Stricter parsing (#8)

Closes issue #4 

* check for leading P, allow for negative
* use switch, add more invalid period string tests, increase parser strictness again
* simplify negative parsing

duration.go

@@ -49,21 +49,26 @@ var (
 // Parse attempts to parse the given duration string into a *Duration,
 // if parsing fails an error is returned instead.
 func Parse(d string) (*Duration, error) {
-	if !strings.Contains(d, "P") {
-		return nil, ErrUnexpectedInput
-	}
-
 	state := parsingPeriod
 	duration := &Duration{}
 	num := ""
 	var err error
 
+	switch {
+	case strings.HasPrefix(d, "P"): // standard duration
+	case strings.HasPrefix(d, "-P"): // negative duration
+		duration.Negative = true
+		d = strings.TrimPrefix(d, "-") // remove the negative sign
+	default:
+		return nil, ErrUnexpectedInput
+	}
+
 	for _, char := range d {
 		switch char {
-		case '-':
-			duration.Negative = true
 		case 'P':
-			state = parsingPeriod
+			if state != parsingPeriod {
+				return nil, ErrUnexpectedInput
+			}
 		case 'T':
 			state = parsingTime
 		case 'Y':

duration_test.go

@@ -23,6 +23,18 @@ func TestParse(t *testing.T) {
 			want:    nil,
 			wantErr: true,
 		},
+		{
+			name:    "invalid-duration-2",
+			args:    args{d: "P-T0S"},
+			want:    nil,
+			wantErr: true,
+		},
+		{
+			name:    "invalid-duration-3",
+			args:    args{d: "PT0SP0D"},
+			want:    nil,
+			wantErr: true,
+		},
 		{
 			name: "period-only",
 			args: args{d: "P4Y"},