#!/usr/bin/python3
"""Convert F83 block files into Unix text files, with shadow screens.

This seems to work okay on *.blk except expand86.blk, which doesn’t
actually have shadow screens.

"""

import sys, os


def read64at(file, offset):
    file.seek(offset)
    return file.read(64)


def blk2unix(file):
    size = os.fstat(file.fileno()).st_size

    halfsize = (size - size % 1024) // 2

    for n in range((size + 127)//128):
        shadowline = read64at(file, halfsize + n*64).rstrip()
        # The \ divider avoids screwing up syntax highlighting.
        yield (read64at(file, n*64)
               + (b' \ ' + shadowline if shadowline else b'')
               ).decode('latin-1').rstrip() + '\n'
        if 15 == n % 16:
            # emit form feed between triads of three screens
            # otherwise just a blank line
            yield '\f\n' if 2 == n // 16 % 3 else '\n'

if __name__ == '__main__':
    sys.stdout.writelines(blk2unix(open(sys.argv[1], 'rb')))