mp4_edit/

chunk_offset_builder.rs

use std::collections::VecDeque;

use crate::atom::{SampleSizeAtom, SampleToChunkAtom};

#[derive(Debug)]
pub struct ChunkInfo {
    pub track_index: usize,
    pub chunk_number: u32,
    pub chunk_size: u64,
    pub sample_indices: Vec<usize>,
    pub sample_sizes: Vec<u32>,
}

#[derive(Debug)]
pub struct ChunkOffset {
    pub track_index: usize,
    pub offset: u64,
}

#[derive(Clone)]
pub struct ChunkOffsetBuilderTrack<'a> {
    stsc: &'a SampleToChunkAtom,
    stsz: &'a SampleSizeAtom,
}

impl<'a> ChunkOffsetBuilderTrack<'a> {
    /// Build chunk information including sizes and sample mappings
    pub fn build_chunk_info(&self, track_index: usize) -> impl Iterator<Item = ChunkInfo> + 'a {
        self.stsc
            .entries
            .iter()
            .zip(
                self.stsc
                    .entries
                    .iter()
                    .skip(1)
                    .map(Some)
                    .chain(std::iter::once(None)),
            )
            .scan(
                (track_index, 0u32),
                |(track_index, sample_index), (entry, next_entry)| {
                    let next_first_chunk = if let Some(next_entry) = next_entry {
                        next_entry.first_chunk
                    } else {
                        let remaining_samples =
                            self.stsz.sample_count.saturating_sub(*sample_index);
                        entry.first_chunk + remaining_samples.div_ceil(entry.samples_per_chunk)
                    };

                    let start_sample_index = *sample_index;
                    *sample_index +=
                        (next_first_chunk - entry.first_chunk) * entry.samples_per_chunk;

                    // Process all chunks for this entry
                    let track_index = *track_index;
                    Some((entry.first_chunk..next_first_chunk).scan(
                        (track_index, start_sample_index),
                        |(track_index, sample_index), chunk_num| {
                            let (sample_indices, sample_sizes, chunk_size) = self
                                .stsz
                                .sample_sizes()
                                .enumerate()
                                .skip(*sample_index as usize)
                                .take(entry.samples_per_chunk as usize)
                                .fold(
                                    (
                                        Vec::with_capacity(entry.samples_per_chunk as usize),
                                        Vec::with_capacity(entry.samples_per_chunk as usize),
                                        0u64,
                                    ),
                                    |(mut sample_indices, mut sample_sizes, mut chunk_size),
                                     (i, size)| {
                                        sample_indices.push(i);
                                        sample_sizes.push(*size);
                                        chunk_size += u64::from(*size);
                                        (sample_indices, sample_sizes, chunk_size)
                                    },
                                );
                            *sample_index += entry.samples_per_chunk;

                            Some(ChunkInfo {
                                track_index: *track_index,
                                chunk_number: chunk_num,
                                chunk_size,
                                sample_indices,
                                sample_sizes,
                            })
                        },
                    ))
                },
            )
            .flatten()
    }
}

#[derive(Debug, Clone)]
#[non_exhaustive]
pub struct BuildMetadata {
    pub total_size: u64,
}

pub struct ChunkOffsetBuilder<'a> {
    tracks: Vec<ChunkOffsetBuilderTrack<'a>>,
}

impl Default for ChunkOffsetBuilder<'_> {
    fn default() -> Self {
        Self::new()
    }
}

impl<'a> ChunkOffsetBuilder<'a> {
    pub fn new() -> Self {
        Self { tracks: Vec::new() }
    }

    pub fn with_capacity(capacity: usize) -> Self {
        Self {
            tracks: Vec::with_capacity(capacity),
        }
    }

    pub fn add_track(&mut self, stsc: &'a SampleToChunkAtom, stsz: &'a SampleSizeAtom) {
        self.tracks.push(ChunkOffsetBuilderTrack { stsc, stsz });
    }

    /// Build interleaved chunk information including sizes and sample mappings
    pub fn build_chunk_info(&self) -> impl Iterator<Item = ChunkInfo> + 'a {
        let mut iters = self
            .tracks
            .clone()
            .into_iter()
            .enumerate()
            .map(|(track_index, track)| track.build_chunk_info(track_index))
            .collect::<VecDeque<_>>();

        // round-robin chunks from each track
        std::iter::from_fn(move || {
            while let Some(mut it) = iters.pop_front() {
                if let Some(item) = it.next() {
                    iters.push_back(it);
                    return Some(item);
                }
            }
            None
        })
    }

