mpegts: add time decoder that works with native timestamp (#150)

Author: aler9

pkg/formats/mpegts/time_decoder.go

@@ -5,9 +5,7 @@ import (
 )
 
 const (
-	maximum           = 0x1FFFFFFFF // 33 bits
-	negativeThreshold = 0x1FFFFFFFF / 2
-	clockRate         = 90000
+	clockRate = 90000
 )
 
 // avoid an int64 overflow and preserve resolution by splitting division into two parts:
@@ -19,31 +17,22 @@ func multiplyAndDivide(v, m, d time.Duration) time.Duration {
 }
 
 // TimeDecoder is a MPEG-TS timestamp decoder.
+//
+// Deprecated: replaced by TimeDecoder2
 type TimeDecoder struct {
-	overall time.Duration
-	prev    int64
+	wrapped *TimeDecoder2
 }
 
 // NewTimeDecoder allocates a TimeDecoder.
+//
+// Deprecated: replaced by NewTimeDecoder2
 func NewTimeDecoder(start int64) *TimeDecoder {
 	return &TimeDecoder{
-		prev: start,
+		wrapped: NewTimeDecoder2(start),
 	}
 }
 
 // Decode decodes a MPEG-TS timestamp.
 func (d *TimeDecoder) Decode(ts int64) time.Duration {
-	diff := (ts - d.prev) & maximum
-
-	// negative difference
-	if diff > negativeThreshold {
-		diff = (d.prev - ts) & maximum
-		d.prev = ts
-		d.overall -= time.Duration(diff)
-	} else {
-		d.prev = ts
-		d.overall += time.Duration(diff)
-	}
-
-	return multiplyAndDivide(d.overall, time.Second, clockRate)
+	return multiplyAndDivide(time.Duration(d.wrapped.Decode(ts)), time.Second, clockRate)
 }

pkg/formats/mpegts/time_decoder2.go

@@ -0,0 +1,36 @@
+package mpegts
+
+const (
+	maximum           = 0x1FFFFFFFF // 33 bits
+	negativeThreshold = 0x1FFFFFFFF / 2
+)
+
+// TimeDecoder2 is a MPEG-TS timestamp decoder.
+type TimeDecoder2 struct {
+	overall int64
+	prev    int64
+}
+
+// NewTimeDecoder2 allocates a TimeDecoder.
+func NewTimeDecoder2(start int64) *TimeDecoder2 {
+	return &TimeDecoder2{
+		prev: start,
+	}
+}
+
+// Decode decodes a MPEG-TS timestamp.
+func (d *TimeDecoder2) Decode(ts int64) int64 {
+	diff := (ts - d.prev) & maximum
+
+	// negative difference
+	if diff > negativeThreshold {
+		diff = (d.prev - ts) & maximum
+		d.prev = ts
+		d.overall -= diff
+	} else {
+		d.prev = ts
+		d.overall += diff
+	}
+
+	return d.overall
+}

pkg/formats/mpegts/time_decoder2_test.go

@@ -0,0 +1,71 @@
+package mpegts
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestTimeDecoder2NegativeDiff(t *testing.T) {
+	d := NewTimeDecoder2(64523434)
+
+	ts := d.Decode(64523434 - 90000)
+	require.Equal(t, int64(-90000), ts)
+
+	ts = d.Decode(64523434)
+	require.Equal(t, int64(0), ts)
+
+	ts = d.Decode(64523434 + 90000*2)
+	require.Equal(t, int64(2*90000), ts)
+
+	ts = d.Decode(64523434 + 90000)
+	require.Equal(t, int64(1*90000), ts)
+}
+
+func TestTimeDecoder2Overflow(t *testing.T) {
+	d := NewTimeDecoder2(0x1FFFFFFFF - 20)
+
+	i := int64(0x1FFFFFFFF - 20)
+	secs := int64(0)
+	const stride = 150
+	lim := int64(uint64(0x1FFFFFFFF - (stride * 90000)))
+
+	for n := 0; n < 100; n++ {
+		// overflow
+		i += 90000 * stride
+		secs += stride
+		ts := d.Decode(i)
+		require.Equal(t, secs*90000, ts)
+
+		// reach 2^32 slowly
+		secs += stride
+		i += 90000 * stride
+		for ; i < lim; i += 90000 * stride {
+			ts = d.Decode(i)
+			require.Equal(t, secs*90000, ts)
+			secs += stride
+		}
+	}
+}
+
+func TestTimeDecoder2OverflowAndBack(t *testing.T) {
+	d := NewTimeDecoder2(0x1FFFFFFFF - 90000 + 1)
+
+	ts := d.Decode(0x1FFFFFFFF - 90000 + 1)
+	require.Equal(t, int64(0), ts)
+
+	ts = d.Decode(90000)
+	require.Equal(t, int64(2*90000), ts)
+
+	ts = d.Decode(0x1FFFFFFFF - 90000 + 1)
+	require.Equal(t, int64(0), ts)
+
+	ts = d.Decode(0x1FFFFFFFF - 90000*2 + 1)
+	require.Equal(t, int64(-1*90000), ts)
+
+	ts = d.Decode(0x1FFFFFFFF - 90000 + 1)
+	require.Equal(t, int64(0), ts)
+
+	ts = d.Decode(90000)
+	require.Equal(t, int64(2*90000), ts)
+}