adding a combinatorics folder with basic algorithms

combinatorics/combinations.py

@@ -0,0 +1,172 @@
+from math import comb
+from math import factorial
+
+def validate_elements_count(total_elements_count: int, selected_elements_count: int) -> None:
+    """ Validate that the number of elements are positive and the total is greater than or equal to selected. """
+    if total_elements_count < selected_elements_count or selected_elements_count < 0:
+        raise ValueError(
+            "Please enter positive integers for total_elements_count and selected_elements_count "
+            "where total_elements_count >= selected_elements_count"
+        )
+
+
+def combinations_iterative(total_elements_count: int, selected_elements_count: int) -> int:
+    """
+    Returns the number of combinations that can be made from a total set of elements.
+
+    Examples:
+    >>> combinations_iterative(10, 5)
+    252
+    >>> combinations_iterative(6, 3)
+    20
+    >>> combinations_iterative(20, 5)
+    15504
+    >>> combinations_iterative(52, 5)
+    2598960
+    >>> combinations_iterative(0, 0)
+    1
+    >>> combinations_iterative(-4, -5)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count
+    """
+    validate_elements_count(total_elements_count, selected_elements_count)
+    combinations_count = 1
+    for i in range(selected_elements_count):
+        combinations_count *= (total_elements_count - i)
+        combinations_count //= (i + 1)
+    return combinations_count
+
+
+def multiset_combinations(total_elements_count: int, selected_elements_count: int) -> int:
+    """
+    Returns the number of combinations from a multiset of elements.
+
+    Examples:
+    >>> multiset_combinations(10, 5)
+    2002
+    >>> multiset_combinations(6, 3)
+    56
+    >>> multiset_combinations(20, 5)
+    42504
+    >>> multiset_combinations(52, 5)
+    3819816
+    >>> multiset_combinations(0, 0)
+    1
+    >>> multiset_combinations(-4, -5)
+    Traceback (most recent call last):
+    ...
+    ValueError: n must be a non-negative integer
+    """
+    validate_elements_count(total_elements_count, selected_elements_count)
+    return comb(total_elements_count + selected_elements_count - 1, selected_elements_count)
+
+
+def combinations_formula(total_elements_count: int, selected_elements_count: int) -> int:
+    """
+    Calculate combinations using the formula for n choose k.
+
+    Examples:
+    >>> combinations_formula(10, 5)
+    252
+    >>> combinations_formula(6, 3)
+    20
+    >>> combinations_formula(20, 5)
+    15504
+    >>> combinations_formula(52, 5)
+    2598960
+    >>> combinations_formula(0, 0)
+    1
+    >>> combinations_formula(-4, -5)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count
+    """
+    validate_elements_count(total_elements_count, selected_elements_count)
+    remaining_elements_count = total_elements_count - selected_elements_count
+    return int(
+        factorial(total_elements_count) /
+        (factorial(selected_elements_count) * factorial(remaining_elements_count))
+    )
+
+
+def combinations_with_repetitions(total_elements_count: int, selected_elements_count: int) -> int:
+    """
+    Calculate combinations with repetitions allowed.
+
+    Examples:
+    >>> combinations_with_repetitions(10, 5)
+    2002
+    >>> combinations_with_repetitions(6, 3)
+    56
+    >>> combinations_with_repetitions(20, 5)
+    42504
+    >>> combinations_with_repetitions(52, 5)
+    3819816
+    >>> combinations_with_repetitions(0, 0)
+    1
+    >>> combinations_with_repetitions(-4, -5)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count
+    """
+    validate_elements_count(total_elements_count, selected_elements_count)
+    if total_elements_count + selected_elements_count == 0:
+        return 1
+    return int(
+        factorial(total_elements_count + selected_elements_count - 1) /
+        (factorial(selected_elements_count) * factorial(total_elements_count - 1))
+    )
+
+
+def permutations(total_elements_count: int, selected_elements_count: int) -> int:
+    """
+    Calculate the number of permutations of selecting k elements from n elements.
+
+    Examples:
+    >>> permutations(10, 5)
+    30240
+    >>> permutations(6, 3)
+    120
+    >>> permutations(20, 5)
+    1860480
+    >>> permutations(52, 5)
+    311875200
+    >>> permutations(0, 0)
+    1
+    >>> permutations(-4, -5)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count
+    """
+    validate_elements_count(total_elements_count, selected_elements_count)
+    remaining_elements_count = total_elements_count - selected_elements_count
+    return int(factorial(total_elements_count) / factorial(remaining_elements_count))
+
+
+def possible_selections(total_elements_count: int, selected_elements_count: int) -> int:
+    """
+    Calculate the number of possible selections of k items from n available items, with replacement.
+
+    Examples:
+    >>> possible_selections(10, 5)
+    100000
+    >>> possible_selections(6, 3)
+    216
+    >>> possible_selections(20, 5)
+    3200000
+    >>> possible_selections(52, 5)
+    380204032
+    >>> possible_selections(0, 0)
+    1
+    >>> possible_selections(-4, -5)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count
+    """
+    validate_elements_count(total_elements_count, selected_elements_count)
+    return int(total_elements_count ** selected_elements_count)
+
+
+if __name__ == "__main__":
+    __import__("doctest").testmod()

