text: # -*- coding: utf-8 -*-
  """
  An implementation of the policyValueNet in PyTorch
  Tested in PyTorch 0.2.0 and 0.3.0
  
  @author: Junxiao Song
  """
  
  import torch
  import torch.nn as nn
  import torch.optim as optim
  import torch.nn.functional as F
  from torch.autograd import Variable
  import numpy as np
  
  
  def set_learning_rate(optimizer, lr):
      """Sets the learning rate to the given value"""
      for param_group in optimizer.param_groups:
          param_group['lr'] = lr
  
  
  class Net(nn.Module):
      """policy-value network module"""
      def __init__(self, board_width, board_height):
          super(Net, self).__init__()
  
          self.board_width = board_width
          self.board_height = board_height
          # common layers
          self.conv1 = nn.Conv2d(4, 32, kernel_size=3, padding=1)
          self.conv2 = nn.Conv2d(32, 64, kernel_size=3, padding=1)
          self.conv3 = nn.Conv2d(64, 128, kernel_size=3, padding=1)
          # action policy layers
          self.act_conv1 = nn.Conv2d(128, 4, kernel_size=1)
          self.act_fc1 = nn.Linear(4*board_width*board_height,
                                   board_width*board_height)
          # state value layers
          self.val_conv1 = nn.Conv2d(128, 2, kernel_size=1)
          self.val_fc1 = nn.Linear(2*board_width*board_height, 64)
          self.val_fc2 = nn.Linear(64, 1)
  
      def forward(self, state_input):
          # common layers
          x = F.relu(self.conv1(state_input))
          x = F.relu(self.conv2(x))
          x = F.relu(self.conv3(x))
          # action policy layers
          x_act = F.relu(self.act_conv1(x))
          x_act = x_act.view(-1, 4*self.board_width*self.board_height)
          x_act = F.log_softmax(self.act_fc1(x_act))
          # state value layers
          x_val = F.relu(self.val_conv1(x))
          x_val = x_val.view(-1, 2*self.board_width*self.board_height)
          x_val = F.relu(self.val_fc1(x_val))
          x_val = F.tanh(self.val_fc2(x_val))
          return x_act, x_val
  
  
  class PolicyValueNet():
      """policy-value network """
      def __init__(self, board_width, board_height,
                   model_file=None, use_gpu=True):
          self.use_gpu = use_gpu
          self.board_width = board_width
          self.board_height = board_height
          self.l2_const = 1e-4  # coef of l2 penalty
          # the policy value net module
          if self.use_gpu:
              self.policy_value_net = Net(board_width, board_height).cuda()
          else:
              self.policy_value_net = Net(board_width, board_height)
          self.optimizer = optim.Adam(self.policy_value_net.parameters(),
                                      weight_decay=self.l2_const)
  
          if model_file:
              net_params = torch.load(model_file)
              self.policy_value_net.load_state_dict(net_params)
  
      def policy_value(self, state_batch):
          """
          input: a batch of states
          output: a batch of action probabilities and state values
          """
          if self.use_gpu:
              state_batch = Variable(torch.FloatTensor(state_batch).cuda())
              log_act_probs, value = self.policy_value_net(state_batch)
              act_probs = np.exp(log_act_probs.data.cpu().numpy())
              return act_probs, value.data.cpu().numpy()
          else:
              state_batch = Variable(torch.FloatTensor(state_batch))
              log_act_probs, value = self.policy_value_net(state_batch)
              act_probs = np.exp(log_act_probs.data.numpy())
              return act_probs, value.data.numpy()
  
      def policy_value_fn(self, board):
          """
          input: board
          output: a list of (action, probability) tuples for each available
          action and the score of the board state
          """
          legal_positions = board.availables
          current_state = np.ascontiguousarray(board.current_state().reshape(
                  -1, 4, self.board_width, self.board_height))
          if self.use_gpu:
              log_act_probs, value = self.policy_value_net(
                      Variable(torch.from_numpy(current_state)).cuda().float())
              act_probs = np.exp(log_act_probs.data.cpu().numpy().flatten())
          else:
              log_act_probs, value = self.policy_value_net(
                      Variable(torch.from_numpy(current_state)).float())
              act_probs = np.exp(log_act_probs.data.numpy().flatten())
          act_probs = zip(legal_positions, act_probs[legal_positions])
          value = value.data[0][0]
          return act_probs, value
  
      def train_step(self, state_batch, mcts_probs, winner_batch, lr):
          """perform a training step"""
          # wrap in Variable
          if self.use_gpu:
              state_batch = Variable(torch.FloatTensor(state_batch).cuda())
              mcts_probs = Variable(torch.FloatTensor(mcts_probs).cuda())
              winner_batch = Variable(torch.FloatTensor(winner_batch).cuda())
          else:
              state_batch = Variable(torch.FloatTensor(state_batch))
              mcts_probs = Variable(torch.FloatTensor(mcts_probs))
              winner_batch = Variable(torch.FloatTensor(winner_batch))
  
          # zero the parameter gradients
          self.optimizer.zero_grad()
          # set learning rate
          set_learning_rate(self.optimizer, lr)
  
          # forward
          log_act_probs, value = self.policy_value_net(state_batch)
          # define the loss = (z - v)^2 - pi^T * log(p) + c||theta||^2
          # Note: the L2 penalty is incorporated in optimizer
          value_loss = F.mse_loss(value.view(-1), winner_batch)
          policy_loss = -torch.mean(torch.sum(mcts_probs*log_act_probs, 1))
          loss = value_loss + policy_loss
          # backward and optimize
          loss.backward()
          self.optimizer.step()
          # calc policy entropy, for monitoring only
          entropy = -torch.mean(
                  torch.sum(torch.exp(log_act_probs) * log_act_probs, 1)
                  )
          #return loss.data[0], entropy.data[0]
          #for pytorch version >= 0.5 please use the following line instead.
          return loss.item(), entropy.item()
  
      def get_policy_param(self):
          net_params = self.policy_value_net.state_dict()
          return net_params
  
      def save_model(self, model_file):
          """ save model params to file """
          net_params = self.get_policy_param()  # get model params
          torch.save(net_params, model_file)
  