shell_sort.py

"""
This is a pure Python implementation of the shell sort algorithm

For doctests run following command:
python -m doctest -v shell_sort.py
or
python3 -m doctest -v shell_sort.py

For manual testing run:
python shell_sort.py
"""


def shell_sort(collection):
    """Pure implementation of shell sort algorithm in Python
    :param collection:  Some mutable ordered collection with heterogeneous
    comparable items inside
    :return:  the same collection ordered by ascending

    >>> shell_sort([0, 5, 3, 2, 2])
    [0, 2, 2, 3, 5]
    >>> shell_sort([])
    []
    >>> shell_sort([-2, -5, -45])
    [-45, -5, -2]
    """
    # Marcin Ciura's gap sequence
    gaps = [701, 301, 132, 57, 23, 10, 4, 1]
    for gap in gaps:
        for i in range(gap, len(collection)):
            insert_value = collection[i]
            j = i - gap
            while j >= 0 and insert_value < collection[j]:
                collection[j + gap] = collection[j]
                j -= gap
            collection[j + gap] = insert_value
    return collection


if __name__ == "__main__":
    from doctest import testmod

    testmod()
    user_input = input("Enter numbers separated by a comma:\n").strip()
    unsorted = [int(item) for item in user_input.split(",")]
    print(shell_sort(unsorted))