combinatorics/derangements.py

@@ -0,0 +1,34 @@
+from math import factorial
+
+"""
+https://en.wikipedia.org/wiki/Derangement
+"""
+
+
+def derangement(elements_count: int) -> int:
+    """
+    Returns the number of different combinations of selected_elements_count length which can
+    be made from total_elements_count values, where total_elements_count >= selected_elements_count.
+
+    Examples:
+    >>> derangement(6)
+    265
+    >>> derangement(7)
+    1854
+    >>> derangement(8)
+    14833
+
+    >>> derangement(-5)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for elements_count
+    """
+    if elements_count < 0:
+        raise ValueError("Please enter positive integers for elements_count")
+
+    derangement = sum([((-1) ** i) / factorial(i) for i in range(elements_count + 1)])
+    return int(factorial(elements_count) * derangement)
+
+
+if __name__ == "__main__":
+    __import__("doctest").testmod()

combinatorics/stirling_second_kind.py

@@ -0,0 +1,53 @@
+"""
+https://en.wikipedia.org/wiki/Stirling_numbers_of_the_second_kind
+"""
+
+
+def validate_elements_count(
+    total_elements_count: int, selected_elements_count: int
+) -> None:
+    # If either of the conditions are true, the function is being asked
+    # to calculate a factorial of a negative number, which is not possible
+    if total_elements_count < selected_elements_count or selected_elements_count < 0:
+        raise ValueError(
+            "Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count"
+        )
+
+
+def stirling_second(total_elements_count: int, selected_elements_count: int) -> None:
+    """
+    Returns the number of different combinations of selected_elements_count length which can
+    be made from total_elements_count values, where total_elements_count >= selected_elements_count.
+
+    Examples:
+    >>> stirling_second(6, 3)
+    122
+    >>> stirling_second(10, 5)
+    50682
+    >>> stirling_second(8, 3)
+    1389
+    >>> stirling_second(-5, 0)
+    Traceback (most recent call last):
+    ...
+    ValueError: Please enter positive integers for total_elements_count and selected_elements_count where total_elements_count >= selected_elements_count
+    """
+
+    validate_elements_count(total_elements_count, selected_elements_count)
+    permutations_count = [
+        [0] * (selected_elements_count + 1) for _ in range(total_elements_count + 1)
+    ]
+
+    for i in range(total_elements_count + 1):
+        permutations_count[i][0] = 1
+
+    for i in range(1, total_elements_count + 1):
+        for j in range(1, selected_elements_count + 1):
+            permutations_count[i][j] = (
+                j * permutations_count[i - 1][j] + permutations_count[i - 1][j - 1]
+            )
+
+    return permutations_count[total_elements_count][selected_elements_count]
+
+
+if __name__ == "__main__":
+    __import__("doctest").testmod()