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
"""
from __future__ import print_function


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:
        i = gap
        while i < len(collection):
            temp = collection[i]
            j = i
            while j >= gap and collection[j - gap] > temp:
                collection[j] = collection[j - gap]
                j -= gap
            collection[j] = temp
            i += 1

    return collection

if __name__ == '__main__':
    import sys
    # For python 2.x and 3.x compatibility: 3.x has not raw_input builtin
    # otherwise 2.x's input builtin function is too "smart"
    if sys.version_info.major < 3:
        input_function = raw_input
    else:
        input_function = input

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