mp4_track/

writer.rs

use crate::parse::validate_tree;
use crate::*;
use std::io::{Seek, SeekFrom, Write};

/// A progressive MP4 writer.
pub struct Mp4Writer<W: Write + Seek> {
    writer: W,
    config: WriterConfig,
    tracks: Vec<WriterTrack>,
    first_sample_written: bool,
    current_track: Option<TrackId>,
    ftyp_offset: u64,
    free_offset: u64,
    mdat_offset: u64,
    data_offset: u64,
}

struct WriterTrack {
    id: TrackId,
    config: TrackConfig,
    samples: Vec<WriterSample>,
    chunks: Vec<WriterChunk>,
    force_co64: bool,
}

#[derive(Clone, Copy)]
struct WriterSample {
    size: u32,
    duration: u32,
    cts_offset: i32,
    is_sync: bool,
}

#[derive(Clone, Copy)]
struct WriterChunk {
    offset: u64,
    sample_count: u32,
}

impl<W: Write + Seek> Mp4Writer<W> {
    /// Creates a writer and emits the file-type and media-data headers.
    pub fn new(mut writer: W, config: WriterConfig) -> Result<Self, Error> {
        if config.movie_timescale == 0 {
            return Err(Error::InvalidTimescale { track: None });
        }
        let ftyp_offset = writer.stream_position()?;
        let ftyp = make_ftyp(&config, false)?;
        writer.write_all(&ftyp)?;
        let free_offset = writer.stream_position()?;
        writer.write_all(&8u32.to_be_bytes())?;
        writer.write_all(b"free")?;
        let mdat_offset = writer.stream_position()?;
        writer.write_all(&0u32.to_be_bytes())?;
        writer.write_all(b"mdat")?;
        let data_offset = writer.stream_position()?;
        Ok(Self {
            writer,
            config,
            tracks: Vec::new(),
            first_sample_written: false,
            current_track: None,
            ftyp_offset,
            free_offset,
            mdat_offset,
            data_offset,
        })
    }

    /// Adds a track before the first sample and returns its identifier.
    pub fn add_track(&mut self, track: TrackConfig) -> Result<TrackId, Error> {
        if self.first_sample_written {
            return Err(Error::WriterState("tracks must be added before samples"));
        }
        validate_track_config(&track)?;
        let next = self
            .tracks
            .len()
            .checked_add(1)
            .ok_or(Error::WriterState("track count overflow"))?;
        let id = TrackId(
            u32::try_from(next).map_err(|_| Error::WriterState("track count exceeds u32"))?,
        );
        self.tracks.push(WriterTrack {
            id,
            config: track,
            samples: Vec::new(),
            chunks: Vec::new(),
            force_co64: self.config.chunk_offsets == ChunkOffsetChoice::Co64,
        });
        Ok(id)
    }

    /// Writes one encoded sample.
    pub fn write_sample(&mut self, id: TrackId, sample: &SampleInput<'_>) -> Result<(), Error> {
        let position = self
            .tracks
            .iter()
            .position(|track| track.id == id)
            .ok_or(Error::NoSuchTrack(id))?;
        let size = u32::try_from(sample.data.len()).map_err(|_| Error::LimitExceeded {
            limit: "sample_size",
            value: u64::try_from(sample.data.len()).unwrap_or(u64::MAX),
            max: u64::from(u32::MAX),
        })?;
        let offset = self.writer.stream_position()?;
        self.writer.write_all(sample.data)?;
        let track = self
            .tracks
            .get_mut(position)
            .ok_or(Error::NoSuchTrack(id))?;
        if self.current_track != Some(id) {
            track.chunks.push(WriterChunk {
                offset,
                sample_count: 0,
            });
            self.current_track = Some(id);
        }
        let chunk = track
            .chunks
            .last_mut()
            .ok_or(Error::WriterState("sample chunk is missing"))?;
        chunk.sample_count = chunk
            .sample_count
            .checked_add(1)
            .ok_or(Error::WriterState("chunk sample count overflow"))?;
        track.samples.push(WriterSample {
            size,
            duration: sample.duration,
            cts_offset: sample.cts_offset,
            is_sync: sample.is_sync,
        });
        self.first_sample_written = true;
        Ok(())
    }

