# 8 mebibytes expressed in bits. 512 bytes leeway for request.
MAX_SIZE=$(( (1024 * 1024 * 8 - 512) * 8 ))

file_name="${1%.*}"
audio_bitrate="$(ffprobe -v error -select_streams a:0 -show_entries stream=bit_rate -of default=noprint_wrappers=1:nokey=1 "$1" | awk '{sum+=$1} END {print sum}')"

if [ -z "$2" ]
then
    seek=""
    duration="$(ffprobe -v error -show_entries format=duration -of default=noprint_wrappers=1:nokey=1 "$1")"
else
    seek="-sseof -$2"
    duration=$(echo $2 | sed -E 's/(.*):(.+):(.+)/\1*3600+\2*60+\3/;s/(.+):(.+)/\1*60+\2/' | bc -l)
fi

video_bitrate=$(echo "(($MAX_SIZE + $duration - 1) / $duration - $audio_bitrate) * 0.98" | bc) # Ceiling division, 2% overhead
bufsize=$(echo "$video_bitrate / 2" | bc)

ffmpeg \
    -hide_banner -loglevel 32 -y \
    $seek \
    -i "$1" \
    -c:v libx264 \
        -x264-params "nal-hrd=cbr" \
        -b:v $video_bitrate \
        -minrate $video_bitrate \
        -maxrate $video_bitrate \
        -bufsize $bufsize \
        -r 30 \
        -vf scale=-1:720 \
        -pass 1 \
        -passlogfile "${file_name}_2pass" \
    -an \
    -f mp4 /dev/null \
&& ffmpeg \
    -hide_banner -loglevel 32 \
    $seek \
    -i "$1" \
    -c:v libx264 \
        -x264-params "nal-hrd=cbr" \
        -b:v $video_bitrate \
        -minrate $video_bitrate \
        -maxrate $video_bitrate \
        -bufsize $bufsize \
        -r 30 \
        -vf scale=-1:720 \
        -pass 2 \
        -passlogfile "${file_name}_2pass" \
    -c:a copy \
    -map 0:v -map 0:a \
    "${file_name}_8mb.mp4"

find . -name "${file_name}_2pass*.log*" -type f -delete