From 27e34124edd89301a00ade4500cdfee185639d2f Mon Sep 17 00:00:00 2001
From: Martin Riedl <martin.riedl.92@googlemail.com>
Date: Thu, 12 Dec 2024 19:23:33 +0100
Subject: [PATCH] feat: chunk offset box

---
 ChunkOffsetBox.go | 87 +++++++++++++++++++++++++++++++++++++++++++++++
 README.md         |  2 +-
 2 files changed, 88 insertions(+), 1 deletion(-)
 create mode 100644 ChunkOffsetBox.go

diff --git a/ChunkOffsetBox.go b/ChunkOffsetBox.go
new file mode 100644
index 0000000..bbeae5e
--- /dev/null
+++ b/ChunkOffsetBox.go
@@ -0,0 +1,87 @@
+// 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"
+)
+
+// ChunkOffsetBox chunk offset box struct
+//
+// 8.7.5 Chunk Offset Box
+//
+// Box Type: ‘stco’, ‘co64’
+// Container: Sample Table Box (‘stbl’)
+// Mandatory: Yes
+// Quantity: Exactly one variant must be present
+//
+// The chunk offset table gives the index of each chunk into the containing file. There are two variants,
+// permitting the use of 32‐bit or 64‐bit offsets. The latter is useful when managing very large
+// presentations. At most one of these variants will occur in any single instance of a sample table.
+//
+// Offsets are file offsets, not the offset into any box within the file (e.g. Media Data Box). This permits
+// referring to media data in files without any box structure. It does also mean that care must be taken
+// when constructing a self‐contained ISO file with its metadata (Movie Box) at the front, as the size of the
+// Movie Box will affect the chunk offsets to the media data.
+type ChunkOffsetBox struct {
+	*FullBox
+	// is an integer that gives the number of entries in the following table
+	EntryCount uint32
+	EntryList  []ChunkOffsetEntry
+}
+
+type ChunkOffsetEntry struct {
+	// is a 32 or 64 bit integer that gives the offset of the start of a chunk into its
+	// containing media file.
+	ChunkOffset uint32
+}
+
+// BoxTypeChunkOffset Chunk Offset Box
+const BoxTypeChunkOffset = "stco"
+
+func init() {
+	BoxDefinitions = append(BoxDefinitions, BoxDefinition{
+		Type:        BoxTypeChunkOffset,
+		ParentTypes: []string{BoxTypeSampleTable},
+		Parser:      ParseChunkOffsetBox,
+	})
+}
+
+// ParseChunkOffsetBox creates a new chunk offset box struct based on bytes
+func ParseChunkOffsetBox(parser *Parser, filePosition uint64, headerSize uint32, content []byte) (any, error) {
+	box := &ChunkOffsetBox{
+		FullBox: newFullBox(&Box{filePosition, headerSize}, content[0:4]),
+	}
+
+	// parse entry counter
+	box.EntryCount = binary.BigEndian.Uint32(content[4:8])
+
+	// parse offset entries
+	position := 8
+	for i := 0; i < int(box.EntryCount); i++ {
+		newOffsetEntry := ChunkOffsetEntry{
+			ChunkOffset: binary.BigEndian.Uint32(content[position+(i*4) : position+4+(i*4)]),
+		}
+		box.EntryList = append(box.EntryList, newOffsetEntry)
+	}
+
+	// validate entries amount
+	if len(box.EntryList) != int(box.EntryCount) {
+		return box, fmt.Errorf("invalid amount of offset entries at %d; got %d but expected %d", filePosition, len(box.EntryList), box.EntryCount)
+	}
+
+	return box, nil
+}
diff --git a/README.md b/README.md
index e318072..4921ee6 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,7 @@ Implementation progress of ISO/IEC 14496-12:2015:
 | 8.7.2 Data Reference Box                           | dref, url, urn |   100% |
 | 8.7.3 Sample Size Boxes                            | stsz, stz2     |    50% |
 | 8.7.4 Sample To Chunk Box                          | stsc           |   100% |
-| 8.7.5 Chunk Offset Box                             | stco, co64     |      - |
+| 8.7.5 Chunk Offset Box                             | stco, co64     |    50% |
 | 8.7.6 Padding Bits Box                             | padb           |      - |
 | 8.7.7 Sub-Sample Information Box                   | subs           |      - |
 | 8.7.8 Sample Auxiliary Information Sizes Box       | saiz           |      - |