From a235613fd67421f34925c489c2a77fd8a6451366 Mon Sep 17 00:00:00 2001
From: aler9 <46489434+aler9@users.noreply.github.com>
Date: Sun, 13 Feb 2022 17:00:13 +0100
Subject: [PATCH] fix wrong timestamp computation that happened at 28h28m

this was caused by a int64 overflow.
---
 pkg/rtptimedec/decoder.go      | 38 ++++++++++++++++++++--------------
 pkg/rtptimedec/decoder_test.go | 29 +++++++++++++++-----------
 2 files changed, 39 insertions(+), 28 deletions(-)

diff --git a/pkg/rtptimedec/decoder.go b/pkg/rtptimedec/decoder.go
index 89d91eae..a2baa526 100644
--- a/pkg/rtptimedec/decoder.go
+++ b/pkg/rtptimedec/decoder.go
@@ -8,9 +8,8 @@ import (
 // Decoder is a RTP timestamp decoder.
 type Decoder struct {
 	clockRate time.Duration
-	tsAdd     int64
-	tsInitial *int64
-	tsPrev    *int64
+	tsAdd     *int64
+	tsPrev    int64
 }
 
 // New allocates a Decoder.
@@ -24,22 +23,29 @@ func New(clockRate int) *Decoder {
 func (d *Decoder) Decode(ts uint32) time.Duration {
 	ts64 := int64(ts)
 
-	if d.tsPrev != nil {
-		diff := ts64 - *d.tsPrev
-		switch {
-		case diff < -0xFFFFFF: // overflow
-			d.tsAdd += 0x100000000
-
-		case diff > 0xFFFFFF: // timestamp overflowed then went back
-			d.tsAdd -= 0x100000000
-		}
+	if d.tsAdd == nil {
+		d.tsPrev = ts64
+		ts64 = -ts64
+		d.tsAdd = &ts64
+		return 0
 	}
 
-	d.tsPrev = &ts64
+	diff := ts64 - d.tsPrev
+	d.tsPrev = ts64
 
-	if d.tsInitial == nil {
-		d.tsInitial = &ts64
+	switch {
+	case diff < -0xFFFFFF: // overflow
+		*d.tsAdd += 0x100000000
+
+	case diff > 0xFFFFFF: // timestamp overflowed then went back
+		*d.tsAdd -= 0x100000000
 	}
 
-	return time.Duration(ts64+d.tsAdd-*d.tsInitial) * time.Second / d.clockRate
+	tot := time.Duration(ts64 + *d.tsAdd)
+
+	// avoid an int64 overflow and preserve resolution by splitting division into two parts:
+	// first add seconds, then the decimal part.
+	secs := tot / d.clockRate
+	dec := tot % d.clockRate
+	return secs*time.Second + dec*time.Second/d.clockRate
 }
diff --git a/pkg/rtptimedec/decoder_test.go b/pkg/rtptimedec/decoder_test.go
index 721ecc1e..dbf7aaa1 100644
--- a/pkg/rtptimedec/decoder_test.go
+++ b/pkg/rtptimedec/decoder_test.go
@@ -11,24 +11,29 @@ func TestOverflow(t *testing.T) {
 	d := New(90000)
 
 	i := uint32(4294877296)
+	secs := time.Duration(0)
 	pts := d.Decode(i)
 	require.Equal(t, time.Duration(0), pts)
 
-	// 1st overflow
-	i += 90000 * 2
-	pts = d.Decode(i)
-	require.Equal(t, 2*time.Second, pts)
+	const stride = 150
+	lim := uint32(uint64(4294967296 - (stride * 90000)))
 
-	// reach 4294890000 slowly
-	for ; i < 4294890000; i += 90000 * 10 {
+	for n := 0; n < 100; n++ {
+		// overflow
+		i += 90000 * stride
+		secs += stride
 		pts = d.Decode(i)
-		require.Equal(t, 2*time.Second+time.Second*time.Duration(i-90000)/90000, pts)
-	}
+		require.Equal(t, secs*time.Second, pts)
 
-	// 2nd overflow
-	i += 90000 * 10
-	pts = d.Decode(i)
-	require.Equal(t, 47732*time.Second, pts)
+		// reach 2^32 slowly
+		secs += stride
+		i += 90000 * stride
+		for ; i < lim; i += 90000 * stride {
+			pts = d.Decode(i)
+			require.Equal(t, secs*time.Second, pts)
+			secs += stride
+		}
+	}
 }
 
 func TestOverflowAndBack(t *testing.T) {