iterating_through_submasks.py

"""
Author : Syed Faizan (3rd Year Student IIIT Pune)
github : faizan2700
You are given a bitmask m and you want to efficiently iterate through all of
its submasks. The mask s is submask of m if only bits that were included in
bitmask are set
"""

from __future__ import annotations


def list_of_submasks(mask: int) -> list[int]:
    """
    Args:
        mask : number which shows mask ( always integer > 0, zero does not have any
            submasks )

    Returns:
        all_submasks : the list of submasks of mask (mask s is called submask of mask
        m if only bits that were included in original mask are set

    Raises:
        AssertionError: mask not positive integer

    >>> list_of_submasks(15)
    [15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1]
    >>> list_of_submasks(13)
    [13, 12, 9, 8, 5, 4, 1]
    >>> list_of_submasks(-7)  # doctest: +ELLIPSIS
    Traceback (most recent call last):
        ...
    AssertionError: mask needs to be positive integer, your input -7
    >>> list_of_submasks(0)  # doctest: +ELLIPSIS
    Traceback (most recent call last):
        ...
    AssertionError: mask needs to be positive integer, your input 0

    """

    assert (
        isinstance(mask, int) and mask > 0
    ), f"mask needs to be positive integer, your input {mask}"

    """
    first submask iterated will be mask itself then operation will be performed
    to get other submasks till we reach empty submask that is zero ( zero is not
    included in final submasks list )
    """
    all_submasks = []
    submask = mask

    while submask:
        all_submasks.append(submask)
        submask = (submask - 1) & mask

    return all_submasks


if __name__ == "__main__":
    import doctest

    doctest.testmod()