL from discovered links.
+// Prefers Atom over RSS.
+func selectFeed(links []feedLink) string {
+	for _, l := range links {
+		if l.Type == "atom" {
+			return l.URL
+		}
+	}
+	for _, l := range links {
+		if l.Type == "rss" {
+			return l.URL
+		}
+	}
+	return ""
+}
+
+func tokenAttrs(z *html.Tokenizer) map[string]string {
+	attrs := make(map[string]string)
+	for {
+		key, val, more := z.TagAttr()
+		if len(key) > 0 {
+			attrs[string(key)] = string(val)
+		}
+		if !more {
+			break
+		}
+	}
+	return attrs
+}