From 1c406419a506fe4c962874cb64b425f1372877c0 Mon Sep 17 00:00:00 2001
From: Martin Riedl <martin.riedl.92@googlemail.com>
Date: Sun, 8 Dec 2024 11:09:57 +0100
Subject: [PATCH] feat: new decoding time to sample box implementation

---
 DecodingTimeToSampleBox.go | 95 ++++++++++++++++++++++++++++++++++++++
 README.md                  |  2 +-
 2 files changed, 96 insertions(+), 1 deletion(-)
 create mode 100644 DecodingTimeToSampleBox.go

diff --git a/DecodingTimeToSampleBox.go b/DecodingTimeToSampleBox.go
new file mode 100644
index 0000000..3bfb3f0
--- /dev/null
+++ b/DecodingTimeToSampleBox.go
@@ -0,0 +1,95 @@
+// Copyright 2024 Martin Riedl
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package gomp4
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+// TimeToSampleBox time to sample box struct
+//
+// 8.6.1.2 Decoding Time to Sample Box
+//
+// Box Type: ‘stts’
+// Container: Sample Table Box (‘stbl’)
+// Mandatory: Yes
+// Quantity: Exactly one
+//
+// This box contains a compact version of a table that allows indexing from decoding time to sample
+// number. Other tables give sample sizes and pointers, from the sample number. Each entry in the table
+// gives the number of consecutive samples with the same time delta, and the delta of those samples. By
+// adding the deltas a complete time‐to‐sample map may be built.
+//
+// The Decoding Time to Sample Box contains decode time delta's: DT(n+1) = DT(n) + STTS(n) where
+// STTS(n) is the (uncompressed) table entry for sample n.
+//
+// The sample entries are ordered by decoding time stamps; therefore the deltas are all non‐negative.
+//
+// The DT axis has a zero origin; DT(i) = SUM(for j=0 to i‐1 of delta(j)), and the sum of all deltas gives the
+// length of the media in the track (not mapped to the overall timescale, and not considering any edit list).
+//
+// The Edit List Box provides the initial CT value if it is non‐empty (non‐zero).
+type TimeToSampleBox struct {
+	*FullBox
+	// is an integer that gives the number of entries in the following table.
+	EntryCount uint32
+	Entries    []TimeToSampleBoxEntry
+}
+
+type TimeToSampleBoxEntry struct {
+	// s an integer that counts the number of consecutive samples that have the given duration.
+	SampleCount uint32
+	// is an integer that gives the delta of these samples in the time‐scale of the media.
+	SampleDelta uint32
+}
+
+// BoxTypeDecodingTimeToSample Decoding Time To Sample Box
+const BoxTypeDecodingTimeToSample = "stts"
+
+func init() {
+	BoxDefinitions = append(BoxDefinitions, BoxDefinition{
+		Type:        BoxTypeDecodingTimeToSample,
+		ParentTypes: []string{BoxTypeSampleTable},
+		Parser:      ParseDecodingTimeToSampleBox,
+	})
+}
+
+// ParseDecodingTimeToSampleBox creates a new decoding time to sample box struct based on bytes
+func ParseDecodingTimeToSampleBox(parser *Parser, filePosition uint64, headerSize uint32, content []byte) (any, error) {
+	box := &TimeToSampleBox{
+		FullBox: newFullBox(&Box{filePosition, headerSize}, content[0:4]),
+	}
+
+	// parse entry counter
+	box.EntryCount = binary.BigEndian.Uint32(content[4:8])
+
+	// parse sample entries
+	position := 8
+	for i := 0; i < int(box.EntryCount); i++ {
+		newSampleEntry := TimeToSampleBoxEntry{
+			SampleCount: binary.BigEndian.Uint32(content[position+(i*8) : position+4+(i*8)]),
+			SampleDelta: binary.BigEndian.Uint32(content[position+4+(i*8) : position+8+(i*8)]),
+		}
+		box.Entries = append(box.Entries, newSampleEntry)
+	}
+
+	// validate entries amount
+	if len(box.Entries) != int(box.EntryCount) {
+		return box, fmt.Errorf("invalid amount of sample entries at %d; got %d but expected %d", filePosition, len(box.Entries), box.EntryCount)
+	}
+
+	return box, nil
+}
diff --git a/README.md b/README.md
index 7018331..b2d4c01 100644
--- a/README.md
+++ b/README.md
@@ -39,7 +39,7 @@ Implementation progress of ISO ICE 14496-12:2015:
 | 8.5.1 Sample Table Box                             | stbl           |   100% |
 | 8.5.2 Sample Description Box                       | stsd           |   100% |
 | 8.5.3 Degradation Priority Box                     | stdp           |      - |
-| 8.6.1.2 Decoding Time to Sample Box                | stts           |      - |
+| 8.6.1.2 Decoding Time to Sample Box                | stts           |   100% |
 | 8.6.1.3 Composition Time to Sample Box             | ctts           |      - |
 | 8.6.1.4 Composition to Decode Box                  | cslg           |      - |
 | 8.6.2 Sync Sample Box                              | stss           |      - |