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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
}
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