    /// Finalizes tables, fixes the media-data header, and returns the stream.
    pub fn finish(mut self) -> Result<W, Error> {
        if self.tracks.is_empty() {
            return Err(Error::WriterState("at least one track is required"));
        }
        let media_end = self.writer.stream_position()?;
        let payload_size = media_end
            .checked_sub(self.data_offset)
            .ok_or(Error::WriterState("media-data size underflow"))?;
        if payload_size <= u64::from(u32::MAX).saturating_sub(8) {
            let size = u32::try_from(payload_size.saturating_add(8))
                .map_err(|_| Error::WriterState("media-data size exceeds u32"))?;
            self.writer.seek(SeekFrom::Start(self.mdat_offset))?;
            self.writer.write_all(&size.to_be_bytes())?;
            self.writer.write_all(b"mdat")?;
        } else {
            let size = payload_size
                .checked_add(16)
                .ok_or(Error::WriterState("media-data size overflow"))?;
            self.writer.seek(SeekFrom::Start(self.free_offset))?;
            self.writer.write_all(&1u32.to_be_bytes())?;
            self.writer.write_all(b"mdat")?;
            self.writer.write_all(&size.to_be_bytes())?;
        }
        let has_avc = self
            .tracks
            .iter()
            .any(|track| matches!(track.config.codec, Codec::H264 { .. }));
        self.writer.seek(SeekFrom::Start(self.ftyp_offset))?;
        self.writer.write_all(&make_ftyp(&self.config, has_avc)?)?;
        self.writer.seek(SeekFrom::Start(media_end))?;
        let moov = make_moov(&self.config, &self.tracks)?;
        validate_tree(
            moov.get(8..).ok_or(Error::WriterState(
                "encoded movie is smaller than its header",
            ))?,
            FourCc(*b"moov"),
            0,
            Limits::default(),
        )?;
        self.writer.write_all(&moov)?;
        self.writer.flush()?;
        Ok(self.writer)
    }
}

fn validate_track_config(track: &TrackConfig) -> Result<(), Error> {
    if track.timescale == 0 {
        return Err(Error::InvalidTimescale { track: None });
    }
    let bytes = track.language.as_bytes();
    if bytes.len() != 3 || !bytes.iter().all(|byte| byte.is_ascii_lowercase()) {
        return Err(Error::WriterState(
            "language must be three lowercase ASCII letters",
        ));
    }
    if !track.width.is_finite()
        || !track.height.is_finite()
        || track.width < 0.0
        || track.height < 0.0
    {
        return Err(Error::WriterState(
            "track dimensions must be finite and nonnegative",
        ));
    }
    if track.width > 65_535.0 || track.height > 65_535.0 {
        return Err(Error::WriterState("track dimensions exceed 16.16 range"));
    }
    Ok(())
}

fn make_box(kind: FourCc, body: Vec<u8>) -> Result<Vec<u8>, Error> {
    let size = body
        .len()
        .checked_add(8)
        .ok_or(Error::WriterState("box size overflow"))?;
    let size = u32::try_from(size).map_err(|_| Error::WriterState("box exceeds u32 size"))?;
    let mut result = Vec::with_capacity(usize::try_from(size).unwrap_or(0));
    result.extend_from_slice(&size.to_be_bytes());
    result.extend_from_slice(&kind.0);
    result.extend_from_slice(&body);
    Ok(result)
}

fn full_box(version: u8, flags: u32, mut body: Vec<u8>) -> Vec<u8> {
    let mut result = Vec::with_capacity(body.len().saturating_add(4));
    result.push(version);
    let bytes = flags.to_be_bytes();
    result.extend_from_slice(bytes.get(1..).unwrap_or_default());
    result.append(&mut body);
    result
}

fn make_ftyp(config: &WriterConfig, has_avc: bool) -> Result<Vec<u8>, Error> {
    let mut brands = config.compatible_brands.clone();
    for required in [FourCc(*b"isom"), FourCc(*b"iso2"), FourCc(*b"mp41")] {
        if !brands.contains(&required) {
            brands.push(required);
        }
    }
    brands.push(if has_avc {
        FourCc(*b"avc1")
    } else {
        config.major_brand
    });
    let mut body = Vec::with_capacity(8usize.saturating_add(brands.len().saturating_mul(4)));
    body.extend_from_slice(&config.major_brand.0);
    body.extend_from_slice(&512u32.to_be_bytes());
    for brand in brands {
        body.extend_from_slice(&brand.0);
    }
    make_box(FourCc(*b"ftyp"), body)
}

