Add feed generating capabilities

`feed.tmpl.xml` should be RSS2.0-conforming.  Ideally each news item
would have a date of publication, but RSS doesn't require it.

I found it easier to copy `genblog.go` to `genfeed.go` than to shoehorn
in feed logic to the blog generator.  As such, `genfeed.go` might have
some unnecessary stuff in it (though the go compiler didn't complain, so
who knows?!).  I also added the necessary machinery in
`generate_homepage`.

Of course, I did very minimal testing.

feed.tmpl.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
+  <channel>
+    <title>tilde.town blog</title>
+    <description>web log of tilde town</description>
+    <link>https://tilde.town/blog.html</link>
+    <atom:link rel="self" type="application/rss+xml" href="https://tilde.town/blog.xml"/>
+    {{ range .News }}
+    <item>
+      <title>{{.Title}}</title>
+      <pubDate>{{.Pubdate}}</pubDate>
+      <description>
+        <![CDATA[{{.Content}}]]>
+      </description>
+      <guid isPermalink="false">{{.Pubdate}}-{{.Title}}</guid>
+    </item>
+    {{ end }}
+  </channel>
+</rss>

generate_homepage

@@ -6,5 +6,6 @@ set -e
 
 cd /town/src/tilde.town
 /usr/bin/go run genblog.go > blog.html
+/usr/bin/go run genfeed.go > blog.xml
 /usr/bin/go run genusers.go > users.html
-/bin/cp index.html blog.html users.html blog.css style.css /var/www/tilde.town/
+/bin/cp index.html blog.html blog.xml users.html blog.css style.css /var/www/tilde.town/

genfeed.go

@@ -0,0 +1,121 @@
+package main
+
+import (
+	"bytes"
+	_ "embed"
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/exec"
+	"sort"
+	"text/template"
+)
+
+const statsPath = "/usr/local/bin/stats"
+
+//go:embed feed.tmpl.xml
+var feedTmpl string
+
+type newsEntry struct {
+	Title   string // Title of entry
+	Pubdate string // Human readable date
+	Content string // HTML of entry
+}
+
+type User struct {
+	Username string
+	Default  bool
+}
+
+type tildeData struct {
+	News        []newsEntry
+	Users       []User
+	ActiveUsers []string `json:"active_users"`
+}
+
+type ByName []User
+
+func (n ByName) Len() int           { return len(n) }
+func (n ByName) Swap(i, j int)      { n[i], n[j] = n[j], n[i] }
+func (n ByName) Less(i, j int) bool { return n[i].Username < n[j].Username }
+
+func _main() error {
+	data, err := stats()
+	if err != nil {
+		return err
+	}
+
+	type tmplData struct {
+		News   []newsEntry
+		Lights string
+	}
+
+	td := &tmplData{
+		News:   data.News,
+		Lights: "",
+	}
+
+	sort.Sort(ByName(data.Users))
+
+	isActive := func(username string) bool {
+		for _, u := range data.ActiveUsers {
+			if u == username {
+				return true
+			}
+		}
+
+		return false
+	}
+
+	for _, u := range data.Users {
+		if isActive(u.Username) {
+			td.Lights += fmt.Sprintf("<a href=\"/~%s\">*</a>", u.Username)
+		} else if !u.Default {
+			td.Lights += fmt.Sprintf("<a href=\"/~%s\">+</a>", u.Username)
+		} else {
+			td.Lights += "."
+		}
+	}
+
+	t, err := template.New("feed").Parse(feedTmpl)
+	if err != nil {
+		return fmt.Errorf("failed to parse the feed template: %w", err)
+	}
+
+	out := bytes.Buffer{}
+	if err = t.Execute(&out, td); err != nil {
+		return fmt.Errorf("failed to render feed template: %w", err)
+	}
+
+	fmt.Println(out.String())
+
+	return nil
+}
+
+func stats() (*tildeData, error) {
+	sout := bytes.Buffer{}
+	cmd := exec.Command(statsPath)
+	cmd.Stdout = &sout
+
+	err := cmd.Run()
+	if err != nil {
+		return nil, err
+	}
+
+	var data tildeData
+
+	err = json.Unmarshal(sout.Bytes(), &data)
+	if err != nil {
+		return nil, err
+	}
+
+	return &data, nil
+}
+
+func main() {
+	err := _main()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: %s\n", err)
+		os.Exit(1)
+	}
+}