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qt_fast_start.ex
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# Pleroma: A lightweight social networking server
# Copyright © 2017-2021 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

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