#!/bin/bash

# See https://video.stackexchange.com/a/32719

set -o nounset
set -o errexit
set -o pipefail

readonly ARGS=( "${@}" );

main()
{
  local _avi_path;
  _avi_path="${1}"; shift;

  local _mkv_path;
  _mkv_path="${1}"; shift;

  local avi_path;
  avi_path="$(readlink --canonicalize-existing "${_avi_path}")";

  local mkv_path;
  mkv_path="$(readlink --canonicalize-missing "${_mkv_path}")";

  # Check whether the mp3 header is missing in the input file
  local missing_count;
  missing_count=0;
  set +o errexit
  missing_count="$(ffprobe -hide_banner -i "${avi_path}" 2>& 1 \
                  | grep -i 'mp3float' \
                  | grep -c -i 'Header missing')";
  set -o errexit

  if [ 0 -eq "${missing_count}" ];
  then
    # Trivial copy
    ffmpeg -i "${avi_path}" -c:v copy -c:a copy "${mkv_path}"
  else
    # Extract the mp3 stream
    local mp3_stream;
    mp3_stream="$(mktemp --suffix='.mp3')";

    ffmpeg -i "${avi_path}" -c:a copy "${mp3_stream}"

    # Combine it back into the original AVI file
    ffmpeg -i "${avi_path}" -i "${mp3_stream}" \
           -c:v libx264 -c:a copy -map 0:v:0 -map 1:a:0 \
           "${mkv_path}"

    rm "${mp3_stream}"
  fi
}

main "${ARGS[@]}"
exit 0