#!/bin/bash

# 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 infile outfile"
    exit 1
fi

set -o pipefail -eux
FORCEFPS="24"
SLOWFILTER="-filter asetrate=46080,aresample=osr=48000:resampler=soxr"

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

    local audiodelay="$(get_minimum_timestamp "$infile" "audio")"
    local videodelay="$(get_minimum_timestamp "$infile" "video")"
    local videotrackid="$(get_track_id "$infile" "video")"

    mkvmerge \
        -o "${outfile}" \
        --default-duration "${videotrackid}:${FORCEFPS}fps" \
        --sync "${videotrackid}:$((videodelay / 1000000))" \
        --no-audio "$infile" \
        --sync "0:$((audiodelay / 1000000))" \
        "$audiofile"
}

function get_track_id {
    mkvmerge -i -F json "$1" | jq -r ".tracks[] | select(.type == \"$2\") | .id"
}

function get_minimum_timestamp {
    mkvmerge -F json -i "$1" | jq -r ".tracks[] | select(.type == \"$2\") | .properties.minimum_timestamp"
}

function encode_audio {
    ffmpeg \
        -i "$1" \
        -vn \
        $SLOWFILTER \
        -c:a libfdk_aac -vbr 3 \
        "$2"
}

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

    encode_audio "$infile" "$audiofile"
    mux_replace_audio "$infile" "$audiofile" "$outfile"
}


infile="$1"
outfile="$2"
tmpdir="$(mktemp -d "${TMPDIR:-/var/tmp}/pal-XXXXXXXX")"
convert_file "$infile" "$outfile"
rm -rf "$tmpdir"