tbc-cut

#!/usr/bin/python3
# Extract a range of fields from a TBC file.

import argparse
import json

def main():
    parser = argparse.ArgumentParser(description='Extract a range of fields from a TBC file')
    parser.add_argument('infile', metavar='infile',
                        help='input TBC file')
    parser.add_argument('outfile', metavar='outfile',
                        help='output TBC file')
    parser.add_argument('-s', '--start', metavar='N', type=int, default=1,
                        help='seqNo of first field to extract (default 1)')
    parser.add_argument('-l', '--length', metavar='N', type=int,
                        help='number of fields to extract (default all)')
    args = parser.parse_args()

    assert args.infile != args.outfile

    # Read input JSON
    with open(args.infile + '.json') as f:
        data = json.load(f)

    num_fields = len(data['fields'])
    assert num_fields == data['videoParameters']['numberOfSequentialFields']

    # Compute start position
    start_idx = args.start - 1
    assert start_idx >= 0
    assert start_idx <= num_fields
    assert data['fields'][start_idx]['isFirstField']

    # Compute stop position
    if args.length is None:
        stop_idx = num_fields
    else:
        stop_idx = start_idx + args.length
    assert stop_idx >= start_idx
    assert stop_idx <= num_fields

    # Build fields array for the output JSON
    new_fields = []
    new_idx = 1
    for idx in range(start_idx, stop_idx):
        field = data['fields'][idx]
        assert field['seqNo'] == idx + 1

        field['seqNo'] = new_idx
        new_idx += 1
        new_fields.append(field)

    # Update JSON
    data['fields'] = new_fields
    data['videoParameters']['numberOfSequentialFields'] = len(new_fields)

    # Write output JSON
    with open(args.outfile + '.json', 'w') as f:
        json.dump(data, f)

    # Copy fields to the new TBC
    field_size = data['videoParameters']['fieldWidth'] * data['videoParameters']['fieldHeight'] * 2
    with open(args.outfile, 'wb') as fout:
        with open(args.infile, 'rb') as fin:
            fin.seek(field_size * start_idx)
            for i in range(start_idx, stop_idx):
                data = fin.read(field_size)
                assert len(data) == field_size
                fout.write(data)

if __name__ == '__main__':
    main()