#!/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, and encoded as AAC, preserving surround sound.

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

set -xe
FORCEFPS="24"
SLOWFILTER="-filter asetrate=46080,aresample=48000"

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

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

function extract_audio {
    local infile="$1"
    local outfile="$2"

    mplayer \
        -noconfig all \
        -novideo \
        -channels 8 \
        -dumpaudio \
        -dumpfile "$outfile" \
        "$infile"
}

function encode_audio {
    ffmpeg \
        -i "$1" \
        $SLOWFILTER \
        -strict experimental \
        "$2"
}

function convert_file {
    local infile="$1"
    local outfile="$2"
    local audio1="${tmpdir}/audio1.ac3"
    local audio2="${tmpdir}/audio2.m4a"

    extract_audio "${infile}" "${audio1}"
    encode_audio "${audio1}" "${audio2}"
    mux_replace_audio "${infile}" "${audio2}" "${outfile}"
}


destdir="$1"
shift

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

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

    tmpdir="$(mktemp -d "${TMPDIR:-/var/tmp}/pal-XXXXXXXX")"
    convert_file "$infile" "$outfile"
    rm -rf "$tmpdir"
done