Help page with shortcut contents

main.go

@@ -17,6 +17,7 @@ import (
 
 type ShortLinks struct {
 	tmpl *template.Template
+	help *template.Template
 	mux  *http.ServeMux
 	db   *sql.DB
 	r    *rand.Rand
@@ -44,6 +45,11 @@ func NewShortLinks(db *sql.DB, domainAliases map[string]string, writableDomains
 		return nil, fmt.Errorf("static/index.html: %w", err)
 	}
 
+	help, err := template.New("help.html").ParseFiles("static/help.html")
+	if err != nil {
+		return nil, fmt.Errorf("static/help.html: %w", err)
+	}
+
 	oai, err := newOAIClientFromEnv()
 	if err != nil {
 		return nil, fmt.Errorf("newOAIClientFromEnv: %w", err)
@@ -51,6 +57,7 @@ func NewShortLinks(db *sql.DB, domainAliases map[string]string, writableDomains
 
 	sl := &ShortLinks{
 		tmpl: tmpl,
+		help: help,
 		mux:  http.NewServeMux(),
 		db:   db,
 		r:    rand.New(rand.NewSource(uint64(time.Now().UnixNano()))),
@@ -127,6 +134,11 @@ func (sl *ShortLinks) serveShort(w http.ResponseWriter, r *http.Request) {
 
 	short := r.PathValue("short")
 
+	if sl.isWritable(r.Host) && short == "_help" {
+		sl.serveHelp(w, r)
+		return
+	}
+
 	long, err := sl.getLong(short, sl.getDomain(r.Host))
 	if err != nil {
 		sl.serveRootWithPath(w, r, short)
@@ -220,6 +232,21 @@ func (sl *ShortLinks) serveSuggest(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+func (sl *ShortLinks) serveHelp(w http.ResponseWriter, r *http.Request) {
+	if !sl.isWritable(r.Host) {
+		sendError(w, http.StatusNotFound, "not found")
+		return
+	}
+
+	err := sl.help.Execute(w, map[string]any{
+		"host": r.Host,
+	})
+	if err != nil {
+		sendError(w, http.StatusInternalServerError, "error executing template: %s", err)
+		return
+	}
+}
+
 func (sl *ShortLinks) genShort(domain string) (string, error) {
 	for chars := 3; chars <= 10; chars++ {
 		b := make([]byte, chars)