feat(mp4): defragment hybrid files with a progressive part

CHANGELOG.md

@@ -43,6 +43,26 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   the output ftyp. Encrypted content, unsupported edit lists, zero
   timescales, truncated byte ranges, and payloads larger than the input
   file are rejected
+- `mp4.Defragment` and `mp4.DefragmentTracks` resolve overlapping fragments:
+  when a fragment's tfdt re-declares an earlier decode time (a
+  retransmission), the fragment appearing later in the file wins and the
+  superseded samples are dropped at sample granularity, whether or not the
+  re-sent bytes are identical. Every abandoned time range must be declared
+  again by surviving fragments, so no declared content is ever silently
+  dropped. Ambiguous overlaps fail closed: cuts inside a sample, abandoned
+  time ranges that no surviving later fragment declares again, and
+  overlapping files whose fragments use absolute base data offsets are
+  rejected
+- `mp4.Defragment` and `mp4.DefragmentTracks` accept hybrid files that carry
+  progressive samples in the moov before the first fragment: the progressive
+  samples come first with their chunk structure preserved (also when stco
+  offsets are not monotone), and the fragment samples are appended.
+  Fragments may supersede progressive samples under the same
+  later-declarer-wins and coverage rules as fragment overlaps, and hostile
+  sample tables are validated before any count-proportional allocation
+- `SttsBox.SampleDurations`, `CttsBox.CompositionTimeOffsets`, and
+  `StssBox.SampleIsSync` expand a whole sample table into one value per
+  sample, validating the entries against the declared sample count
 - `mp4ff-defragment` command line tool exposing the defragmentation with
   optional track selection
 

mp4/ctts.go

@@ -134,6 +134,35 @@ func (b *CttsBox) AddSampleCountsAndOffset(counts []uint32, offsets []int32) err
 	return nil
 }
 
+// CompositionTimeOffsets returns the offset of each of nrSamples samples,
+// validating that the entries cover exactly that many samples. A box
+// without entries covers no samples.
+func (b *CttsBox) CompositionTimeOffsets(nrSamples uint32) ([]int32, error) {
+	ctss := make([]int32, nrSamples)
+	if len(b.EndSampleNr) < 2 {
+		if nrSamples == 0 {
+			return ctss, nil
+		}
+		return nil, fmt.Errorf("ctts covers 0 samples, not the %d declared", nrSamples)
+	}
+	if b.EndSampleNr[len(b.EndSampleNr)-1] != nrSamples {
+		return nil, fmt.Errorf("ctts covers %d samples, not the %d declared",
+			b.EndSampleNr[len(b.EndSampleNr)-1], nrSamples)
+	}
+	for i := 0; i < b.NrSampleCount(); i++ {
+		// The decoded EndSampleNr accumulation can wrap uint32, so the final
+		// entry alone proves nothing about the intermediate ones.
+		if b.EndSampleNr[i] > b.EndSampleNr[i+1] || b.EndSampleNr[i+1] > nrSamples {
+			return nil, fmt.Errorf("ctts entry %d covers samples (%d, %d], outside the %d declared samples",
+				i+1, b.EndSampleNr[i], b.EndSampleNr[i+1], nrSamples)
+		}
+		for nr := b.EndSampleNr[i]; nr < b.EndSampleNr[i+1]; nr++ {
+			ctss[nr] = b.SampleOffset[i]
+		}
+	}
+	return ctss, nil
+}
+
 // GetCompositionTimeOffset - composition time offset for (one-based) sampleNr in track timescale.
 // Returns 0 for a sampleNr beyond the samples covered by this box.
 func (b *CttsBox) GetCompositionTimeOffset(sampleNr uint32) int32 {

mp4/ctts_test.go

@@ -78,3 +78,30 @@ func TestGetCompositionTimeOffsetOutsideBox(t *testing.T) {
 		t.Errorf("decoded empty ctts: got cto %d instead of 0", cto)
 	}
 }
+
+func TestCttsCompositionTimeOffsets(t *testing.T) {
+	ctts := &mp4.CttsBox{}
+	if err := ctts.AddSampleCountsAndOffset([]uint32{2, 1}, []int32{100, -200}); err != nil {
+		t.Fatal(err)
+	}
+	offsets, err := ctts.CompositionTimeOffsets(3)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []int32{100, 100, -200}
+	for i := range want {
+		if offsets[i] != want[i] {
+			t.Errorf("offset %d is %d, want %d", i, offsets[i], want[i])
+		}
+	}
+	if _, err := ctts.CompositionTimeOffsets(2); err == nil {
+		t.Error("entries covering more samples than declared must be an error")
+	}
+	empty := &mp4.CttsBox{}
+	if offsets, err := empty.CompositionTimeOffsets(0); err != nil || len(offsets) != 0 {
+		t.Errorf("empty box with 0 samples gave %v, %v", offsets, err)
+	}
+	if _, err := empty.CompositionTimeOffsets(3); err == nil {
+		t.Error("an empty box covering declared samples must be an error")
+	}
+}