fn make_moov(config: &WriterConfig, tracks: &[WriterTrack]) -> Result<Vec<u8>, Error> {
    let movie_duration = tracks.iter().try_fold(0u64, |maximum, track| {
        Ok::<u64, Error>(maximum.max(track_movie_duration(config.movie_timescale, track)?))
    })?;
    let mut body = make_mvhd(config.movie_timescale, movie_duration, tracks.len())?;
    for track in tracks {
        body.extend_from_slice(&make_trak(config.movie_timescale, track)?);
    }
    make_box(FourCc(*b"moov"), body)
}

fn track_media_duration(track: &WriterTrack) -> Result<u64, Error> {
    track.samples.iter().try_fold(0u64, |sum, sample| {
        sum.checked_add(u64::from(sample.duration))
            .ok_or(Error::WriterState("track duration overflow"))
    })
}

fn track_movie_duration(movie_timescale: u32, track: &WriterTrack) -> Result<u64, Error> {
    if !track.config.edit_list.is_empty() {
        return track.config.edit_list.iter().try_fold(0u64, |sum, edit| {
            sum.checked_add(edit.segment_duration)
                .ok_or(Error::WriterState("edit duration overflow"))
        });
    }
    let media = u128::from(track_media_duration(track)?);
    let scaled = media
        .checked_mul(u128::from(movie_timescale))
        .ok_or(Error::WriterState("movie duration overflow"))?
        .checked_div(u128::from(track.config.timescale))
        .ok_or(Error::InvalidTimescale {
            track: Some(track.id),
        })?;
    u64::try_from(scaled).map_err(|_| Error::WriterState("movie duration exceeds u64"))
}

fn make_mvhd(timescale: u32, duration: u64, track_count: usize) -> Result<Vec<u8>, Error> {
    let version = if duration > u64::from(u32::MAX) { 1 } else { 0 };
    let mut body = Vec::new();
    if version == 1 {
        body.extend_from_slice(&0u64.to_be_bytes());
        body.extend_from_slice(&0u64.to_be_bytes());
        body.extend_from_slice(&timescale.to_be_bytes());
        body.extend_from_slice(&duration.to_be_bytes());
    } else {
        body.extend_from_slice(&0u32.to_be_bytes());
        body.extend_from_slice(&0u32.to_be_bytes());
        body.extend_from_slice(&timescale.to_be_bytes());
        body.extend_from_slice(&u32::try_from(duration).unwrap_or(u32::MAX).to_be_bytes());
    }
    body.extend_from_slice(&0x0001_0000u32.to_be_bytes());
    body.extend_from_slice(&0x0100u16.to_be_bytes());
    body.extend_from_slice(&[0; 10]);
    append_matrix(&mut body);
    body.extend_from_slice(&[0; 24]);
    let next = u32::try_from(track_count)
        .unwrap_or(u32::MAX)
        .saturating_add(1);
    body.extend_from_slice(&next.to_be_bytes());
    make_box(FourCc(*b"mvhd"), full_box(version, 0, body))
}

fn append_matrix(body: &mut Vec<u8>) {
    for value in [0x0001_0000u32, 0, 0, 0, 0x0001_0000, 0, 0, 0, 0x4000_0000] {
        body.extend_from_slice(&value.to_be_bytes());
    }
}

fn make_trak(movie_timescale: u32, track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let duration = track_movie_duration(movie_timescale, track)?;
    let mut body = make_tkhd(duration, track)?;
    if !track.config.edit_list.is_empty() {
        body.extend_from_slice(&make_edts(&track.config.edit_list)?);
    }
    body.extend_from_slice(&make_mdia(track)?);
    make_box(FourCc(*b"trak"), body)
}

fn fixed_16_16(value: f64) -> Result<u32, Error> {
    let scaled = value * 65_536.0;
    if !(0.0..=f64::from(u32::MAX)).contains(&scaled) {
        return Err(Error::WriterState("fixed-point value is out of range"));
    }
    Ok(scaled.round() as u32)
}

