Add Josephus Problem

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()