add --length option to cut_bs.py

SYNC_WORDS = {b"!k", b" k"}

def cut_bs(fp, fp_out, start_frame=0, start_with_sid=False):

def cut_to_length(fp, fp_out, length):

    assert length > 0

    fr_cnt = 0

    for f in range(length):

        sync_word = fp.read(2)

        if sync_word == b"":

            return (fr_cnt, cut_cnt)

        if sync_word not in SYNC_WORDS:

            raise ValueError("Bad Sync word!")

        n_bits_bs = fp.read(2)

        n_bits = struct.unpack("h", n_bits_bs)[0]

        fp_out.write(sync_word)

        fp_out.write(n_bits_bs)

        fp_out.write(fp.read(n_bits * 2))

        fr_cnt += 1

    return fr_cnt

def cut_from_start(fp, fp_out, start_frame=0, start_with_sid=False):

    # cut until start frame

    fr_cnt = 0

    cut_cnt = 0

if __name__ == "__main__":

    my_parser = argparse.ArgumentParser(

        description="Cut frames from the beginning of a G.192 bit stream file"

        description="Cut a G.192 bit stream file"

    )

    my_parser.add_argument(

        "--sid", "-s", help="Cut until the first SID frame", action="store_true"

        "--sid", "-s", help="Cut away all frames before the first SID frame", action="store_true"

    )

    my_parser.add_argument(

        "--frame", "-f", type=int, help="Number of frames to cut.", default=0

        "--frame", "-f", type=int, help="Number of frames to cut from the start of the file.", default=0

    )

    my_parser.add_argument("--length", "-l", type=int, help="Cut bitstream to this length (in frames)", default=-1)

    my_parser.add_argument("bs_in", type=str, help="G.192 bit stream file name to cut")

    my_parser.add_argument("bs_out", type=str, help="Cut G.192 bit stream file name")

    my_args = my_parser.parse_args()

    with open(my_args.bs_in, "rb") as fp_in:

        with open(my_args.bs_out, "wb") as fp_out:

            fr_cnt, cut_cnt = cut_bs(

            if my_args.sid or my_args.frame:

                fr_cnt, cut_cnt = cut_from_start(

                    fp_in, fp_out, start_frame=my_args.frame, start_with_sid=my_args.sid

                )

                if my_args.sid and (fr_cnt == cut_cnt):

                    print("Warning! No SID frame found in bitstream!")

                print(f"Cut {cut_cnt} of {fr_cnt} frames from {my_args.bs_in}")

            elif my_args.length:

                fr_cnt = cut_to_length(fp_in, fp_out, my_args.length)

                if fr_cnt != my_args.length:

                    print(f"Warning! Could not cut to length {my_args.length} as bitstream only contained {fr_cnt} frames!")

                print(f"Cut {my_args.bs_in} to {fr_cnt} frames")