Fix unnecessary early overflow in NewUTCSpliceTime, add tests

pkg/scte35/scte35.go

@@ -39,7 +39,7 @@ const (
 	TicksPerNanosecond = 0.00009
 	// unixEpochToGPSEpoch is the number of seconds between 1970-01-01T00:00:00Z
 	// (Unix Epoch) and 1980-01-06T00:00:00Z (GPS Epoch).
-	unixEpochToGPSEpoch = uint32(315964800)
+	unixEpochToGPSEpoch = 315964800
 )
 
 var (
@@ -123,10 +123,10 @@ func (bb *Bytes) UnmarshalText(b []byte) error {
 }
 
 // NewUTCSpliceTime creates a UTCSpliceTime representing the number of seconds
-// from GPS Epoch (01 Jan 1980, 00:00:00 UTC)
+// from GPS Epoch (06 Jan 1980, 00:00:00 UTC)
 func NewUTCSpliceTime(sec uint32) UTCSpliceTime {
 	return UTCSpliceTime{
-		time.Unix(int64(sec+unixEpochToGPSEpoch), 0),
+		time.Unix(int64(sec)+unixEpochToGPSEpoch, 0).UTC(),
 	}
 }
 
@@ -137,7 +137,7 @@ type UTCSpliceTime struct {
 
 // GPSSeconds returns the seconds since GPS Epoch
 func (t UTCSpliceTime) GPSSeconds() uint32 {
-	return uint32(t.Time.Unix()) - unixEpochToGPSEpoch
+	return uint32(t.Time.Unix() - unixEpochToGPSEpoch)
 }
 
 // readerError returns the readers error state, if any.

pkg/scte35/scte35_test.go

@@ -849,6 +849,57 @@ func TestDurationToTicks(t *testing.T) {
 	}
 }
 
+var commonGPSTimeTestCases = map[string]struct {
+	gpsSeconds    uint32
+	utcSpliceTime scte35.UTCSpliceTime
+}{
+	"GPS Second 0": {
+		gpsSeconds: 0,
+		utcSpliceTime: scte35.UTCSpliceTime{
+			Time: time.Date(1980, time.January, 6, 0, 0, 0, 0, time.UTC),
+		},
+	},
+	// last value before a uint32 based calculation would overflow
+	"GPS Second 3979002495": {
+		gpsSeconds: 3979002495,
+		utcSpliceTime: scte35.UTCSpliceTime{
+			Time: time.Date(2106, time.February, 7, 6, 28, 15, 0, time.UTC),
+		},
+	},
+	// in a uint32 based calculation this would overflow
+	"GPS Second 3979002496": {
+		gpsSeconds: 3979002496,
+		utcSpliceTime: scte35.UTCSpliceTime{
+			Time: time.Date(2106, time.February, 7, 6, 28, 16, 0, time.UTC),
+		},
+	},
+	// highest possible value
+	"GPS Second 4294967295": {
+		gpsSeconds: 4294967295,
+		utcSpliceTime: scte35.UTCSpliceTime{
+			Time: time.Date(2116, time.February, 12, 6, 28, 15, 0, time.UTC),
+		},
+	},
+}
+
+func TestNewUTCSpliceTime(t *testing.T) {
+	for k, c := range commonGPSTimeTestCases {
+		t.Run(k, func(t *testing.T) {
+			utcSpliceTime := scte35.NewUTCSpliceTime(c.gpsSeconds)
+			assert.Equal(t, c.utcSpliceTime.Time, utcSpliceTime.Time)
+		})
+	}
+}
+
+func TestUTCSpliceTimeGPSSeconds(t *testing.T) {
+	for k, c := range commonGPSTimeTestCases {
+		t.Run(k, func(t *testing.T) {
+			gpsSeconds := c.utcSpliceTime.GPSSeconds()
+			assert.Equal(t, c.gpsSeconds, gpsSeconds)
+		})
+	}
+}
+
 // helper func to make test life a bit easier
 
 func toBytes(i uint64) []byte {