franustring.py

def franustring(n: int) -> str:
    """Generate a franustring of length n.

    A franustring is a self-measuring string that allows you to
    eyeball the length of a string by looking at the last part of
    the string and doing a little math. It's sort of like a self
    describing "ruler" for strings.

    e.g.
        n=1  len=1  '1'
        n=2  len=2  '12'
        n=4  len=4  '1234'
        n=8  len=8  '12345678'
        n=16 len=16 '12345678(10)3456'
    
    Positions are 1-indexed. At position p the character is p % 10
    (rendered as a digit), except where a milestone marker "(N)" is
    inserted. A marker for milestone N (every positive multiple of 10)
    sits with "(" at position N-1, the digits of N starting at
    position N, and ")" immediately after.

    A marker that starts before n but would extend past it is
    truncated, so the returned string is always exactly n chars.
    """
    chars: list[str] = []
    p = 1
    while p <= n:
        if (p + 1) % 10 == 0:
            for c in f"({p + 1})":
                if p > n:
                    break
                chars.append(c)
                p += 1
        else:
            chars.append(str(p % 10))
            p += 1
    return "".join(chars)


if __name__ == "__main__":
    import sys

    if len(sys.argv) > 1:
        print(franustring(int(sys.argv[1])))
    else:
        print(franustring.__doc__)
        for k in range(9):
            n = 2 ** k
            s = franustring(n)
            print(f"n={n:4} len={len(s):4}  {s!r}")