akkoma/lib/pleroma/helpers/qt_fast_start.ex
href dfceb03cf4 Rewrite MP4/MOV binaries to be faststart
In some cases, MP4/MOV files can have the data _before_ the meta-data.

Thus, ffmpeg (and all similar tools) cannot really process the input if
it's given over stdin/streaming/pipes.

BUT I REALLY DON'T WANT TO MAKE TEMPORARY FILES

so here we go, an implementation of qtfaststart in elixir.
2020-08-28 21:14:28 +02:00

131 lines
4.2 KiB
Elixir

# Pleroma: A lightweight social networking server
# Copyright © 2017-2020 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(binary = <<0x00, 0x00, 0x00, _, 0x66, 0x74, 0x79, 0x70, _::binary>>) do
index = fix(binary, binary, 0, [])
case index do
[{"ftyp", _, _, _, _}, {"mdat", _, _, _, _} | _] -> faststart(index)
[{"ftyp", _, _, _, _}, {"free", _, _, _, _}, {"mdat", _, _, _, _} | _] -> faststart(index)
_ -> binary
end
end
def fix(binary) do
binary
end
defp fix(<<>>, _bin, _pos, acc) do
:lists.reverse(acc)
end
defp fix(
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), rest::binary>>,
bin,
pos,
acc
) do
if fourcc == "mdat" && size == 0 do
# mdat with 0 size means "seek to the end" -- also, in that case the file is probably OK.
acc = [
{fourcc, pos, byte_size(bin) - pos, byte_size(bin) - pos,
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), rest::binary>>}
| acc
]
fix(<<>>, bin, byte_size(bin), acc)
else
full_size = size - 8
<<data::binary-size(full_size), rest::binary>> = rest
acc = [
{fourcc, pos, pos + size, size,
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), data::binary>>}
| acc
]
fix(rest, bin, pos + size, acc)
end
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::binary-size(8), moov_data::binary>> = moov
new_moov = fix_moov(moov_data, offset)
<<ftyp::binary, moov_head::binary, new_moov::binary, rest::binary>>
end
defp fix_moov(moov, offset) do
fix_moov(moov, offset, <<>>)
end
defp fix_moov(<<>>, _, acc), do: acc
defp fix_moov(
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), rest::binary>>,
offset,
acc
) do
full_size = size - 8
<<data::binary-size(full_size), rest::binary>> = rest
data =
cond do
fourcc in ["trak", "mdia", "minf", "stbl"] ->
# Theses contains sto or co64 part
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4),
fix_moov(data, offset, <<>>)::binary>>
fourcc in ["stco", "co64"] ->
# fix the damn thing
<<version::integer-big-size(4)-unit(8), count::integer-big-size(4)-unit(8),
rest::binary>> = data
entry_size =
case fourcc do
"stco" -> 4
"co64" -> 8
end
{_, result} =
Enum.reduce(1..count, {rest, <<>>}, fn _,
{<<pos::integer-big-size(entry_size)-unit(8),
rest::binary>>, acc} ->
{rest, <<acc::binary, pos + offset::integer-big-size(entry_size)-unit(8)>>}
end)
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4),
version::integer-big-size(4)-unit(8), count::integer-big-size(4)-unit(8),
result::binary>>
true ->
<<size::integer-big-size(4)-unit(8), fourcc::binary-size(4), data::binary>>
end
acc = <<acc::binary, data::binary>>
fix_moov(rest, offset, acc)
end
end