Python/backtracking/minimax.py

"""
Minimax helps to achieve maximum score in a game by checking all possible moves
depth is current depth in game tree.

nodeIndex is index of current node in scores[].
if move is of maximizer return true else false
leaves of game tree is stored in scores[]
height is maximum height of Game tree
"""
from __future__ import annotations

import math


def minimax(
    depth: int, node_index: int, is_max: bool, scores: list[int], height: float
) -> int:
    """
    This function implements the minimax algorithm, which helps achieve the optimal
    score for a player in a two-player game by checking all possible moves.
    If the player is the maximizer, then the score is maximized.
    If the player is the minimizer, then the score is minimized.

    Parameters:
    - depth: Current depth in the game tree.
    - node_index: Index of the current node in the scores list.
    - is_max: A boolean indicating whether the current move
              is for the maximizer (True) or minimizer (False).
    - scores: A list containing the scores of the leaves of the game tree.
    - height: The maximum height of the game tree.

    Returns:
    - An integer representing the optimal score for the current player.

    >>> import math
    >>> scores = [90, 23, 6, 33, 21, 65, 123, 34423]
    >>> height = math.log(len(scores), 2)
    >>> minimax(0, 0, True, scores, height)
    65
    >>> minimax(-1, 0, True, scores, height)
    Traceback (most recent call last):
        ...
    ValueError: Depth cannot be less than 0
    >>> minimax(0, 0, True, [], 2)
    Traceback (most recent call last):
        ...
    ValueError: Scores cannot be empty
    >>> scores = [3, 5, 2, 9, 12, 5, 23, 23]
    >>> height = math.log(len(scores), 2)
    >>> minimax(0, 0, True, scores, height)
    12
    """

    if depth < 0:
        raise ValueError("Depth cannot be less than 0")
    if len(scores) == 0:
        raise ValueError("Scores cannot be empty")

    # Base case: If the current depth equals the height of the tree,
    # return the score of the current node.
    if depth == height:
        return scores[node_index]

    # If it's the maximizer's turn, choose the maximum score
    # between the two possible moves.
    if is_max:
        return max(
            minimax(depth + 1, node_index * 2, False, scores, height),
            minimax(depth + 1, node_index * 2 + 1, False, scores, height),
        )

    # If it's the minimizer's turn, choose the minimum score
    # between the two possible moves.
    return min(
        minimax(depth + 1, node_index * 2, True, scores, height),
        minimax(depth + 1, node_index * 2 + 1, True, scores, height),
    )


def main() -> None:
    # Sample scores and height calculation
    scores = [90, 23, 6, 33, 21, 65, 123, 34423]
    height = math.log(len(scores), 2)

    # Calculate and print the optimal value using the minimax algorithm
    print("Optimal value : ", end="")
    print(minimax(0, 0, True, scores, height))


if __name__ == "__main__":
    import doctest

    doctest.testmod()
    main()