akkoma/lib/pleroma/helpers/qt_fast_start.ex

# Pleroma: A lightweight social networking server
# Copyright © 2017-2023 Pleroma Authors <https://pleroma.social/>
# SPDX-License-Identifier: AGPL-3.0-only

defmodule Pleroma.Helpers.QtFastStart do
  @moduledoc """
  (WIP) Converts a "slow start" (data before metadatas) mov/mp4 file to a "fast start" one (metadatas before data).
  """

  # TODO: Cleanup and optimizations
  # Inspirations: https://www.ffmpeg.org/doxygen/3.4/qt-faststart_8c_source.html
  #               https://github.com/danielgtaylor/qtfaststart/blob/master/qtfaststart/processor.py
  #               ISO/IEC 14496-12:2015, ISO/IEC 15444-12:2015
  #               Paracetamol

  def fix(<<0x00, 0x00, 0x00, _, 0x66, 0x74, 0x79, 0x70, _::bits>> = binary) do
    index = fix(binary, 0, nil, nil, [])

    case index do
      :abort -> binary
      [{"ftyp", _, _, _, _}, {"mdat", _, _, _, _} | _] -> faststart(index)
      [{"ftyp", _, _, _, _}, {"free", _, _, _, _}, {"mdat", _, _, _, _} | _] -> faststart(index)
      _ -> binary
    end
  end

  def fix(binary) do
    binary
  end

  # MOOV have been seen before MDAT- abort
  defp fix(<<_::bits>>, _, true, false, _) do
    :abort
  end

  defp fix(
         <<size::integer-big-size(32), fourcc::bits-size(32), rest::bits>>,
         pos,
         got_moov,
         got_mdat,
         acc
       ) do
    full_size = (size - 8) * 8
    <<data::bits-size(full_size), rest::bits>> = rest

    acc = [
      {fourcc, pos, pos + size, size,
       <<size::integer-big-size(32), fourcc::bits-size(32), data::bits>>}
      | acc
    ]

    fix(rest, pos + size, got_moov || fourcc == "moov", got_mdat || fourcc == "mdat", acc)
  end

  defp fix(<<>>, _pos, _, _, acc) do
    :lists.reverse(acc)
  end

  defp faststart(index) do
    {{_ftyp, _, _, _, ftyp}, index} = List.keytake(index, "ftyp", 0)

    # Skip re-writing the free fourcc as it's kind of useless.
    # Why stream useless bytes when you can do without?
    {free_size, index} =
      case List.keytake(index, "free", 0) do
        {{_, _, _, size, _}, index} -> {size, index}
        _ -> {0, index}
      end

    {{_moov, _, _, moov_size, moov}, index} = List.keytake(index, "moov", 0)
    offset = -free_size + moov_size
    rest = for {_, _, _, _, data} <- index, do: data, into: []
    <<moov_head::bits-size(64), moov_data::bits>> = moov
    [ftyp, moov_head, fix_moov(moov_data, offset, []), rest]
  end

  defp fix_moov(
         <<size::integer-big-size(32), fourcc::bits-size(32), rest::bits>>,
         offset,
         acc
       ) do
    full_size = (size - 8) * 8
    <<data::bits-size(full_size), rest::bits>> = rest

    data =
      cond do
        fourcc in ["trak", "mdia", "minf", "stbl"] ->
          # Theses contains sto or co64 part
          [<<size::integer-big-size(32), fourcc::bits-size(32)>>, fix_moov(data, offset, [])]

        fourcc in ["stco", "co64"] ->
          # fix the damn thing
          <<version::integer-big-size(32), count::integer-big-size(32), rest::bits>> = data

          entry_size =
            case fourcc do
              "stco" -> 32
              "co64" -> 64
            end

          [
            <<size::integer-big-size(32), fourcc::bits-size(32), version::integer-big-size(32),
              count::integer-big-size(32)>>,
            rewrite_entries(entry_size, offset, rest, [])
          ]

        true ->
          [<<size::integer-big-size(32), fourcc::bits-size(32)>>, data]
      end

    acc = [acc | data]
    fix_moov(rest, offset, acc)
  end

  defp fix_moov(<<>>, _, acc), do: acc

  for size <- [32, 64] do
    defp rewrite_entries(
           unquote(size),
           offset,
           <<pos::integer-big-size(unquote(size)), rest::bits>>,
           acc
         ) do
      rewrite_entries(unquote(size), offset, rest, [
        acc | <<pos + offset::integer-big-size(unquote(size))>>
      ])
    end
  end

  defp rewrite_entries(_, _, <<>>, acc), do: acc
end