symphonia-format-isomp4 0.5.2

// Symphonia
// Copyright (c) 2019-2022 The Project Symphonia Developers.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at https://mozilla.org/MPL/2.0/.

use symphonia_core::errors::{decode_error, Result};
use symphonia_core::io::ReadBytes;

use crate::atoms::{Atom, AtomHeader};

#[derive(Debug)]
pub struct StscEntry {
    pub first_chunk: u32,
    pub first_sample: u32,
    pub samples_per_chunk: u32,
    pub sample_desc_index: u32,
}

/// Sample to Chunk Atom
#[derive(Debug)]
pub struct StscAtom {
    /// Atom header.
    header: AtomHeader,
    /// Entries.
    pub entries: Vec<StscEntry>,
}

impl StscAtom {
    /// Finds the `StscEntry` for the sample indicated by `sample_num`. Note, `sample_num` is indexed
    /// relative to the `StscAtom`. Complexity is O(log2 N).
    pub fn find_entry_for_sample(&self, sample_num: u32) -> Option<&StscEntry> {
        let mut left = 1;
        let mut right = self.entries.len();

        while left < right {
            let mid = left + (right - left) / 2;

            let entry = self.entries.get(mid).unwrap();

            if entry.first_sample < sample_num {
                left = mid + 1;
            }
            else {
                right = mid;
            }
        }

        // The index found above (left) is the exclusive upper bound of all entries where
        // first_sample < sample_num. Therefore, the entry to return has an index of left-1. The
        // index will never equal 0 so this is safe. If the table were empty, left == 1, thus calling
        // get with an index of 0, and safely returning None.
        self.entries.get(left - 1)
    }
}

impl Atom for StscAtom {
    fn header(&self) -> AtomHeader {
        self.header
    }

    fn read<B: ReadBytes>(reader: &mut B, header: AtomHeader) -> Result<Self> {
        let (_, _) = AtomHeader::read_extra(reader)?;

        let entry_count = reader.read_be_u32()?;

        // TODO: Apply a limit.
        let mut entries = Vec::with_capacity(entry_count as usize);

        for _ in 0..entry_count {
            entries.push(StscEntry {
                first_chunk: reader.read_be_u32()? - 1,
                first_sample: 0,
                samples_per_chunk: reader.read_be_u32()?,
                sample_desc_index: reader.read_be_u32()?,
            });
        }

        // Post-process entries to check for errors and calculate the file sample.
        if entry_count > 0 {
            for i in 0..entry_count as usize - 1 {
                // Validate that first_chunk is monotonic across all entries.
                if entries[i + 1].first_chunk < entries[i].first_chunk {
                    return decode_error("isomp4: stsc entry first chunk not monotonic");
                }

                // Validate that samples per chunk is > 0. Could the entry be ignored?
                if entries[i].samples_per_chunk == 0 {
                    return decode_error("isomp4: stsc entry has 0 samples per chunk");
                }

                let n = entries[i + 1].first_chunk - entries[i].first_chunk;

                entries[i + 1].first_sample =
                    entries[i].first_sample + (n * entries[i].samples_per_chunk);
            }

            // Validate that samples per chunk is > 0. Could the entry be ignored?
            if entries[entry_count as usize - 1].samples_per_chunk == 0 {
                return decode_error("isomp4: stsc entry has 0 samples per chunk");
            }
        }

        Ok(StscAtom { header, entries })
    }
}