Id should always represent position in file

sort.go

@@ -37,9 +37,6 @@ func (todolist *TodoList) Sort(sortFlag int) error {
 }
 func (todolist *TodoList) refresh() {
 	todolist.Sort(todolist.SortFlag)
-	for i, t := range todolist.Todos {
-		t.Id = i
-	}
 }
 
 type todolistSort struct {

todolist.go

@@ -121,6 +121,16 @@ func (todolist *TodoList) GetIncompleteTasks() *TodoList {
 	return &t
 }
 
+func (todolist *TodoList) GetNextId() int {
+	nextId := 0
+	for _, v := range todolist.Todos {
+		if v.Id > nextId {
+			nextId = v.Id
+		}
+	}
+	return nextId + 1
+}
+
 // String returns a complete list of tasks in todo.txt format.
 func (todolist *TodoList) String() string {
 	var ret string
@@ -132,14 +142,16 @@ func (todolist *TodoList) String() string {
 
 // AddTodo prepends a Todo to the current TodoList and takes care to set the Todo.Id correctly
 func (todolist *TodoList) AddTodo(todo *Todo) {
+	todo.Id = todolist.GetNextId()
 	todolist.Todos = append(todolist.Todos, todo)
 	todolist.refresh()
 }
 
 // AddTimers adds all passed in timers to the list, sorts the list, then updates the Timer.Id values.
 func (todolist *TodoList) AddTodos(todos []*Todo) {
-	todolist.Todos = append(todolist.Todos, todos...)
-	todolist.Sort(SortCompletedDateDesc)
+	for _, v := range todos {
+		todolist.AddTodo(v)
+	}
 }
 func (todolist *TodoList) Combine(other *TodoList) { todolist.AddTodos(other.Todos) }
 
@@ -222,6 +234,14 @@ func (todolist *TodoList) Filter(predicate func(*Todo) bool) *TodoList {
 	return &newList
 }
 
+func (todolist *TodoList) ResetIds() {
+	todoId := 1
+	for _, v := range todolist.Todos {
+		v.Id = todoId
+		todoId++
+	}
+}
+
 // LoadFromFile loads a TodoList from *os.File.
 // Note: This will clear the current TodoList and overwrite it's contents with whatever is in *os.File.
 func (todolist *TodoList) LoadFromFile(file *os.File) error {
@@ -254,7 +274,11 @@ func (todolist *TodoList) WriteToFile(file *os.File) error {
 	writer := bufio.NewWriter(file)
 	_, err := writer.WriteString(todolist.String())
 	writer.Flush()
-	return err
+	if err != nil {
+		return err
+	}
+	todolist.ResetIds()
+	return nil
 }
 
 // LoadFromFilename loads a TodoList from the filename.
@@ -269,7 +293,11 @@ func (todolist *TodoList) LoadFromFilename(filename string) error {
 
 // WriteToFilename writes a TodoList to the specified file (most likely called "todo.txt").
 func (todolist *TodoList) WriteToFilename(filename string) error {
-	return ioutil.WriteFile(filename, []byte(todolist.String()), 0640)
+	if err := ioutil.WriteFile(filename, []byte(todolist.String()), 0640); err != nil {
+		return err
+	}
+	todolist.ResetIds()
+	return nil
 }
 
 // LoadFromFile loads and returns a TodoList from *os.File.