tumblrandom: use better random

go/tumblrandom/tumblrandom.go

@@ -12,7 +12,7 @@ import (
 	"io"
 	"io/fs"
 	"log"
-	mathrand "math/rand"
+	"math/big"
 	"net"
 	"net/http"
 	"os"
@@ -289,7 +289,12 @@ func (a *app) random(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	n := mathrand.Intn(len(u.Likes))
+	bigN, err := rand.Int(rand.Reader, big.NewInt(int64(len(u.Likes))))
+	if err != nil {
+		http.Error(rw, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	n := int(bigN.Int64())
 	l := u.Likes[n]
 	http.Redirect(rw, r, l.PostURL, http.StatusSeeOther)
 }