add pttodo.NewRootFromFile, pttodo.NewTodosFromFile

pttodo/root.go

@@ -1,11 +1,41 @@
 package pttodo
 
+import (
+	"os"
+
+	yaml "gopkg.in/yaml.v2"
+)
+
 type Root struct {
 	Todo      []Todo
 	Scheduled []Todo
 	Done      []Todo
 }
 
+func VerifyRootFromFile(p string) error {
+	_, err := NewRootFromFile(p)
+	return err
+}
+
+func NewRootFromFile(p string) (Root, error) {
+	f, err := os.Open(p)
+	if os.IsNotExist(err) {
+		return Root{}, nil
+	}
+	if err != nil {
+		return Root{}, err
+	}
+	defer f.Close()
+
+	var result Root
+	if err := yaml.NewDecoder(f).Decode(&result); err != nil {
+		return Root{}, err
+	}
+
+	result.AutoMove()
+	return result, nil
+}
+
 func (root Root) Equals(other Root) bool {
 	for i, slice := range [][2][]Todo{
 		[2][]Todo{root.Todo, other.Todo},