# Copyright 2018 DeepMind Technologies Limited. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

"""Transition adders.

This implements an N-step transition adder which collapses trajectory sequences
into a single transition, simplifying to a simple transition adder when N=1.
"""

import copy
from typing import Optional, Tuple

from acme import specs
from acme import types
from acme.adders.reverb import base
from acme.adders.reverb import utils
from acme.utils import tree_utils

import numpy as np
import reverb
import tree


class NStepTransitionAdder(base.ReverbAdder):
  """An N-step transition adder.

  This will buffer a sequence of N timesteps in order to form a single N-step
  transition which is added to reverb for future retrieval.

  For N=1 the data added to replay will be a standard one-step transition which
  takes the form:

        (s_t, a_t, r_t, d_t, s_{t+1}, e_t)

  where:

    s_t = state observation at time t
    a_t = the action taken from s_t
    r_t = reward ensuing from action a_t
    d_t = environment discount ensuing from action a_t. This discount is
        applied to future rewards after r_t.
    e_t [Optional] = extra data that the agent persists in replay.

  For N greater than 1, transitions are of the form:

        (s_t, a_t, R_{t:t+n}, D_{t:t+n}, s_{t+N}, e_t),

  where:

    s_t = State (observation) at time t.
    a_t = Action taken from state s_t.
    g = the additional discount, used by the agent to discount future returns.
    R_{t:t+n} = N-step discounted return, i.e. accumulated over N rewards:
          R_{t:t+n} := r_t + g * d_t * r_{t+1} + ...
                           + g^{n-1} * d_t * ... * d_{t+n-2} * r_{t+n-1}.
    D_{t:t+n}: N-step product of agent discounts g_i and environment
      "discounts" d_i.
          D_{t:t+n} := g^{n-1} * d_{t} * ... * d_{t+n-1},
      For most environments d_i is 1 for all steps except the last,
      i.e. it is the episode termination signal.
    s_{t+n}: The "arrival" state, i.e. the state at time t+n.
    e_t [Optional]: A nested structure of any 'extras' the user wishes to add.

  Notes:
    - At the beginning and end of episodes, shorter transitions are added.
      That is, at the beginning of the episode, it will add:
            (s_0 -> s_1), (s_0 -> s_2), ..., (s_0 -> s_n), (s_1 -> s_{n+1})

      And at the end of the episode, it will add:
            (s_{T-n+1} -> s_T), (s_{T-n+2} -> s_T), ... (s_{T-1} -> s_T).
    - We add the *first* `extra` of each transition, not the *last*, i.e.
        if extras are provided, we get e_t, not e_{t+n}.
  """

  def __init__(
      self,
      client: reverb.Client,
      n_step: int,
      discount: float,
      *,
      priority_fns: Optional[base.PriorityFnMapping] = None,
      max_in_flight_items: int = 5,
  ):
    """Creates an N-step transition adder.

    Args:
      client: A `reverb.Client` to send the data to replay through.
      n_step: The "N" in N-step transition. See the class docstring for the
        precise definition of what an N-step transition is. `n_step` must be at
        least 1, in which case we use the standard one-step transition, i.e.
        (s_t, a_t, r_t, d_t, s_t+1, e_t).
      discount: Discount factor to apply. This corresponds to the agent's
        discount in the class docstring.
      priority_fns: See docstring for BaseAdder.
      max_in_flight_items: The maximum number of items allowed to be "in flight"
        at the same time. See `block_until_num_items` in
        `reverb.TrajectoryWriter.flush` for more info.

    Raises:
      ValueError: If n_step is less than 1.
    """
    # Makes the additional discount a float32, which means that it will be
    # upcast if rewards/discounts are float64 and left alone otherwise.
    self.n_step = n_step
    self._discount = tree.map_structure(np.float32, discount)
    self._first_idx = 0
    self._last_idx = 0

    super().__init__(
        client=client,
        max_sequence_length=n_step + 1,
        priority_fns=priority_fns,
        max_in_flight_items=max_in_flight_items)

  def add(self, *args, **kwargs):
    # Increment the indices for the start and end of the window for computing
    # n-step returns.
    if self._writer.episode_steps >= self.n_step:
      self._first_idx += 1
    self._last_idx += 1

    super().add(*args, **kwargs)

  def reset(self):  # pytype: disable=signature-mismatch  # overriding-parameter-count-checks
    super().reset()
    self._first_idx = 0
    self._last_idx = 0

  @property
  def _n_step(self) -> int:
    """Effective n-step, which may vary at starts and ends of episodes."""
    return self._last_idx - self._first_idx

  def _write(self):
    # Convenient getters for use in tree operations.
    get_first = lambda x: x[self._first_idx]
    get_last = lambda x: x[self._last_idx]
    # Note: this getter is meant to be used on a TrajectoryWriter.history to
    # obtain its numpy values.
    get_all_np = lambda x: x[self._first_idx:self._last_idx].numpy()

    # Get the state, action, next_state, as well as possibly extras for the
    # transition that is about to be written.
    history = self._writer.history
    s, a = tree.map_structure(get_first,
                              (history['observation'], history['action']))
    s_ = tree.map_structure(get_last, history['observation'])

    # Maybe get extras to add to the transition later.
    if 'extras' in history:
      extras = tree.map_structure(get_first, history['extras'])

    # Note: at the beginning of an episode we will add the initial N-1
    # transitions (of size 1, 2, ...) and at the end of an episode (when
    # called from write_last) we will write the final transitions of size (N,
    # N-1, ...). See the Note in the docstring.
    # Get numpy view of the steps to be fed into the priority functions.
    reward, discount = tree.map_structure(
        get_all_np, (history['reward'], history['discount']))

