#!/bin/bash

# usage : 
# ./reunite.sh video-name.part1.mp4

filename=$(basename -- "$1")
extension="${filename##*.}"
filename=$(echo "${filename%.*}" | sed -r 's/(.+)\.part[0-4]{1}$/\1/g')

# Extract audio track from one of the parts

ffmpeg -i $1 -acodec copy $filename.aac

# Let's use more explicit names for manipulation

mv $filename.part1.$extension even_odd.mp4
mv $filename.part2.$extension even_even.mp4
mv $filename.part3.$extension odd_odd.mp4
mv $filename.part4.$extension odd_even.mp4

# Rotate the 4 sub-videos clockwise in order to merge them

ffmpeg -i even_odd.mp4 -vf "transpose=1" even_90cw_odd.mp4
ffmpeg -i even_even.mp4 -vf "transpose=1" even_90cw_even.mp4

ffmpeg -i odd_odd.mp4 -vf "transpose=1" odd_90cw_odd.mp4 
ffmpeg -i odd_even.mp4 -vf "transpose=1" odd_90cw_even.mp4


# Merge the 4 sub-videos into 2

ffmpeg -i even_90cw_odd.mp4 -i even_90cw_even.mp4 -filter_complex "[0][1]vstack,il=l=i:c=i" even_90cw.mp4
ffmpeg -i odd_90cw_odd.mp4 -i odd_90cw_even.mp4 -filter_complex "[0][1]vstack,il=l=i:c=i" odd_90cw.mp4

# Rotate the 2 videos back counter-clockwise

ffmpeg -i odd_90cw.mp4 -vf "transpose=2" odd.mp4
ffmpeg -i even_90cw.mp4 -vf "transpose=2" even.mp4

# Merge the 2 videos

ffmpeg -i odd.mp4 -i even.mp4 -filter_complex "[0][1]vstack,il=l=i:c=i" $filename.merged_no_sound.mp4

# Merge video with audio track

ffmpeg -i $filename.aac -i $filename.merged_no_sound.mp4 $filename.merged.mp4

# Remove manipulation files

rm even_odd.mp4 even_even.mp4 odd_odd.mp4 odd_even.mp4 even_90cw_odd.mp4 even_90cw_even.mp4 odd_90cw_odd.mp4 odd_90cw_even.mp4 even_90cw.mp4 odd_90cw.mp4 odd.mp4 even.mp4 $filename.merged_no_sound.mp4 $filename.aac