Sofdec2(USM) subtitle file converter

crisub_conv.py

import struct
import sys

desc = """
Sofdec2(USM) subtitle file converter
crisub_conv.py filename
"""

if len(sys.argv) != 2:
    print(desc)
    quit()

filename = sys.argv[1]
newfilename = '.'.join(filename.split('.')[:-1])

def millisec_to_timestamp(ms):
    h = (ms // (3600*1000))
    tmp = (ms % (3600*1000))
    m = (tmp // (60*1000))
    tmp = (ms % (60*1000))
    s = (tmp // (1000))
    ms = (ms % (1000))
    return f'{h:02d}:{m:02d}:{s:02d},{ms:03d}'

subs = {}

with open(filename, 'rb') as f:
    while rawdata := f.read(20):
        substream_num, chk, pos, duration, txt_len = struct.unpack('<5I', rawdata)
        assert chk == 1000
        txt = struct.unpack(f'<{txt_len}s', f.read(txt_len))[0]
        if subs.get(substream_num) == None:
            subs[substream_num] = []
        subs[substream_num].append({
            'pos': pos,
            'duration': duration,
            'text': txt.rstrip(b'\x00').decode('utf-8')
        })

for idx, sub in subs.items():
    with open(f'{newfilename}.{idx}.srt', 'w') as f:
        for n, i in enumerate(sub):
            f.write(f'{n+1}\n')
            f.write(f'{millisec_to_timestamp(i["pos"])} --> {millisec_to_timestamp(i["pos"] + i["duration"])}\n')
            f.write(i['text'])
            f.write('\n\n')