"""PyTorch policy class used for R2D2."""

from typing import Dict, Tuple

import gym
import ray
from ray.rllib.agents.dqn.dqn_tf_policy import (PRIO_WEIGHTS,
                                                postprocess_nstep_and_prio)
from ray.rllib.agents.dqn.dqn_torch_policy import adam_optimizer, \
    build_q_model_and_distribution, compute_q_values
from ray.rllib.agents.dqn.r2d2_tf_policy import \
    get_distribution_inputs_and_class
from ray.rllib.agents.dqn.simple_q_torch_policy import TargetNetworkMixin
from ray.rllib.models.modelv2 import ModelV2
from ray.rllib.models.torch.torch_action_dist import \
    TorchDistributionWrapper
from ray.rllib.policy.policy import Policy
from ray.rllib.policy.policy_template import build_policy_class
from ray.rllib.policy.sample_batch import SampleBatch
from ray.rllib.policy.torch_policy import LearningRateSchedule
from ray.rllib.utils.framework import try_import_torch
from ray.rllib.utils.torch_utils import apply_grad_clipping, \
    concat_multi_gpu_td_errors, FLOAT_MIN, huber_loss, sequence_mask
from ray.rllib.utils.typing import TensorType, TrainerConfigDict

torch, nn = try_import_torch()
F = None
if nn:
    F = nn.functional


def build_r2d2_model_and_distribution(
    policy: Policy, obs_space: gym.spaces.Space,
    action_space: gym.spaces.Space,
    config: TrainerConfigDict) -> \
        Tuple[ModelV2, TorchDistributionWrapper]:
    """Build q_model and target_model for DQN

    Args:
        policy (Policy): The policy, which will use the model for optimization.
        obs_space (gym.spaces.Space): The policy's observation space.
        action_space (gym.spaces.Space): The policy's action space.
        config (TrainerConfigDict):

    Returns:
        (q_model, TorchCategorical)
            Note: The target q model will not be returned, just assigned to
            `policy.target_model`.
    """

    # Create the policy's models and action dist class.
    model, distribution_cls = build_q_model_and_distribution(
        policy, obs_space, action_space, config)

    # Assert correct model type by checking the init state to be present.
    # For attention nets: These don't necessarily publish their init state via
    # Model.get_initial_state, but may only use the trajectory view API
    # (view_requirements).
    assert (model.get_initial_state() != [] or
            model.view_requirements.get("state_in_0") is not None), \
        "R2D2 requires its model to be a recurrent one! Try using " \
        "`model.use_lstm` or `model.use_attention` in your config " \
        "to auto-wrap your model with an LSTM- or attention net."

    return model, distribution_cls


def r2d2_loss(policy: Policy, model, _,
              train_batch: SampleBatch) -> TensorType:
    """Constructs the loss for R2D2TorchPolicy.

    Args:
        policy (Policy): The Policy to calculate the loss for.
        model (ModelV2): The Model to calculate the loss for.
        train_batch (SampleBatch): The training data.

    Returns:
        TensorType: A single loss tensor.
    """
    target_model = policy.target_models[model]
    config = policy.config

    # Construct internal state inputs.
    i = 0
    state_batches = []
    while "state_in_{}".format(i) in train_batch:
        state_batches.append(train_batch["state_in_{}".format(i)])
        i += 1
    assert state_batches

    # Q-network evaluation (at t).
    q, _, _, _ = compute_q_values(
        policy,
        model,
        train_batch,
        state_batches=state_batches,
        seq_lens=train_batch.get(SampleBatch.SEQ_LENS),
        explore=False,
        is_training=True)

    # Target Q-network evaluation (at t+1).
    q_target, _, _, _ = compute_q_values(
        policy,
        target_model,
        train_batch,
        state_batches=state_batches,
        seq_lens=train_batch.get(SampleBatch.SEQ_LENS),
        explore=False,
        is_training=True)

    actions = train_batch[SampleBatch.ACTIONS].long()
    dones = train_batch[SampleBatch.DONES].float()
    rewards = train_batch[SampleBatch.REWARDS]
    weights = train_batch[PRIO_WEIGHTS]

