Source code for igp2.planning.policy

import abc
from typing import Any, Tuple

import numpy as np
from math import sqrt

from igp2.planning.node import Node


class Policy(abc.ABC):
    """ Abstract class for implementing various selection policies """

    def select(self, node: Node) -> Tuple[Any, int]:
        """ Select an action from the node's list of actions using its Q-values.

         Returns:
             the action and its index in the list of actions of the node
         """
        raise NotImplementedError


class MaxPolicy(Policy):
    """ Policy selecting the action with highest Q-value at a node. """

    def select(self, node: Node):
        idx = np.argmax(node.q_values)
        return node.actions[idx], idx


class UCB1(Policy):
    """ Policy implementing the UCB1 selection policy.
    Ref: https://en.wikipedia.org/wiki/Monte_Carlo_tree_search#Exploration_and_exploitation"""

    def __init__(self, c: float = sqrt(2)):
        """ Initialise new UCB1 policy

        Args:
            c: the exploration parameter
        """
        self.c = c

    def select(self, node: Node):
        with np.errstate(divide="ignore"):
            values = node.q_values + self.c * np.sqrt(np.log(node.state_visits) / node.action_visits)
            idx = np.argmax(values)
            return node.actions[idx], idx