fn make_tkhd(duration: u64, track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let version = if duration > u64::from(u32::MAX) { 1 } else { 0 };
    let mut body = Vec::new();
    if version == 1 {
        body.extend_from_slice(&[0; 16]);
        body.extend_from_slice(&track.id.0.to_be_bytes());
        body.extend_from_slice(&0u32.to_be_bytes());
        body.extend_from_slice(&duration.to_be_bytes());
    } else {
        body.extend_from_slice(&[0; 8]);
        body.extend_from_slice(&track.id.0.to_be_bytes());
        body.extend_from_slice(&0u32.to_be_bytes());
        body.extend_from_slice(&u32::try_from(duration).unwrap_or(u32::MAX).to_be_bytes());
    }
    body.extend_from_slice(&[0; 8]);
    body.extend_from_slice(&0i16.to_be_bytes());
    body.extend_from_slice(&0i16.to_be_bytes());
    let volume = if track.config.kind == TrackKind::Audio {
        0x0100u16
    } else {
        0
    };
    body.extend_from_slice(&volume.to_be_bytes());
    body.extend_from_slice(&0u16.to_be_bytes());
    append_matrix(&mut body);
    body.extend_from_slice(&fixed_16_16(track.config.width)?.to_be_bytes());
    body.extend_from_slice(&fixed_16_16(track.config.height)?.to_be_bytes());
    make_box(FourCc(*b"tkhd"), full_box(version, 3, body))
}

fn make_edts(edits: &[Edit]) -> Result<Vec<u8>, Error> {
    let version = if edits.iter().any(|edit| {
        edit.segment_duration > u64::from(u32::MAX)
            || edit
                .media_time
                .is_some_and(|value| value > i64::from(i32::MAX) as u64)
    }) {
        1
    } else {
        0
    };
    let mut body = Vec::new();
    body.extend_from_slice(
        &u32::try_from(edits.len())
            .map_err(|_| Error::WriterState("edit count exceeds u32"))?
            .to_be_bytes(),
    );
    for edit in edits {
        if version == 1 {
            body.extend_from_slice(&edit.segment_duration.to_be_bytes());
            let media_time = edit
                .media_time
                .map(|value| i64::try_from(value).unwrap_or(i64::MAX))
                .unwrap_or(-1);
            body.extend_from_slice(&media_time.to_be_bytes());
        } else {
            body.extend_from_slice(
                &u32::try_from(edit.segment_duration)
                    .unwrap_or(u32::MAX)
                    .to_be_bytes(),
            );
            let media_time = edit
                .media_time
                .map(|value| i32::try_from(value).unwrap_or(i32::MAX))
                .unwrap_or(-1);
            body.extend_from_slice(&media_time.to_be_bytes());
        }
        let integer = edit.media_rate.trunc();
        let fraction = ((edit.media_rate - integer) * 65_536.0).round();
        body.extend_from_slice(&(integer as i16).to_be_bytes());
        body.extend_from_slice(&(fraction as u16).to_be_bytes());
    }
    let elst = make_box(FourCc(*b"elst"), full_box(version, 0, body))?;
    make_box(FourCc(*b"edts"), elst)
}

fn make_mdia(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let mut body = make_mdhd(track)?;
    body.extend_from_slice(&make_hdlr(track)?);
    body.extend_from_slice(&make_minf(track)?);
    make_box(FourCc(*b"mdia"), body)
}

fn pack_language(language: &str) -> Result<u16, Error> {
    let bytes = language.as_bytes();
    let first = u16::from(
        *bytes
            .first()
            .ok_or(Error::WriterState("language is empty"))?,
    )
    .saturating_sub(0x60);
    let second = u16::from(
        *bytes
            .get(1)
            .ok_or(Error::WriterState("language is too short"))?,
    )
    .saturating_sub(0x60);
    let third = u16::from(
        *bytes
            .get(2)
            .ok_or(Error::WriterState("language is too short"))?,
    )
    .saturating_sub(0x60);
    Ok((first << 10) | (second << 5) | third)
}

