// 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
}