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"""Reinforcement Learning (Chapter 21)

"""

from collections import defaultdict

from utils import argmax

from mdp import MDP, policy_evaluation

import random

class PassiveADPAgent:

"""Passive (non-learning) agent that uses adaptive dynamic programming

on a given MDP and policy. [Figure 21.2]"""

class ModelMDP(MDP):

""" Class for implementing modifed Version of input MDP with

an editable transition model P and a custom function T. """

def __init__(self, init, actlist, terminals, gamma, states):

super().__init__(init, actlist, terminals, gamma)

nested_dict = lambda: defaultdict(nested_dict)

# StackOverflow:whats-the-best-way-to-initialize-a-dict-of-dicts-in-python

self.P = nested_dict()

def T(self, s, a):

"""Returns a list of tuples with probabilities for states

based on the learnt model P. """

return [(prob, res) for (res, prob) in self.P[(s, a)].items()]

def __init__(self, pi, mdp):

self.pi = pi

self.mdp = PassiveADPAgent.ModelMDP(mdp.init, mdp.actlist,

mdp.terminals, mdp.gamma, mdp.states)

self.U = {}

self.Nsa = defaultdict(int)

self.Ns1_sa = defaultdict(int)

self.s = None

self.a = None

def __call__(self, percept):

s1, r1 = percept

self.mdp.states.add(s1) # Model keeps track of visited states.

R, P, mdp, pi = self.mdp.reward, self.mdp.P, self.mdp, self.pi

s, a, Nsa, Ns1_sa, U = self.s, self.a, self.Nsa, self.Ns1_sa, self.U

if s1 not in R: # Reward is only available for visted state.

U[s1] = R[s1] = r1

if s is not None:

Nsa[(s, a)] += 1

Ns1_sa[(s1, s, a)] += 1

# for each t such that Ns′|sa [t, s, a] is nonzero

for t in [res for (res, state, act), freq in Ns1_sa.items()

if (state, act) == (s, a) and freq != 0]:

P[(s, a)][t] = Ns1_sa[(t, s, a)] / Nsa[(s, a)]

U = policy_evaluation(pi, U, mdp)

if s1 in mdp.terminals:

self.s = self.a = None

else:

self.s, self.a = s1, self.pi[s1]

return self.a

def update_state(self, percept):

''' To be overridden in most cases. The default case

assumes th percept to be of type (state, reward)'''

return percept

class PassiveTDAgent:

"""The abstract class for a Passive (non-learning) agent that uses

temporal differences to learn utility estimates. Override update_state

method to convert percept to state and reward. The mdp being provided

should be an instance of a subclass of the MDP Class.[Figure 21.4]

"""

def __init__(self, pi, mdp, alpha=None):

self.pi = pi

self.U = {s: 0. for s in mdp.states}

self.Ns = {s: 0 for s in mdp.states}

self.s = None

self.a = None

self.r = None

self.gamma = mdp.gamma

self.terminals = mdp.terminals

if alpha:

self.alpha = alpha

else:

self.alpha = lambda n: 1./(1+n) # udacity video

def __call__(self, percept):

s1, r1 = self.update_state(percept)

pi, U, Ns, s, a, r = self.pi, self.U, self.Ns, self.s, self.a, self.r

alpha, gamma, terminals = self.alpha, self.gamma, self.terminals

if not Ns[s1]:

U[s1] = r1

if s is not None:

Ns[s] += 1

U[s] += alpha(Ns[s]) * (r + gamma * U[s1] - U[s])

if s1 in terminals:

self.s = self.a = self.r = None

else:

self.s, self.a, self.r = s1, pi[s1], r1

return self.a

def update_state(self, percept):

''' To be overridden in most cases. The default case

assumes th percept to be of type (state, reward)'''

return percept

class QLearningAgent:

""" An exploratory Q-learning agent. It avoids having to learn the transition

model because the Q-value of a state can be related directly to those of

its neighbors. [Figure 21.8]

"""

def __init__(self, mdp, Ne, Rplus, alpha=None):

self.gamma = mdp.gamma

self.terminals = mdp.terminals

self.all_act = mdp.actlist

self.Ne = Ne # iteration limit in exploration function

self.Rplus = Rplus # large value to assign before iteration limit

self.Q = defaultdict(float)

self.Nsa = defaultdict(float)

self.s = None

self.a = None

self.r = None

if alpha:

self.alpha = alpha

else:

self.alpha = lambda n: 1./(1+n) # udacity video

def f(self, u, n):

""" Exploration function. Returns fixed Rplus untill

agent has visited state, action a Ne number of times.

Same as ADP agent in book."""

if n < self.Ne:

return self.Rplus

else:

return u

def actions_in_state(self, state):

""" Returns actions possible in given state.

Useful for max and argmax. """

if state in self.terminals:

return [None]

else:

return self.all_act

def __call__(self, percept):

s1, r1 = self.update_state(percept)

Q, Nsa, s, a, r = self.Q, self.Nsa, self.s, self.a, self.r

alpha, gamma, terminals, actions_in_state = self.alpha, self.gamma, self.terminals, self.actions_in_state

if s1 in terminals:

Q[s1, None] = r1

if s is not None:

Nsa[s, a] += 1

Q[s, a] += alpha(Nsa[s, a]) * (r + gamma * max(Q[s1, a1] for a1 in actions_in_state(s1))

- Q[s, a])

if s1 in terminals:

self.s = self.a = self.r = None

else:

self.s, self.r = s1, r1

self.a = argmax(actions_in_state(s1), key=lambda a1: self.f(Q[s1, a1], Nsa[s1, a1]))

return self.a

def update_state(self, percept):

''' To be overridden in most cases. The default case

assumes the percept to be of type (state, reward)'''

return percept

def run_single_trial(agent_program, mdp):

''' Execute trial for given agent_program

and mdp. mdp should be an instance of subclass

of mdp.MDP '''

def take_single_action(mdp, s, a):

'''

Selects outcome of taking action a

in state s. Weighted Sampling.

'''

x = random.uniform(0, 1)

cumulative_probability = 0.0

for probability_state in mdp.T(s, a):

probability, state = probability_state

cumulative_probability += probability

if x < cumulative_probability:

break

return state

current_state = mdp.init

while True:

current_reward = mdp.R(current_state)

percept = (current_state, current_reward)

next_action = agent_program(percept)

if next_action is None:

break

current_state = take_single_action(mdp, current_state, next_action)

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