simplify marshal/ummarshal logic, make parser stricter, update tests,…

… fix readme (#5)

duration.go

@@ -1,10 +1,12 @@
 package duration
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"math"
 	"strconv"
+	"strings"
 	"time"
 	"unicode"
 )
@@ -47,6 +49,10 @@ 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 := ""
@@ -277,25 +283,24 @@ func (duration *Duration) String() string {
 	return d
 }
 
+// MarshalJSON satisfies the Marshaler interface by return a valid JSON string representation of the duration
 func (duration Duration) MarshalJSON() ([]byte, error) {
-	return []byte("\"" + duration.String() + "\""), nil
+	return json.Marshal(duration.String())
 }
 
+// UnmarshalJSON satisfies the Unmarshaler interface by return a valid JSON string representation of the duration
 func (duration *Duration) UnmarshalJSON(source []byte) error {
-	strVal := string(source)
-	if len(strVal) < 2 {
-		return fmt.Errorf("invalid ISO 8601 duration: %s", strVal)
-	}
-	strVal = strVal[1 : len(strVal)-1]
-
-	if strVal == "null" {
-		return nil
+	durationString := ""
+	err := json.Unmarshal(source, &durationString)
+	if err != nil {
+		return err
 	}
 
-	parsed, err := Parse(strVal)
+	parsed, err := Parse(durationString)
 	if err != nil {
-		return fmt.Errorf("invalid ISO 8601 duration: %s", strVal)
+		return fmt.Errorf("failed to parse duration: %w", err)
 	}
+
 	*duration = *parsed
 	return nil
 }

duration_test.go

@@ -17,6 +17,12 @@ func TestParse(t *testing.T) {
 		want    *Duration
 		wantErr bool
 	}{
+		{
+			name:    "invalid-duration-1",
+			args:    args{d: "T0S"},
+			want:    nil,
+			wantErr: true,
+		},
 		{
 			name: "period-only",
 			args: args{d: "P4Y"},
@@ -27,7 +33,7 @@ func TestParse(t *testing.T) {
 		},
 		{
 			name: "time-only-decimal",
-			args: args{d: "T2.5S"},
+			args: args{d: "PT2.5S"},
 			want: &Duration{
 				Seconds: 2.5,
 			},
@@ -245,7 +251,7 @@ func TestDuration_String(t *testing.T) {
 		t.Errorf("expected: %s, got: %s", "P3Y6M4DT12H30M33.3333S", duration.String())
 	}
 
-	smallDuration, err := Parse("T0.0000000000001S")
+	smallDuration, err := Parse("PT0.0000000000001S")
 	if err != nil {
 		t.Fatal(err)
 	}

readme.md

@@ -36,7 +36,7 @@ func main() {
 	fmt.Println(d.Minutes) // 30
 	fmt.Println(d.Seconds) // 5.5
 
-	d, err = duration.Parse("T33.3S")
+	d, err = duration.Parse("PT33.3S")
 	if err != nil {
 		panic(err)
 	}