    /// Build interleaved chunk offsets for each track given a starting offset
    pub fn build_chunk_offsets(&self, start_offset: u64) -> (Vec<Vec<u64>>, BuildMetadata) {
        let tracks: Vec<Vec<u64>> = (0..self.tracks.len()).map(|_| Vec::new()).collect();

        let (_, chunk_offsets, total_size) = self.build_chunk_info().fold(
            (start_offset, tracks, 0),
            |(mut current_offset, mut tracks, size), chunk| {
                let chunk_offset = current_offset;
                current_offset += chunk.chunk_size;
                tracks[chunk.track_index].push(chunk_offset);
                (current_offset, tracks, size + chunk.chunk_size)
            },
        );

        (chunk_offsets, BuildMetadata { total_size })
    }

    /// Build interleaved chunk offsets preserving the original order based on input chunk offsets
    pub fn build_chunk_offsets_ordered(
        &self,
        original_chunk_offsets: Vec<&[u64]>,
        start_offset: u64,
    ) -> (Vec<Vec<u64>>, BuildMetadata) {
        struct ChunkOffsetSortable {
            original_offset: u64,
            track_index: usize,
            chunk_size: u64,
        }

        // Allocate intermediate and output `Vec`s
        let (total_chunks, mut tracks): (usize, Vec<Vec<u64>>) =
            original_chunk_offsets.iter().fold(
                (0, Vec::with_capacity(original_chunk_offsets.len())),
                |(mut total, mut tracks), offsets| {
                    total += offsets.len();
                    tracks.push(Vec::with_capacity(offsets.len()));
                    (total, tracks)
                },
            );
        let mut all_chunks = Vec::with_capacity(total_chunks);

        // Collect chunk order information
        for (track_index, track) in self.tracks.iter().enumerate() {
            for (chunk_idx, chunk) in track.build_chunk_info(track_index).enumerate() {
                let original_offset = original_chunk_offsets[track_index][chunk_idx];
                all_chunks.push(ChunkOffsetSortable {
                    original_offset,
                    track_index: chunk.track_index,
                    chunk_size: chunk.chunk_size,
                });
            }
        }

        // Sort by original offset to maintain interleaving order
        all_chunks.sort_unstable_by_key(|chunk| chunk.original_offset);

        // Calculate new offsets maintaining the original interleaving order
        let mut total_size = 0;
        let mut current_offset = start_offset;
        for chunk in all_chunks {
            tracks[chunk.track_index].push(current_offset);
            current_offset += chunk.chunk_size;
            total_size += chunk.chunk_size;
        }

        (tracks, BuildMetadata { total_size })
    }
}

#[cfg(test)]
mod tests {
    use crate::atom::stsc::SampleToChunkEntry;

    use super::*;

    #[test]
    fn test_chunk_offset_calculation() {
        let stsc_entries = vec![
            SampleToChunkEntry {
                first_chunk: 1,
                samples_per_chunk: 2,
                sample_description_index: 1,
            },
            SampleToChunkEntry {
                first_chunk: 3,
                samples_per_chunk: 3,
                sample_description_index: 1,
            },
        ];

        let stsc = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries.into(),
        };

        let stsz = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 7,
            entry_sizes: vec![100, 200, 150, 250, 300, 400, 500].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(1);
        builder.add_track(&stsc, &stsz);
        let (offsets, meta) = builder.build_chunk_offsets(0);
        let offsets = &offsets[0];

        // Expected:
        // Chunk 1: samples 0,1 (100+200=300 bytes) -> offset 0
        // Chunk 2: samples 2,3 (150+250=400 bytes) -> offset 300
        // Chunk 3: samples 4,5,6 (300+400+500=1200 bytes) -> offset 700

        assert_eq!(offsets.len(), 3);
        assert_eq!(offsets[0], 0); // Chunk 1 starts at 0
        assert_eq!(offsets[1], 300); // Chunk 2 starts at 300
        assert_eq!(offsets[2], 700); // Chunk 3 starts at 700
        assert_eq!(
            meta.total_size,
            stsz.entry_sizes.iter().map(|s| *s as u64).sum::<u64>()
        );
    }

