#!/bin/bash -e

# Many DVDs released in Australia are sped up from 24fps to 25fps.
# This script reverses the procedure, correcting the audio pitch.
# The video framerate is adjusted without re-encoding. The audio is
# slowed, volume normalised, down-mixed to stereo and encoded as mp3.

if [ -z "$1" -o -z "$2" ]; then
    echo "Usage: $0 destdir infile [infile ...]"
    exit 1
fi

set -xe
FORCEFPS="24"
SLOWDOWN="0.96"

function mux_replace_audio {
    infile="$1"
    audiofile="$2"
    outfile="$3"

    trackid="$(mkvmerge -i "$infile" | grep video | sed 's/^Track ID \(.\):.*$/\1/')"
    exec mkvmerge -o "${outfile}" --default-duration "${trackid}:${FORCEFPS}fps" --no-audio "$infile" "$audiofile"
}

function extract_audio {
    infile="$1"

    exec mpv \
        --no-terminal \
        --no-video \
        --ao pcm:waveheader:file=/dev/stdout \
        "$infile"
}

function slow_audio {
    exec sox \
        --temp "$tmpdir" \
        - \
        -t wav - \
        speed "${SLOWDOWN}" \
        gain -n \
        channels 2
}

function encode_audio {
    outfile="$1"
    exec lame \
        --preset standard \
        - \
        "${outfile}"
}

function convert_file {
    infile="$1"
    outfile="$2"
    tmpdir="$(mktemp -d "${TMPDIR:-/var/tmp}/pal-XXXXXXXX")"
    audiofile="${tmpdir}/audio.mp3"

    extract_audio "${infile}" | slow_audio | encode_audio "${audiofile}"
    mux_replace_audio "${infile}" "${audiofile}" "${outfile}"

    rm -rf "$tmpdir"
}


destdir="$1"
shift

for infile in "$@"; do
    outfile="$destdir/$(basename "$infile")"

    if [ -f "$outfile" ]; then
        echo "Not overwriting $outfile"
        continue
    fi

    convert_file "$infile" "$outfile"
done