diff --git a/maths/josephus_problem.py b/maths/josephus_problem.py new file mode 100644 index 000000000..271292ba1 --- /dev/null +++ b/maths/josephus_problem.py @@ -0,0 +1,130 @@ +""" +The Josephus problem is a famous theoretical problem related to a certain +counting-out game. This module provides functions to solve the Josephus problem +for num_people and a step_size. + +The Josephus problem is defined as follows: +- num_people are standing in a circle. +- Starting with a specified person, you count around the circle, + skipping a fixed number of people (step_size). +- The person at which you stop counting is eliminated from the circle. +- The counting continues until only one person remains. + +For more information about the Josephus problem, refer to: +https://en.wikipedia.org/wiki/Josephus_problem +""" + + +def josephus_recursive(num_people: int, step_size: int) -> int: + """ + Solve the Josephus problem for num_people and a step_size recursively. + + Args: + num_people: A positive integer representing the number of people. + step_size: A positive integer representing the step size for elimination. + + Returns: + The position of the last person remaining. + + Raises: + ValueError: If num_people or step_size is not a positive integer. + + Examples: + >>> josephus_recursive(7, 3) + 3 + >>> josephus_recursive(10, 2) + 4 + >>> josephus_recursive(0, 2) + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + >>> josephus_recursive(1.9, 2) + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + >>> josephus_recursive(-2, 2) + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + >>> josephus_recursive(7, 0) + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + >>> josephus_recursive(7, -2) + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + >>> josephus_recursive(1_000, 0.01) + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + >>> josephus_recursive("cat", "dog") + Traceback (most recent call last): + ... + ValueError: num_people or step_size is not a positive integer. + """ + if ( + not isinstance(num_people, int) + or not isinstance(step_size, int) + or num_people <= 0 + or step_size <= 0 + ): + raise ValueError("num_people or step_size is not a positive integer.") + + if num_people == 1: + return 0 + + return (josephus_recursive(num_people - 1, step_size) + step_size) % num_people + + +def find_winner(num_people: int, step_size: int) -> int: + """ + Find the winner of the Josephus problem for num_people and a step_size. + + Args: + num_people (int): Number of people. + step_size (int): Step size for elimination. + + Returns: + int: The position of the last person remaining (1-based index). + + Examples: + >>> find_winner(7, 3) + 4 + >>> find_winner(10, 2) + 5 + """ + return josephus_recursive(num_people, step_size) + 1 + + +def josephus_iterative(num_people: int, step_size: int) -> int: + """ + Solve the Josephus problem for num_people and a step_size iteratively. + + Args: + num_people (int): The number of people in the circle. + step_size (int): The number of steps to take before eliminating someone. + + Returns: + int: The position of the last person standing. + + Examples: + >>> josephus_iterative(5, 2) + 3 + >>> josephus_iterative(7, 3) + 4 + """ + circle = list(range(1, num_people + 1)) + current = 0 + + while len(circle) > 1: + current = (current + step_size - 1) % len(circle) + circle.pop(current) + + return circle[0] + + +if __name__ == "__main__": + import doctest + + doctest.testmod()