    #[test]
    fn test_chunk_info_generation() {
        let stsc_entries = vec![
            SampleToChunkEntry {
                first_chunk: 1,
                samples_per_chunk: 2,
                sample_description_index: 1,
            },
            SampleToChunkEntry {
                first_chunk: 3,
                samples_per_chunk: 1,
                sample_description_index: 1,
            },
        ];

        let stsc = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries.into(),
        };

        let stsz = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 5,
            entry_sizes: vec![100, 200, 300, 400, 500].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(1);
        builder.add_track(&stsc, &stsz);
        let chunk_info = builder.build_chunk_info().collect::<Vec<_>>();

        assert_eq!(chunk_info.len(), 3);

        // Chunk 1: 2 samples (0, 1)
        assert_eq!(chunk_info[0].chunk_number, 1);
        assert_eq!(chunk_info[0].chunk_size, 300); // 100 + 200
        assert_eq!(chunk_info[0].sample_indices, vec![0, 1]);

        // Chunk 2: 2 samples (2, 3)
        assert_eq!(chunk_info[1].chunk_number, 2);
        assert_eq!(chunk_info[1].chunk_size, 700); // 300 + 400
        assert_eq!(chunk_info[1].sample_indices, vec![2, 3]);

        // Chunk 3: 1 sample (4)
        assert_eq!(chunk_info[2].chunk_number, 3);
        assert_eq!(chunk_info[2].chunk_size, 500); // 500
        assert_eq!(chunk_info[2].sample_indices, vec![4]);
    }

    #[test]
    fn test_edge_case_empty_samples() {
        let stsc_entries = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries.into(),
        };

        let stsz = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 0,
            entry_sizes: vec![].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(1);
        builder.add_track(&stsc, &stsz);
        let chunk_info = builder.build_chunk_info().collect::<Vec<_>>();

        assert_eq!(chunk_info.len(), 0);
    }

    #[test]
    fn test_track_interleaving() {
        // Track 1: 2 chunks with 2 samples each
        let stsc_entries_1 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 2,
            sample_description_index: 1,
        }];

        let stsc_1 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_1.into(),
        };

        let stsz_1 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 4,
            entry_sizes: vec![100, 200, 150, 250].into(),
        };

        // Track 2: 3 chunks with 1 sample each
        let stsc_entries_2 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc_2 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_2.into(),
        };

        let stsz_2 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 3,
            entry_sizes: vec![300, 400, 500].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(2);
        builder.add_track(&stsc_1, &stsz_1);
        builder.add_track(&stsc_2, &stsz_2);

        let chunk_info = builder.build_chunk_info().collect::<Vec<_>>();

        // Expected interleaving: T1C1, T2C1, T1C2, T2C2, T2C3
        assert_eq!(chunk_info.len(), 5);

        // Track 1, Chunk 1: samples 0,1 (track_index=0)
        assert_eq!(chunk_info[0].track_index, 0);
        assert_eq!(chunk_info[0].chunk_number, 1);
        assert_eq!(chunk_info[0].chunk_size, 300); // 100 + 200
        assert_eq!(chunk_info[0].sample_indices, vec![0, 1]);

        // Track 2, Chunk 1: sample 0 (track_index=1)
        assert_eq!(chunk_info[1].track_index, 1);
        assert_eq!(chunk_info[1].chunk_number, 1);
        assert_eq!(chunk_info[1].chunk_size, 300); // 300
        assert_eq!(chunk_info[1].sample_indices, vec![0]);

        // Track 1, Chunk 2: samples 2,3 (track_index=0)
        assert_eq!(chunk_info[2].track_index, 0);
        assert_eq!(chunk_info[2].chunk_number, 2);
        assert_eq!(chunk_info[2].chunk_size, 400); // 150 + 250
        assert_eq!(chunk_info[2].sample_indices, vec![2, 3]);

        // Track 2, Chunk 2: sample 1 (track_index=1)
        assert_eq!(chunk_info[3].track_index, 1);
        assert_eq!(chunk_info[3].chunk_number, 2);
        assert_eq!(chunk_info[3].chunk_size, 400); // 400
        assert_eq!(chunk_info[3].sample_indices, vec![1]);

        // Track 2, Chunk 3: sample 2 (track_index=1)
        assert_eq!(chunk_info[4].track_index, 1);
        assert_eq!(chunk_info[4].chunk_number, 3);
        assert_eq!(chunk_info[4].chunk_size, 500); // 500
        assert_eq!(chunk_info[4].sample_indices, vec![2]);

        // Test chunk offsets are calculated correctly with interleaving
        let (offsets, meta) = builder.build_chunk_offsets(0);

