Add counting sort

Author: andrealmeid

sorts/counting_sort.py

@@ -0,0 +1,72 @@
+"""
+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__':
+    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(counting_sort(unsorted))