#!/bin/bash
#
# speedcode - reencode audio to mp3, scaling the tempo
# usage: speedcode [-f factor] [-b bitrate] outfile file1 [file2 ...]

factor=1.75
bitrate='%%AUTO%%'
mono='-af pan=1:0.5:0.5,format=s16le'
while getopts b:f:s opt;do
 case $opt in
  b)    bitrate=$OPTARG;;
  f)    factor=$OPTARG;;
  s)	mono=;;
 esac
done
shift $(expr $OPTIND - 1)

outfile="$1"
shift

if [[ -e "$outfile" ]];then 
  echo "Output file $outfile exists.  Will not overwrite." >&2
  exit 1
fi

nearest_standard_mp3_rate () {
  # This doesn't take the sample rate into account =/
  # standard_rates='8 16 24 32 40 48 56 64 80 96 112 128 144 160 192 224 256 320'
  standard_rates='8 16 24 32 40 48 56 64 80 96 112 128' # I don't like high rates
  tr ' ' \\n <<< "$standard_rates" |
    awk -v "rate=$1" '{ print $1 - rate, $1 }' |
    tr -d -- - |
    sort -n |
    head -n1 |
    cut -d' ' -f2
}

if [[ "$bitrate" = '%%AUTO%%' ]];then
  # Use the maximum rate of the input files, multiplied by the speed factor,
  # rounded to the nearest standard mp3 bitrate
  bitrate="$(nearest_standard_mp3_rate $(
    dc <<< "$(
        # Use the first input file to determine the bitrate
        empirical-bitrate-calculator "$1" | sort -nr | cut -f1)
      1024 8k/ $factor *p"))"
  echo "Autoselected bitrate: $bitrate"
fi

  soundstretch <(
    exec mplayer -quiet -really-quiet $mono -ao pcm:waveheader:fast:file=/dev/stdout "$@"
  ) >(
    exec lame --cbr -b "$bitrate" - "$outfile"
  ) -tempo=$(echo "9k $factor 1 - 100 * p" | dc) 

  # Copy the id3 tag of the first input file
  if [[ "$1" = *.mp3 ]];then
    id3cp "$1" "$outfile"
  fi