fn make_mdhd(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let duration = track_media_duration(track)?;
    let version = if duration > u64::from(u32::MAX) { 1 } else { 0 };
    let mut body = Vec::new();
    if version == 1 {
        body.extend_from_slice(&[0; 16]);
        body.extend_from_slice(&track.config.timescale.to_be_bytes());
        body.extend_from_slice(&duration.to_be_bytes());
    } else {
        body.extend_from_slice(&[0; 8]);
        body.extend_from_slice(&track.config.timescale.to_be_bytes());
        body.extend_from_slice(&u32::try_from(duration).unwrap_or(u32::MAX).to_be_bytes());
    }
    body.extend_from_slice(&pack_language(&track.config.language)?.to_be_bytes());
    body.extend_from_slice(&0u16.to_be_bytes());
    make_box(FourCc(*b"mdhd"), full_box(version, 0, body))
}

fn handler_type(kind: TrackKind) -> FourCc {
    match kind {
        TrackKind::Video => FourCc(*b"vide"),
        TrackKind::Audio => FourCc(*b"soun"),
        TrackKind::Subtitle => FourCc(*b"subt"),
        TrackKind::Other(value) => value,
    }
}

fn make_hdlr(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let mut body = Vec::new();
    body.extend_from_slice(&0u32.to_be_bytes());
    body.extend_from_slice(&handler_type(track.config.kind).0);
    body.extend_from_slice(&[0; 12]);
    body.extend_from_slice(track.config.handler_name.as_bytes());
    body.push(0);
    make_box(FourCc(*b"hdlr"), full_box(0, 0, body))
}

fn make_minf(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let mut body = match track.config.kind {
        TrackKind::Video => {
            let mut fields = Vec::new();
            fields.extend_from_slice(&0u16.to_be_bytes());
            fields.extend_from_slice(&[0; 6]);
            make_box(FourCc(*b"vmhd"), full_box(0, 1, fields))?
        }
        TrackKind::Audio => {
            let mut fields = Vec::new();
            fields.extend_from_slice(&0i16.to_be_bytes());
            fields.extend_from_slice(&0u16.to_be_bytes());
            make_box(FourCc(*b"smhd"), full_box(0, 0, fields))?
        }
        TrackKind::Subtitle | TrackKind::Other(_) => {
            make_box(FourCc(*b"nmhd"), full_box(0, 0, Vec::new()))?
        }
    };
    body.extend_from_slice(&make_dinf()?);
    body.extend_from_slice(&make_stbl(track)?);
    make_box(FourCc(*b"minf"), body)
}

fn make_dinf() -> Result<Vec<u8>, Error> {
    let url = make_box(FourCc(*b"url "), full_box(0, 1, Vec::new()))?;
    let mut dref_body = Vec::new();
    dref_body.extend_from_slice(&1u32.to_be_bytes());
    dref_body.extend_from_slice(&url);
    let dref = make_box(FourCc(*b"dref"), full_box(0, 0, dref_body))?;
    make_box(FourCc(*b"dinf"), dref)
}

fn make_stbl(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let mut body = make_stsd(track)?;
    body.extend_from_slice(&make_stts(track)?);
    if track.samples.iter().any(|sample| sample.cts_offset != 0) {
        body.extend_from_slice(&make_ctts(track)?);
    }
    if track.samples.iter().any(|sample| !sample.is_sync) {
        body.extend_from_slice(&make_stss(track)?);
    }
    body.extend_from_slice(&make_stsc(track)?);
    body.extend_from_slice(&make_stsz(track)?);
    body.extend_from_slice(&make_chunk_offsets(track)?);
    make_box(FourCc(*b"stbl"), body)
}

fn make_stsd(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let entry = make_sample_entry(track)?;
    let mut body = Vec::new();
    body.extend_from_slice(&1u32.to_be_bytes());
    body.extend_from_slice(&entry);
    make_box(FourCc(*b"stsd"), full_box(0, 0, body))
}

fn visual_fields(width: f64, height: f64) -> Result<Vec<u8>, Error> {
    let mut body = Vec::with_capacity(78);
    body.extend_from_slice(&[0; 6]);
    body.extend_from_slice(&1u16.to_be_bytes());
    body.extend_from_slice(&[0; 16]);
    body.extend_from_slice(
        &u16::try_from(width.round() as u64)
            .map_err(|_| Error::WriterState("video width exceeds u16"))?
            .to_be_bytes(),
    );
    body.extend_from_slice(
        &u16::try_from(height.round() as u64)
            .map_err(|_| Error::WriterState("video height exceeds u16"))?
            .to_be_bytes(),
    );
    body.extend_from_slice(&0x0048_0000u32.to_be_bytes());
    body.extend_from_slice(&0x0048_0000u32.to_be_bytes());
    body.extend_from_slice(&0u32.to_be_bytes());
    body.extend_from_slice(&1u16.to_be_bytes());
    body.extend_from_slice(&[0; 32]);
    body.extend_from_slice(&0x0018u16.to_be_bytes());
    body.extend_from_slice(&u16::MAX.to_be_bytes());
    Ok(body)
}

