Python/sorts/counting_sort.py

"""
This is pure python implementation of counting sort algorithm
For doctests run following command:
python -m doctest -v counting_sort.py
or
python3 -m doctest -v counting_sort.py
For manual testing run:
python counting_sort.py
"""

from __future__ import print_function


def counting_sort(collection):
    """Pure implementation of counting sort algorithm in Python
    :param collection: some mutable ordered collection with heterogeneous
    comparable items inside
    :return: the same collection ordered by ascending
    Examples:
    >>> counting_sort([0, 5, 3, 2, 2])
    [0, 2, 2, 3, 5]
    >>> counting_sort([])
    []
    >>> counting_sort([-2, -5, -45])
    [-45, -5, -2]
    """
    # if the collection is empty, returns empty
    if collection == []:
        return []

    # get some information about the collection
    coll_len = len(collection)
    coll_max = max(collection)
    coll_min = min(collection)

    # create the counting array
    counting_arr_length = coll_max + 1 - coll_min
    counting_arr = [0] * counting_arr_length

    # count how much a number appears in the collection
    for number in collection:
        counting_arr[number - coll_min] += 1

    # sum each position with it's predecessors. now, counting_arr[i] tells
    # us how many elements <= i has in the collection
    for i in range(1, counting_arr_length):
        counting_arr[i] = counting_arr[i] + counting_arr[i-1]

    # create the output collection
    ordered = [0] * coll_len

    # place the elements in the output, respecting the original order (stable
    # sort) from end to begin, updating counting_arr
    for i in reversed(range(0, coll_len)):
        ordered[counting_arr[collection[i] - coll_min]-1] = collection[i]
        counting_arr[collection[i] - coll_min] -= 1

    return ordered


if __name__ == '__main__':
    try:
        raw_input          # Python 2
    except NameError:
        raw_input = input  # Python 3

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