        // Track 1 offsets: [0, 600] (T1C1 at 0, T1C2 at 0+300+300=600)
        assert_eq!(offsets[0], vec![0, 600]);

        // Track 2 offsets: [300, 1000, 1400] (T2C1 at 300, T2C2 at 300+300+400=1000, T2C3 at 1000+400=1400)
        assert_eq!(offsets[1], vec![300, 1000, 1400]);

        assert_eq!(
            meta.total_size,
            stsz_1
                .entry_sizes
                .iter()
                .cloned()
                .chain(stsz_2.entry_sizes.iter().cloned())
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered() {
        // Track 1: 2 chunks
        let stsc_entries_1 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 2,
            sample_description_index: 1,
        }];

        let stsc_1 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_1.into(),
        };

        let stsz_1 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 4,
            entry_sizes: vec![100, 200, 150, 250].into(),
        };

        // Track 2: 2 chunks
        let stsc_entries_2 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc_2 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_2.into(),
        };

        let stsz_2 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 2,
            entry_sizes: vec![300, 400].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(2);
        builder.add_track(&stsc_1, &stsz_1);
        builder.add_track(&stsc_2, &stsz_2);

        // Original chunk offsets in a specific order:
        // Track 1: chunks at offsets [1000, 2000]
        // Track 2: chunks at offsets [500, 1500]
        // This means the order should be: T2C1 (500), T1C1 (1000), T2C2 (1500), T1C2 (2000)
        let original_offsets_track_1 = vec![1000u64, 2000u64];
        let original_offsets_track_2 = vec![500u64, 1500u64];
        let original_offsets = vec![
            original_offsets_track_1.as_slice(),
            original_offsets_track_2.as_slice(),
        ];

        let (new_offsets, meta) = builder.build_chunk_offsets_ordered(original_offsets, 0);

        // With the original ordering (T2C1, T1C1, T2C2, T1C2) and chunk sizes:
        // T2C1: 300 bytes -> offset 0
        // T1C1: 300 bytes -> offset 300
        // T2C2: 400 bytes -> offset 600
        // T1C2: 400 bytes -> offset 1000

        // Track 1 chunks should be at offsets [300, 1000]
        assert_eq!(new_offsets[0], vec![300, 1000]);

        // Track 2 chunks should be at offsets [0, 600]
        assert_eq!(new_offsets[1], vec![0, 600]);

        assert_eq!(
            meta.total_size,
            stsz_1
                .entry_sizes
                .iter()
                .cloned()
                .chain(stsz_2.entry_sizes.iter().cloned())
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered_single_track() {
        let stsc_entries = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 3,
            sample_description_index: 1,
        }];

        let stsc = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries.into(),
        };

        let stsz = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 6,
            entry_sizes: vec![100, 200, 300, 150, 250, 350].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(1);
        builder.add_track(&stsc, &stsz);

        // Single track with 2 chunks at original offsets [5000, 10000]
        let original_offsets_track_1 = vec![5000u64, 10000u64];
        let original_offsets = vec![original_offsets_track_1.as_slice()];

        let (new_offsets, meta) = builder.build_chunk_offsets_ordered(original_offsets, 0);

        // Chunk 1: samples 0,1,2 -> sizes 100+200+300 = 600
        // Chunk 2: samples 3,4,5 -> sizes 150+250+350 = 750
        // Sequential placement: Chunk 1 at 0, Chunk 2 at 600
        assert_eq!(new_offsets[0], vec![0, 600]);

        assert_eq!(
            meta.total_size,
            stsz.entry_sizes
                .iter()
                .cloned()
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered_non_zero_start() {
        let stsc_entries = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries.into(),
        };

        let stsz = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 3,
            entry_sizes: vec![100, 200, 300].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(1);
        builder.add_track(&stsc, &stsz);

        let original_offsets_track_1 = vec![1000u64, 2000u64, 3000u64];
        let original_offsets = vec![original_offsets_track_1.as_slice()];
        let start_offset = 50000u64;

        let (new_offsets, meta) =
            builder.build_chunk_offsets_ordered(original_offsets, start_offset);

