{ ... }:
{
  text = ''
    # Mux audio into containers. File names in sound and current dirrectories must match.
    # Usage: ffmpeg_mux_audio <SOUND> <OUTPUT DIR>
    function ffmpeg_mux_audio() {
      if [[ "''${1}" = "" ]]; then
        help ffmpeg_mux_audio
        return 2
      fi

      for file in *; do ffmpeg -i "$file" -i "$1"/"$file" -c copy -map 0:v:0 -map 1:a:0 -shortest "$2"/"$file"; done
    }

    # Mux cover into music file.
    # Usage: ffmpeg_mux_cover <FORMAT> <COVER>
    function ffmpeg_mux_cover() {
      if [[ "''${1}" = "" ]]; then
        help ffmpeg_mux_cover
        return 2
      fi

      local format="''${1}"
      local cover="''${2}"

      mkdir out

      case "''${format}" in
        # "mka"|"mkv")
        #   for file in *.''${format}; do
        #     ffmpeg -i "''${file}" -attach "''${cover}" -map 0 -c copy -metadata:s:t mimetype="image/''${cover##*.}" -metadata:s:t:0 filename="cover.''${cover##*.}" "./out/''${file}" || return 1
        #   done
        #   ;;
        *)
          for file in *.''${format}; do
            # ffmpeg -i "''${file}" -i "''${cover}" -map 0 -map 0:-v? -map 1 -codec copy -metadata:s:v title="Album cover" -metadata:s:v comment="Cover (front)" -disposition:v attached_pic ./out/"''${file}" || return 1
            ffmpeg -i "''${file}" -i "''${cover}" -map 0 -map 1 -codec copy -metadata:s:v title="Album cover" -metadata:s:v comment="Cover (front)" -disposition:v attached_pic ./out/"''${file}" || return 1
          done
        ;;
      esac

      mv out/* .
      rm -d out/ && rm "''${2}"
    }

    # Generate music metadata from directory structure.
    # Top dir is the Artist name like this: `The_Beatles`.
    # Next are albums like this: `2010_My_love`.
    # Inside are songs like this: `01_sample.flac`.
    # Usage: ffmpeg_music_meta <FORMAT>
    function ffmpeg_music_meta() {
      if [[ "''${1}" = "" ]]; then
        help ffmpeg_music_meta
        return 2
      fi

      local format="''${1}"

      ls *.''${format} &> /dev/null || return 1

      local artist="''${PWD%/*}"; artist="''${artist##*/}"; artist="''${artist//_/ }"
      local album="''${PWD##*/}"; album="''${album#*_}"; album="''${album//_/ }"
      local year="''${PWD##*/}"; year="''${year%%_*}"
      # local total=$(ls *.''${format} | wc -l)

      mkdir out

      for file in *.''${format}; do
        local track="''${file%%_*}"; track=$((10#''${track})); [[ "''${track}" = "" ]] && track=0
        local title="''${file#*_}"; title="''${title%.*}"; title="''${title//_/ }"

        # echo "''${artist}; ''${album}; ''${year}; ''${track}; ''${title}"
        # TODO: make it format-specific.
        ffmpeg -i "''${file}" -map 0 -c copy -metadata "artists=" -metadata "artist=''${artist}" -metadata "album_artist=''${artist}" -metadata "album=''${album}" -metadata "date=''${year}" -metadata "year=''${year}" -metadata "date_released=''${year}" -metadata "track=''${track}" -metadata "part_number=''${track}" -metadata "title=''${title}" ./out/"''${file}" || return 1
      done

      mv out/* .
      rm -d out/
    }

    # Rotate the video clock-wise.
    # Usage: ffmpeg_rotate <ANGLE> <TARGET>
    function ffmpeg_rotate() {
      if [[ "''${2}" = "" ]]; then
        help ffmpeg_rotate
      fi

      local angle="''${1}"
      local target="''${2}"

      ffmpeg -display_rotation ''${angle} -i ''${target} -c copy _''${target} && mv _''${target} ''${target} || rm _''${target}
    }

    # Get video FPS.
    function _ffprobe_fps() {
      local fps=$(ffprobe -v 0 -of csv=p=0 -select_streams v:0 -show_entries stream=r_frame_rate "''${1}")
      [[ "''${fps}" = "" ]] && fps=30 || fps=$((fps))
      echo "''${fps}"
    }

    # Get recommended keyframe interval for a file.
    function _ffprobe_keyint() {
      local fps=$(_ffprobe_fps "''${1}")
      echo $((fps*5))
    }

    # Get audio bitrage. 128 by default.
    function _ffprobe_ba() {
      local ba=$(ffprobe -v error -select_streams a:0 -show_entries stream=bit_rate -of default=noprint_wrappers=1:nokey=1 "''${1}")
      [[ "''${ba}" != "N/A" ]] && echo $((ba/1024)) || echo 128
    }
  '';
}