fn audio_fields(channels: u16, sample_rate: u32) -> Vec<u8> {
    let mut body = Vec::with_capacity(28);
    body.extend_from_slice(&[0; 6]);
    body.extend_from_slice(&1u16.to_be_bytes());
    body.extend_from_slice(&[0; 8]);
    body.extend_from_slice(&channels.to_be_bytes());
    body.extend_from_slice(&16u16.to_be_bytes());
    body.extend_from_slice(&0u16.to_be_bytes());
    body.extend_from_slice(&0u16.to_be_bytes());
    // The sound sample entry stores a 16.16 rate. A rate above 65535 does
    // not fit that field and stays in the media timescale, so the entry
    // stores zero.
    let fixed_rate = if sample_rate > 65_535 {
        0
    } else {
        sample_rate.saturating_mul(65_536)
    };
    body.extend_from_slice(&fixed_rate.to_be_bytes());
    body
}

fn append_sidecars(body: &mut Vec<u8>, sidecars: &[SidecarBox]) -> Result<(), Error> {
    for sidecar in sidecars {
        body.extend_from_slice(&make_box(sidecar.kind, sidecar.payload.clone())?);
    }
    Ok(())
}

fn make_sample_entry(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    match &track.config.codec {
        Codec::H264 { entry, config } => {
            let mut body = visual_fields(track.config.width, track.config.height)?;
            body.extend_from_slice(&make_box(FourCc(*b"avcC"), config.raw.clone())?);
            append_sidecars(&mut body, &config.sidecars)?;
            make_box(*entry, body)
        }
        Codec::H265 { entry, config } => {
            let mut body = visual_fields(track.config.width, track.config.height)?;
            body.extend_from_slice(&make_box(FourCc(*b"hvcC"), config.raw.clone())?);
            append_sidecars(&mut body, &config.sidecars)?;
            make_box(*entry, body)
        }
        Codec::Aac { config } => {
            let mut body = audio_fields(config.channels, config.sample_rate);
            body.extend_from_slice(&make_box(FourCc(*b"esds"), config.raw_esds.clone())?);
            append_sidecars(&mut body, &config.sidecars)?;
            make_box(FourCc(*b"mp4a"), body)
        }
        Codec::Opus { config } => {
            let mut body = audio_fields(u16::from(config.channels), track.config.timescale);
            body.extend_from_slice(&make_box(FourCc(*b"dOps"), config.raw.clone())?);
            append_sidecars(&mut body, &config.sidecars)?;
            make_box(FourCc(*b"Opus"), body)
        }
        Codec::Other { entry, raw } => make_box(*entry, raw.clone()),
    }
}

fn rle_u32<I>(values: I) -> Vec<(u32, u32)>
where
    I: IntoIterator<Item = u32>,
{
    let mut result: Vec<(u32, u32)> = Vec::new();
    for value in values {
        if let Some(last) = result.last_mut() {
            if last.1 == value {
                last.0 = last.0.saturating_add(1);
                continue;
            }
        }
        result.push((1, value));
    }
    result
}

fn make_stts(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let entries = rle_u32(track.samples.iter().map(|sample| sample.duration));
    let mut body = Vec::new();
    body.extend_from_slice(
        &u32::try_from(entries.len())
            .map_err(|_| Error::WriterState("time entry count exceeds u32"))?
            .to_be_bytes(),
    );
    for (count, delta) in entries {
        body.extend_from_slice(&count.to_be_bytes());
        body.extend_from_slice(&delta.to_be_bytes());
    }
    make_box(FourCc(*b"stts"), full_box(0, 0, body))
}