        // Starting at 50000, chunks of sizes 100, 200, 300
        assert_eq!(new_offsets[0], vec![50000, 50100, 50300]);

        assert_eq!(
            meta.total_size,
            stsz.entry_sizes
                .iter()
                .cloned()
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered_interleaving() {
        // Track 1: 3 chunks with different sample counts
        let stsc_entries_1 = vec![
            SampleToChunkEntry {
                first_chunk: 1,
                samples_per_chunk: 1,
                sample_description_index: 1,
            },
            SampleToChunkEntry {
                first_chunk: 2,
                samples_per_chunk: 2,
                sample_description_index: 1,
            },
        ];

        let stsc_1 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_1.into(),
        };

        let stsz_1 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 5,
            entry_sizes: vec![100, 150, 200, 250, 300].into(),
        };

        // Track 2: 2 chunks
        let stsc_entries_2 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 2,
            sample_description_index: 1,
        }];

        let stsc_2 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_2.into(),
        };

        let stsz_2 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 4,
            entry_sizes: vec![80, 120, 160, 240].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(2);
        builder.add_track(&stsc_1, &stsz_1);
        builder.add_track(&stsc_2, &stsz_2);

        // Original offsets creating interleaving:
        // Track 1: [100, 300, 700] (chunk 1: sample 0, chunk 2: samples 1-2, chunk 3: samples 3-4)
        // Track 2: [200, 600] (chunk 1: samples 0-1, chunk 2: samples 2-3)
        // Order should be: T1C1 (100), T2C1 (200), T1C2 (300), T2C2 (600), T1C3 (700)
        let original_offsets_track_1 = vec![100u64, 300u64, 700u64];
        let original_offsets_track_2 = vec![200u64, 600u64];
        let original_offsets = vec![
            original_offsets_track_1.as_slice(),
            original_offsets_track_2.as_slice(),
        ];

        let (new_offsets, meta) = builder.build_chunk_offsets_ordered(original_offsets, 0);

        // Chunk sizes:
        // T1C1: 100 bytes (sample 0)
        // T2C1: 200 bytes (samples 0-1: 80+120)
        // T1C2: 350 bytes (samples 1-2: 150+200)
        // T2C2: 400 bytes (samples 2-3: 160+240)
        // T1C3: 550 bytes (samples 3-4: 250+300)
        //
        // Sequential placement:
        // T1C1 at 0, T2C1 at 100, T1C2 at 300, T2C2 at 650, T1C3 at 1050

        assert_eq!(new_offsets[0], vec![0, 300, 1050]);
        assert_eq!(new_offsets[1], vec![100, 650]);

        assert_eq!(
            meta.total_size,
            stsz_1
                .entry_sizes
                .iter()
                .cloned()
                .chain(stsz_2.entry_sizes.iter().cloned())
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered_different_chunk_sizes() {
        // Track 1: Large chunks
        let stsc_entries_1 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 4,
            sample_description_index: 1,
        }];

        let stsc_1 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_1.into(),
        };

        let stsz_1 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 8,
            entry_sizes: vec![1000; 8].into(), // All samples are 1000 bytes
        };

        // Track 2: Small chunks
        let stsc_entries_2 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc_2 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_2.into(),
        };

        let stsz_2 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 4,
            entry_sizes: vec![50; 4].into(), // All samples are 50 bytes
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(2);
        builder.add_track(&stsc_1, &stsz_1);
        builder.add_track(&stsc_2, &stsz_2);

        // Interleave small chunks between large chunks
        // T1: [1000, 10000] (chunks of 4000 bytes each)
        // T2: [2000, 3000, 4000, 5000] (chunks of 50 bytes each)
        let original_offsets_track_1 = vec![1000u64, 10000u64];
        let original_offsets_track_2 = vec![2000u64, 3000u64, 4000u64, 5000u64];
        let original_offsets = vec![
            original_offsets_track_1.as_slice(),
            original_offsets_track_2.as_slice(),
        ];

        let (new_offsets, meta) = builder.build_chunk_offsets_ordered(original_offsets, 0);

        // Order: T1C1 (1000), T2C1 (2000), T2C2 (3000), T2C3 (4000), T2C4 (5000), T1C2 (10000)
        // Sizes: 4000, 50, 50, 50, 50, 4000
        // Offsets: 0, 4000, 4050, 4100, 4150, 4200
        assert_eq!(new_offsets[0], vec![0, 4200]);
        assert_eq!(new_offsets[1], vec![4000, 4050, 4100, 4150]);

        assert_eq!(
            meta.total_size,
            stsz_1
                .entry_sizes
                .iter()
                .cloned()
                .chain(stsz_2.entry_sizes.iter().cloned())
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered_empty_track_handling() {
        // Track 1: Has chunks
        let stsc_entries_1 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 2,
            sample_description_index: 1,
        }];

        let stsc_1 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_1.into(),
        };

        let stsz_1 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 4,
            entry_sizes: vec![100, 200, 300, 400].into(),
        };