    # Compute discounted return and geometric discount over n steps.
    n_step_return, total_discount = self._compute_cumulative_quantities(
        reward, discount)

    # Append the computed n-step return and total discount.
    # Note: if this call to _write() is within a call to _write_last(), then
    # this is the only data being appended and so it is not a partial append.
    self._writer.append(
        dict(n_step_return=n_step_return, total_discount=total_discount),
        partial_step=self._writer.episode_steps <= self._last_idx)
    # This should be done immediately after self._writer.append so the history
    # includes the recently appended data.
    history = self._writer.history

    # Form the n-step transition by using the following:
    # the first observation and action in the buffer, along with the cumulative
    # reward and discount computed above.
    n_step_return, total_discount = tree.map_structure(
        lambda x: x[-1], (history['n_step_return'], history['total_discount']))
    transition = types.Transition(
        observation=s,
        action=a,
        reward=n_step_return,
        discount=total_discount,
        next_observation=s_,
        extras=(extras if 'extras' in history else ()))

    # Calculate the priority for this transition.
    table_priorities = utils.calculate_priorities(self._priority_fns,
                                                  transition)

    # Insert the transition into replay along with its priority.
    for table, priority in table_priorities.items():
      self._writer.create_item(
          table=table, priority=priority, trajectory=transition)
      self._writer.flush(self._max_in_flight_items)

  def _write_last(self):
    # Write the remaining shorter transitions by alternating writing and
    # incrementingfirst_idx. Note that last_idx will no longer be incremented
    # once we're in this method's scope.
    self._first_idx += 1
    while self._first_idx < self._last_idx:
      self._write()
      self._first_idx += 1

  def _compute_cumulative_quantities(
      self, rewards: types.NestedArray, discounts: types.NestedArray
  ) -> Tuple[types.NestedArray, types.NestedArray]:

    # Give the same tree structure to the n-step return accumulator,
    # n-step discount accumulator, and self.discount, so that they can be
    # iterated in parallel using tree.map_structure.
    rewards, discounts, self_discount = tree_utils.broadcast_structures(
        rewards, discounts, self._discount)
    flat_rewards = tree.flatten(rewards)
    flat_discounts = tree.flatten(discounts)
    flat_self_discount = tree.flatten(self_discount)

    # Copy total_discount as it is otherwise read-only.
    total_discount = [np.copy(a[0]) for a in flat_discounts]

    # Broadcast n_step_return to have the broadcasted shape of
    # reward * discount.
    n_step_return = [
        np.copy(np.broadcast_to(r[0],
                                np.broadcast(r[0], d).shape))
        for r, d in zip(flat_rewards, total_discount)
    ]

    # NOTE: total_discount will have one less self_discount applied to it than
    # the value of self._n_step. This is so that when the learner/update uses
    # an additional discount we don't apply it twice. Inside the following loop
    # we will apply this right before summing up the n_step_return.
    for i in range(1, self._n_step):
      for nsr, td, r, d, sd in zip(n_step_return, total_discount, flat_rewards,
                                   flat_discounts, flat_self_discount):
        # Equivalent to: `total_discount *= self._discount`.
        td *= sd
        # Equivalent to: `n_step_return += reward[i] * total_discount`.
        nsr += r[i] * td
        # Equivalent to: `total_discount *= discount[i]`.
        td *= d[i]

    n_step_return = tree.unflatten_as(rewards, n_step_return)
    total_discount = tree.unflatten_as(rewards, total_discount)
    return n_step_return, total_discount

  # TODO(bshahr): make this into a standalone method. Class methods should be
  # used as alternative constructors or when modifying some global state,
  # neither of which is done here.
  @classmethod
  def signature(cls,
                environment_spec: specs.EnvironmentSpec,
                extras_spec: types.NestedSpec = ()):

    # This function currently assumes that self._discount is a scalar.
    # If it ever becomes a nested structure and/or a np.ndarray, this method
    # will need to know its structure / shape. This is because the signature
    # discount shape is the environment's discount shape and this adder's
    # discount shape broadcasted together. Also, the reward shape is this
    # signature discount shape broadcasted together with the environment
    # reward shape. As long as self._discount is a scalar, it will not affect
    # either the signature discount shape nor the signature reward shape, so we
    # can ignore it.

    rewards_spec, step_discounts_spec = tree_utils.broadcast_structures(
        environment_spec.rewards, environment_spec.discounts)
    rewards_spec = tree.map_structure(_broadcast_specs, rewards_spec,
                                      step_discounts_spec)
    step_discounts_spec = tree.map_structure(copy.deepcopy, step_discounts_spec)

    transition_spec = types.Transition(
        environment_spec.observations,
        environment_spec.actions,
        rewards_spec,
        step_discounts_spec,
        environment_spec.observations,  # next_observation
        extras_spec)

    return tree.map_structure_with_path(base.spec_like_to_tensor_spec,
                                        transition_spec)


def _broadcast_specs(*args: specs.Array) -> specs.Array:
  """Like np.broadcast, but for specs.Array.

  Args:
    *args: one or more specs.Array instances.

  Returns:
    A specs.Array with the broadcasted shape and dtype of the specs in *args.
  """
  bc_info = np.broadcast(*tuple(a.generate_value() for a in args))
  dtype = np.result_type(*tuple(a.dtype for a in args))
  return specs.Array(shape=bc_info.shape, dtype=dtype)
