implement diff/parent/%d endpoint

DB.go

@@ -165,6 +165,21 @@ func (this *WikiDB) GetRevisionHistory(title string) ([]Article, error) {
 	return ret, nil
 }
 
+func (this *WikiDB) GetNextOldestRevision(revision int) (int, error) {
+	row := this.db.QueryRow(
+		`SELECT articles.id FROM articles WHERE articles.article = (SELECT article FROM articles WHERE id = ?) AND id < ? ORDER BY id DESC LIMIT 1`,
+		revision, revision,
+	)
+
+	var ret int
+	err := row.Scan(&ret)
+	if err != nil {
+		return 0, err
+	}
+
+	return ret, nil
+}
+
 func (this *WikiDB) GetRecentChanges(offset int, limit int) ([]Article, error) {
 	rows, err := this.db.Query(
 		`SELECT articles.id, articles.modified, articles.author, titles.title FROM articles JOIN titles ON articles.article=titles.id ORDER BY modified DESC ` +

WikiServer.go

@@ -194,6 +194,22 @@ func (this *WikiServer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 			this.serveRedirect(w, this.opts.ExpectBaseURL+`diff/`+r.URL.Query().Get("f")+`/`+r.URL.Query().Get("t"))
 			return
 
+		} else if strings.HasPrefix(remainingPath, "diff/parent/") {
+			toRev, err := strconv.Atoi(remainingPath[len("diff/parent/"):])
+			if err != nil {
+				this.serveErrorMessage(w, err)
+				return
+			}
+
+			fromRev, err := this.db.GetNextOldestRevision(toRev)
+			if err != nil {
+				this.serveErrorMessage(w, err)
+				return
+			}
+
+			this.serveRedirect(w, this.opts.ExpectBaseURL+`diff/`+fmt.Sprintf("%d/%d", fromRev, toRev))
+			return
+
 		} else if match := this.rxDiff.FindStringSubmatch(remainingPath); len(match) == 3 {
 
 			fromRev, err := strconv.Atoi(match[1])