        // Track 2: Empty (no samples)
        let stsc_entries_2 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc_2 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_2.into(),
        };

        let stsz_2 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 0,
            entry_sizes: vec![].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(2);
        builder.add_track(&stsc_1, &stsz_1);
        builder.add_track(&stsc_2, &stsz_2);

        let original_offsets_track_1 = vec![1000u64, 2000u64];
        let original_offsets_track_2: Vec<u64> = vec![];
        let original_offsets = vec![
            original_offsets_track_1.as_slice(),
            original_offsets_track_2.as_slice(), // Empty track has no chunks
        ];

        let (new_offsets, meta) = builder.build_chunk_offsets_ordered(original_offsets, 0);

        // Only track 1 has chunks: chunk 1 (300 bytes), chunk 2 (700 bytes)
        assert_eq!(new_offsets[0], vec![0, 300]);
        assert_eq!(new_offsets[1], vec![]); // Empty track remains empty

        assert_eq!(
            meta.total_size,
            stsz_1
                .entry_sizes
                .iter()
                .cloned()
                .chain(stsz_2.entry_sizes.iter().cloned())
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }

    #[test]
    fn test_build_chunk_offsets_ordered_non_round_robin_interleaving() {
        // Track 1: 5 chunks
        let stsc_entries_1 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 1,
            sample_description_index: 1,
        }];

        let stsc_1 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_1.into(),
        };

        let stsz_1 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 5,
            entry_sizes: vec![100, 150, 200, 250, 300].into(),
        };

        // Track 2: 4 chunks
        let stsc_entries_2 = vec![SampleToChunkEntry {
            first_chunk: 1,
            samples_per_chunk: 2,
            sample_description_index: 1,
        }];

        let stsc_2 = SampleToChunkAtom {
            version: 0,
            flags: [0, 0, 0],
            entries: stsc_entries_2.into(),
        };

        let stsz_2 = SampleSizeAtom {
            version: 0,
            flags: [0u8; 3],
            sample_size: 0,
            sample_count: 8,
            entry_sizes: vec![80, 120, 90, 110, 70, 130, 60, 140].into(),
        };

        let mut builder = ChunkOffsetBuilder::with_capacity(2);
        builder.add_track(&stsc_1, &stsz_1);
        builder.add_track(&stsc_2, &stsz_2);

        // Create non-round-robin interleaving pattern:
        // T1 chunks: [100, 300, 500, 800, 1000]
        // T2 chunks: [200, 400, 600, 900]
        // Expected order: T1C1(100), T1C2(300), T2C1(200), T1C3(500), T2C2(400), T2C3(600), T1C4(800), T2C4(900), T1C5(1000)
        // Pattern: T1, T2, T1, T1, T2, T2, T1, T2, T1 (non-round-robin)
        let original_offsets_track_1 = vec![100u64, 300u64, 500u64, 800u64, 1000u64];
        let original_offsets_track_2 = vec![200u64, 400u64, 600u64, 900u64];
        let original_offsets = vec![
            original_offsets_track_1.as_slice(),
            original_offsets_track_2.as_slice(),
        ];

        let (new_offsets, meta) = builder.build_chunk_offsets_ordered(original_offsets, 0);

        // Calculate expected chunk sizes:
        // T1: [100, 150, 200, 250, 300] (individual samples)
        // T2: [(80+120)=200, (90+110)=200, (70+130)=200, (60+140)=200] (2 samples per chunk)
        //
        // Sequential placement based on original ordering:
        // T1C1(100) at 0, T2C1(200) at 100, T1C2(150) at 300, T2C2(200) at 450,
        // T1C3(200) at 650, T2C3(200) at 850, T1C4(250) at 1050, T2C4(200) at 1300, T1C5(300) at 1500

        assert_eq!(new_offsets[0], vec![0, 300, 650, 1050, 1500]); // Track 1 chunks
        assert_eq!(new_offsets[1], vec![100, 450, 850, 1300]); // Track 2 chunks

        assert_eq!(
            meta.total_size,
            stsz_1
                .entry_sizes
                .iter()
                .cloned()
                .chain(stsz_2.entry_sizes.iter().cloned())
                .map(|s| s as u64)
                .sum::<u64>()
        );
    }
}