#!/bin/bash
Usage="shift timestamps and adjust time between (change speed), print result to stdout

Usage: $(basename "$0") filename timefactor timedivisor timeoffset

filename:
    plaintext file containing timestamps in the format \"(H)H:mm:ss.SS\"
    e.g. ASS/SSA, SRT or chapter files
timefactor / timedivisor:
    factor to multiply to each timestamp (as an integer fraction)
timeoffset:
    absolute amount of time to shift all timestamps (in 10s of ms)

Examples:
    $(basename "$0") input.ssa 24024 24000 0
        stretch time between timestamps by 24024/24000
        i.e. adjust the speed of subtitles from 24 to 23.976 fps

    $(basename "$0") input.ssa 1 1 57 > output.ssa
        add a flat delay of 570 ms to all timestamps, don't change speed
        and save the result in output.ssa

    $(basename "$0") input.ssa 24000 24024 -100
        adjust the speed of subtitles from 23.976 to 24 fps
        and shift all timestamps 1 second towards the start
"

if ! [[ $# -eq 4 && "$2$3$4" =~ ^[-0-9]+$ ]]
    then
    echo "$Usage" >&2
    exit 1
    fi

Sourcefile=$1
Timefactor=$2
Timedivisor=$3
Timeoffset=$4

if [ -f "$Sourcefile" ]
    then
    while   read -r Oldline
            do
            Newline=$Oldline
            while   read Oldstamp
                    do
                    IFS=’:.,’ read -ra Oldarray <<< "${Oldstamp}"
                    let "Oldtimesum = 10#${Oldarray[0]}*360000 + 10#${Oldarray[1]}*6000 + 10#${Oldarray[2]}*100 + 10#${Oldarray[3]}"
                    let "Newtimesum = $Oldtimesum*$Timefactor/$Timedivisor +$Timeoffset"
                    let "Newarray[0] = $Newtimesum/360000"
                    let "Newarray[1] = ($Newtimesum%360000)/6000"
                    let "Newarray[2] = (($Newtimesum%360000)%6000)/100"
                    let "Newarray[3] = (($Newtimesum%360000)%6000)%100"
                    HH=$( printf "%0${#Oldarray[0]}d" "${Newarray[0]}" )
                    MM=$( printf "%02d" "${Newarray[1]}" )
                    SS=$( printf "%02d" "${Newarray[2]}" )
                    CS=$( printf "%02d" "${Newarray[3]}" )
                    Newline=${Newline/$Oldstamp/$HH:$MM:$SS.$CS}
                    done <<< $(echo $Oldline | grep -o -E "[0-9]+:[0-9]{2}:[0-9]{2}[.,][0-9]{2}" | sort -r)
            echo "$Newline"
            done < "$Sourcefile"
    else    echo "File not found: $Sourcefile"
    fi