From fc3383946dc5982cfe2178ad6628aa95769139c2 Mon Sep 17 00:00:00 2001
From: Nazar Kanaev <nkanaev@live.com>
Date: Thu, 18 Mar 2021 00:38:44 +0000
Subject: [PATCH] crawl

---
 src/worker/crawl.go | 109 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)
 create mode 100644 src/worker/crawl.go

diff --git a/src/worker/crawl.go b/src/worker/crawl.go
new file mode 100644
index 0000000..df29d14
--- /dev/null
+++ b/src/worker/crawl.go
@@ -0,0 +1,109 @@
+package worker
+
+import (
+	"net/url"
+	"strings"
+
+	"golang.org/x/net/html"
+)
+
+func getAttr(node *html.Node, key string) string {
+	for _, a := range node.Attr {
+		if a.Key == key {
+			return a.Val
+		}
+	}
+	return ""
+}
+
+func getText(node *html.Node) string {
+	text := make([]string, 0)
+	isTextNode := func(n *html.Node) bool {
+		return n.Type == html.TextNode
+	}
+	for _, n := range getNodes(node, isTextNode) {
+		text = append(text, strings.TrimSpace(n.Data))
+	}
+	return strings.Join(text, " ")
+}
+
+func getNodes(node *html.Node, match func(*html.Node) bool) []*html.Node {
+	nodes := make([]*html.Node, 0)
+
+	queue := make([]*html.Node, 0)
+	queue = append(queue, node)
+	for len(queue) > 0 {
+		queue, n := queue[1:], queue[0]
+		if match(n) {
+			nodes = append(nodes, n)
+		}
+		for c := n.FirstChild; c != nil; c = c.NextSibling {
+			queue = append(queue, c)
+		}
+	}
+	return nodes
+}
+
+func FindFeeds(doc *html.Node, baseUrl *url.URL) []*FeedSource {
+	candidates := make(map[string]string)
+
+	linkTypes := []string{"application/atom+xml", "application/rss+xml", "application/json"}
+	isFeedLink := func(n *html.Node) bool {
+		if n.Type == html.ElementNode && n.Data == "link" {
+			t := getAttr(n, "type")
+			for _, tt := range linkTypes {
+				if tt == t {
+					return true
+				}
+			}
+		}
+		return false
+	}
+	for _, node := range getNodes(doc, isFeedLink) {
+		href := getAttr(node, "href")
+		name := getAttr(node, "title")
+		link := baseUrl.ResolveReference(href).String()
+
+		if href != "" {
+			candidates[link] = name
+		}
+	}
+
+	if len(candidates) == 0 {
+		// guess by hyperlink properties:
+		// - a[href="feed"]
+		// - a:contains("rss")
+		// ...etc
+		feedHrefs := []string{"feed", "feed.xml", "rss.xml", "atom.xml"}
+		feedTexts := []string{"rss", "feed"}
+		isFeedHyperLink := func(n *html.Node) bool {
+			if n.Type == html.ElementNode && n.Data == "a" {
+				href := strings.Trim(getAttr(n, "href"), "/")
+				text := strings.Lower(getText(n))
+
+				for _, feedHref := range feedHrefs {
+					if strings.HasSuffix(href, feedHref) {
+						return true
+					}
+				}
+				for _, feedText := range feedTexts {
+					if strings.Contains(text, feedText) {
+						return true
+					}
+				}
+			}
+			return false
+		}
+		for _, node := range getNodes(doc, isFeedHyperLink) {
+			href := getAttr(node, "href")
+			link := baseUrl.ResolveReference(href).String()
+			candidates[link] = ""
+		}
+	}
+
+	sources := make([]*FeedSource, 0, len(candidates))
+	for url, title := range candidates {
+		sources = append(sources, &FeedSource{Title: title, Url: url})
+	}
+	return sources
+}