    B = state_batches[0].shape[0]
    T = q.shape[0] // B

    # Q scores for actions which we know were selected in the given state.
    one_hot_selection = F.one_hot(actions, policy.action_space.n)
    q_selected = torch.sum(
        torch.where(q > FLOAT_MIN, q, torch.tensor(0.0, device=q.device)) *
        one_hot_selection, 1)

    if config["double_q"]:
        best_actions = torch.argmax(q, dim=1)
    else:
        best_actions = torch.argmax(q_target, dim=1)

    best_actions_one_hot = F.one_hot(best_actions, policy.action_space.n)
    q_target_best = torch.sum(
        torch.where(q_target > FLOAT_MIN, q_target,
                    torch.tensor(0.0, device=q_target.device)) *
        best_actions_one_hot,
        dim=1)

    if config["num_atoms"] > 1:
        raise ValueError("Distributional R2D2 not supported yet!")
    else:
        q_target_best_masked_tp1 = (1.0 - dones) * torch.cat([
            q_target_best[1:],
            torch.tensor([0.0], device=q_target_best.device)
        ])

        if config["use_h_function"]:
            h_inv = h_inverse(q_target_best_masked_tp1,
                              config["h_function_epsilon"])
            target = h_function(
                rewards + config["gamma"]**config["n_step"] * h_inv,
                config["h_function_epsilon"])
        else:
            target = rewards + \
                config["gamma"] ** config["n_step"] * q_target_best_masked_tp1

        # Seq-mask all loss-related terms.
        seq_mask = sequence_mask(train_batch[SampleBatch.SEQ_LENS], T)[:, :-1]
        # Mask away also the burn-in sequence at the beginning.
        burn_in = policy.config["burn_in"]
        if burn_in > 0 and burn_in < T:
            seq_mask[:, :burn_in] = False

        num_valid = torch.sum(seq_mask)

        def reduce_mean_valid(t):
            return torch.sum(t[seq_mask]) / num_valid

        # Make sure use the correct time indices:
        # Q(t) - [gamma * r + Q^(t+1)]
        q_selected = q_selected.reshape([B, T])[:, :-1]
        td_error = q_selected - target.reshape([B, T])[:, :-1].detach()
        td_error = td_error * seq_mask
        weights = weights.reshape([B, T])[:, :-1]
        total_loss = reduce_mean_valid(weights * huber_loss(td_error))

        # Store values for stats function in model (tower), such that for
        # multi-GPU, we do not override them during the parallel loss phase.
        model.tower_stats["total_loss"] = total_loss
        model.tower_stats["mean_q"] = reduce_mean_valid(q_selected)
        model.tower_stats["min_q"] = torch.min(q_selected)
        model.tower_stats["max_q"] = torch.max(q_selected)
        model.tower_stats["mean_td_error"] = reduce_mean_valid(td_error)
        # Store per time chunk (b/c we need only one mean
        # prioritized replay weight per stored sequence).
        model.tower_stats["td_error"] = torch.mean(td_error, dim=-1)

    return total_loss


def h_function(x, epsilon=1.0):
    """h-function to normalize target Qs, described in the paper [1].

    h(x) = sign(x) * [sqrt(abs(x) + 1) - 1] + epsilon * x

    Used in [1] in combination with h_inverse:
      targets = h(r + gamma * h_inverse(Q^))
    """
    return torch.sign(x) * (torch.sqrt(torch.abs(x) + 1.0) - 1.0) + epsilon * x


def h_inverse(x, epsilon=1.0):
    """Inverse if the above h-function, described in the paper [1].

