{ ... }: {
	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
		}
	'';
}