fn make_ctts(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let version = if track.samples.iter().any(|sample| sample.cts_offset < 0) {
        1
    } else {
        0
    };
    let mut entries: Vec<(u32, i32)> = Vec::new();
    for sample in &track.samples {
        if let Some(last) = entries.last_mut() {
            if last.1 == sample.cts_offset {
                last.0 = last.0.saturating_add(1);
                continue;
            }
        }
        entries.push((1, sample.cts_offset));
    }
    let mut body = Vec::new();
    body.extend_from_slice(
        &u32::try_from(entries.len())
            .map_err(|_| Error::WriterState("composition entry count exceeds u32"))?
            .to_be_bytes(),
    );
    for (count, offset) in entries {
        body.extend_from_slice(&count.to_be_bytes());
        if version == 1 {
            body.extend_from_slice(&offset.to_be_bytes());
        } else {
            body.extend_from_slice(
                &u32::try_from(offset)
                    .map_err(|_| Error::WriterState("negative version-zero composition offset"))?
                    .to_be_bytes(),
            );
        }
    }
    make_box(FourCc(*b"ctts"), full_box(version, 0, body))
}

fn make_stss(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let count = track.samples.iter().filter(|sample| sample.is_sync).count();
    let mut body = Vec::new();
    body.extend_from_slice(
        &u32::try_from(count)
            .map_err(|_| Error::WriterState("sync sample count exceeds u32"))?
            .to_be_bytes(),
    );
    for (index, sample) in track.samples.iter().enumerate() {
        if sample.is_sync {
            let number = u32::try_from(index)
                .map_err(|_| Error::WriterState("sample index exceeds u32"))?
                .saturating_add(1);
            body.extend_from_slice(&number.to_be_bytes());
        }
    }
    make_box(FourCc(*b"stss"), full_box(0, 0, body))
}

fn make_stsc(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let mut entries: Vec<(u32, u32)> = Vec::new();
    for (index, chunk) in track.chunks.iter().enumerate() {
        if entries
            .last()
            .is_some_and(|entry| entry.1 == chunk.sample_count)
        {
            continue;
        }
        entries.push((
            u32::try_from(index)
                .map_err(|_| Error::WriterState("chunk index exceeds u32"))?
                .saturating_add(1),
            chunk.sample_count,
        ));
    }
    let mut body = Vec::new();
    body.extend_from_slice(
        &u32::try_from(entries.len())
            .map_err(|_| Error::WriterState("chunk map count exceeds u32"))?
            .to_be_bytes(),
    );
    for (first_chunk, samples_per_chunk) in entries {
        body.extend_from_slice(&first_chunk.to_be_bytes());
        body.extend_from_slice(&samples_per_chunk.to_be_bytes());
        body.extend_from_slice(&1u32.to_be_bytes());
    }
    make_box(FourCc(*b"stsc"), full_box(0, 0, body))
}

fn make_stsz(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let fixed = track
        .samples
        .first()
        .map(|first| first.size)
        .filter(|size| track.samples.iter().all(|sample| sample.size == *size))
        .unwrap_or(0);
    let mut body = Vec::new();
    body.extend_from_slice(&fixed.to_be_bytes());
    body.extend_from_slice(
        &u32::try_from(track.samples.len())
            .map_err(|_| Error::WriterState("sample count exceeds u32"))?
            .to_be_bytes(),
    );
    if fixed == 0 {
        for sample in &track.samples {
            body.extend_from_slice(&sample.size.to_be_bytes());
        }
    }
    make_box(FourCc(*b"stsz"), full_box(0, 0, body))
}

fn make_chunk_offsets(track: &WriterTrack) -> Result<Vec<u8>, Error> {
    let use_co64 = track.force_co64
        || track
            .chunks
            .iter()
            .any(|chunk| chunk.offset > u64::from(u32::MAX));
    let mut body = Vec::new();
    body.extend_from_slice(
        &u32::try_from(track.chunks.len())
            .map_err(|_| Error::WriterState("chunk count exceeds u32"))?
            .to_be_bytes(),
    );
    if use_co64 {
        for chunk in &track.chunks {
            body.extend_from_slice(&chunk.offset.to_be_bytes());
        }
        make_box(FourCc(*b"co64"), full_box(0, 0, body))
    } else {
        for chunk in &track.chunks {
            body.extend_from_slice(
                &u32::try_from(chunk.offset)
                    .unwrap_or(u32::MAX)
                    .to_be_bytes(),
            );
        }
        make_box(FourCc(*b"stco"), full_box(0, 0, body))
    }
}