    If x > 0.0:
    h-1(x) = [2eps * x + (2eps + 1) - sqrt(4eps x + (2eps + 1)^2)] /
        (2 * eps^2)

    If x < 0.0:
    h-1(x) = [2eps * x + (2eps + 1) + sqrt(-4eps x + (2eps + 1)^2)] /
        (2 * eps^2)
    """
    two_epsilon = epsilon * 2
    if_x_pos = (two_epsilon * x + (two_epsilon + 1.0) -
                torch.sqrt(4.0 * epsilon * x +
                           (two_epsilon + 1.0)**2)) / (2.0 * epsilon**2)
    if_x_neg = (two_epsilon * x - (two_epsilon + 1.0) +
                torch.sqrt(-4.0 * epsilon * x +
                           (two_epsilon + 1.0)**2)) / (2.0 * epsilon**2)
    return torch.where(x < 0.0, if_x_neg, if_x_pos)


class ComputeTDErrorMixin:
    """Assign the `compute_td_error` method to the R2D2TorchPolicy

    This allows us to prioritize on the worker side.
    """

    def __init__(self):
        def compute_td_error(obs_t, act_t, rew_t, obs_tp1, done_mask,
                             importance_weights):
            input_dict = self._lazy_tensor_dict({SampleBatch.CUR_OBS: obs_t})
            input_dict[SampleBatch.ACTIONS] = act_t
            input_dict[SampleBatch.REWARDS] = rew_t
            input_dict[SampleBatch.NEXT_OBS] = obs_tp1
            input_dict[SampleBatch.DONES] = done_mask
            input_dict[PRIO_WEIGHTS] = importance_weights

            # Do forward pass on loss to update td error attribute
            r2d2_loss(self, self.model, None, input_dict)

            return self.model.tower_stats["td_error"]

        self.compute_td_error = compute_td_error


def build_q_stats(policy: Policy, batch: SampleBatch) -> Dict[str, TensorType]:

    return {
        "cur_lr": policy.cur_lr,
        "total_loss": torch.mean(
            torch.stack(policy.get_tower_stats("total_loss"))),
        "mean_q": torch.mean(torch.stack(policy.get_tower_stats("mean_q"))),
        "min_q": torch.mean(torch.stack(policy.get_tower_stats("min_q"))),
        "max_q": torch.mean(torch.stack(policy.get_tower_stats("max_q"))),
        "mean_td_error": torch.mean(
            torch.stack(policy.get_tower_stats("mean_td_error"))),
    }


def setup_early_mixins(policy: Policy, obs_space, action_space,
                       config: TrainerConfigDict) -> None:
    LearningRateSchedule.__init__(policy, config["lr"], config["lr_schedule"])


def before_loss_init(policy: Policy, obs_space: gym.spaces.Space,
                     action_space: gym.spaces.Space,
                     config: TrainerConfigDict) -> None:
    ComputeTDErrorMixin.__init__(policy)
    TargetNetworkMixin.__init__(policy)


def grad_process_and_td_error_fn(policy: Policy,
                                 optimizer: "torch.optim.Optimizer",
                                 loss: TensorType) -> Dict[str, TensorType]:
    # Clip grads if configured.
    return apply_grad_clipping(policy, optimizer, loss)


def extra_action_out_fn(policy: Policy, input_dict, state_batches, model,
                        action_dist) -> Dict[str, TensorType]:
    return {"q_values": policy.q_values}


R2D2TorchPolicy = build_policy_class(
    name="R2D2TorchPolicy",
    framework="torch",
    loss_fn=r2d2_loss,
    get_default_config=lambda: ray.rllib.agents.dqn.r2d2.DEFAULT_CONFIG,
    make_model_and_action_dist=build_r2d2_model_and_distribution,
    action_distribution_fn=get_distribution_inputs_and_class,
    stats_fn=build_q_stats,
    postprocess_fn=postprocess_nstep_and_prio,
    optimizer_fn=adam_optimizer,
    extra_grad_process_fn=grad_process_and_td_error_fn,
    extra_learn_fetches_fn=concat_multi_gpu_td_errors,
    extra_action_out_fn=extra_action_out_fn,
    before_init=setup_early_mixins,
    before_loss_init=before_loss_init,
    mixins=[
        TargetNetworkMixin,
        ComputeTDErrorMixin,
        LearningRateSchedule,
    ])
