#!/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.
# - Chapters and subtitles are also adjusted to match the new timing.

if [ -z "$1" ] || [ -z "$2" ]; then
    echo "Usage: $0 infile outfile"
    exit 1
fi

set -o pipefail -eux
OLDFPS="25"
NEWFPS="24"
SLOWFILTER=("-filter" "asetrate=46080,aresample=osr=48000:resampler=soxr")

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

    local tmpdir=""
    tmpdir="$(mktemp -d "${TMPDIR:-/var/tmp}/pal-XXXXXXXX")"
    local audiofile="${tmpdir}/audiofile.m4a"

    encode_audio "$infile" "$audiofile"
    remux_file "$infile" "$audiofile" "$outfile"

    rm -rf "$tmpdir"
}

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

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

    local audiodelay=""
    audiodelay="$(get_minimum_timestamp "$infile" "audio")"

    local videodelay=""
    videodelay="$(get_minimum_timestamp "$infile" "video")"

    local videotrackid=""
    videotrackid="$(get_track_id "$infile" "video")"

    local suboptions=()
    local subtitletrackid=""
    while read -r subtitletrackid; do
        suboptions+=("--sync" "${subtitletrackid}:0,${OLDFPS}/${NEWFPS}")
    done < <(get_track_id "$infile" "subtitles")

    local chapteroptions=()
    if [ "$(get_chapter_count "$infile")" -gt 0 ]; then
        chapteroptions=("--chapter-sync" "0,${OLDFPS}/${NEWFPS}")
    fi

    mkvmerge \
        -o "${outfile}" \
        --default-duration "${videotrackid}:${NEWFPS}fps" \
        --sync "${videotrackid}:$((videodelay / 1000000))" \
        "${chapteroptions[@]}" \
        "${suboptions[@]}" \
        --no-audio "$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 -i -F json "$1" | jq -r ".tracks[] | select(.type == \"$2\") | .properties.minimum_timestamp"
}

function get_chapter_count {
    mkvmerge -i -F json "$1" | jq -r ".chapters | length"
}

main "$@"