Length of n-th look-and-say number

look_and_say.py

"""
Script to calculate 'look-and-say' sequence numbers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Started off with good old `for` loops but wanted to have fun with some
functional programming recipes and play some code golf :)
"""

import functools as _ft
import itertools as _it


def _next(val: str) -> str:
    """ Calculate the next 'look-and-say' number. """
    return "".join([f"{len(list(group))}{key}"
                    for key, group in _it.groupby(val)])


def length(seed: str, index: int) -> int:
    """
    Calculate the length of a specific number of the 'look-and-say' sequence.

    Args:
        seed: Value of the initial number in the sequence.
        index: Index of the desired number in the sequence.

    Returns:
        Length of the number at index `index`, given initial value of `seed`.

    """
    return len(_ft.reduce(lambda x, _: _next(x), range(index - 1), seed))


if __name__ == '__main__':
    print(length('1', 30))