feat: add Format() to timestamp (#30)

Signed-off-by: Patrick Zheng <patrickzheng@microsoft.com>

timestamp.go

@@ -13,7 +13,10 @@
 
 package tspclient
 
-import "time"
+import (
+	"fmt"
+	"time"
+)
 
 // Timestamp denotes the time at which the timestamp token was created by the TSA
 //
@@ -43,3 +46,11 @@ func (t *Timestamp) BoundedAfter(u time.Time) bool {
 	timestampLowerLimit := t.Value.Add(-t.Accuracy)
 	return timestampLowerLimit.After(u) || timestampLowerLimit.Equal(u)
 }
+
+// Format returns a string of t in format layout.
+// The output is a timestamp range calculated with its accuracy.
+func (t *Timestamp) Format(layout string) string {
+	lowerBound := t.Value.Add(-t.Accuracy)
+	upperBound := t.Value.Add(t.Accuracy)
+	return fmt.Sprintf("[%s, %s]", lowerBound.Format(layout), upperBound.Format(layout))
+}

timestamp_test.go

@@ -83,3 +83,18 @@ func TestTimestamp(t *testing.T) {
 		t.Fatal("timestamp.BoundedAfter expected false, but got true")
 	}
 }
+
+func TestString(t *testing.T) {
+	// timestamp range:
+	// [time.Date(2021, time.September, 17, 14, 9, 8, 0, time.UTC),
+	//  time.Date(2021, time.September, 17, 14, 9, 12, 0, time.UTC)]
+	timestamp := Timestamp{
+		Value:    time.Date(2021, time.September, 17, 14, 9, 10, 0, time.UTC),
+		Accuracy: 2 * time.Second,
+	}
+
+	expectedStr := "[2021-09-17T14:09:08Z, 2021-09-17T14:09:12Z]"
+	if timestamp.Format(time.RFC3339) != expectedStr {
+		t.Fatalf("expected %s, but got %s", expectedStr, timestamp.Format(time.RFC3339))
+	}
+}