# Copyright The PyTorch Lightning team.
#
# 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.
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# limitations under the License.
from copy import deepcopy
from typing import Any, Dict, Optional, Union

import torch
from torch import Tensor
from torch.nn import ModuleList

from torchmetrics.metric import Metric
from torchmetrics.utilities import apply_to_collection


def _bootstrap_sampler(
    size: int,
    sampling_strategy: str = "poisson",
) -> Tensor:
    """Resample a tensor along its first dimension with replacement.

    Args:
        size: number of samples
        sampling_strategy: the strategy to use for sampling, either ``'poisson'`` or ``'multinomial'``

    Returns:
        resampled tensor
    """
    if sampling_strategy == "poisson":
        p = torch.distributions.Poisson(1)
        n = p.sample((size,))
        return torch.arange(size).repeat_interleave(n.long(), dim=0)
    if sampling_strategy == "multinomial":
        idx = torch.multinomial(torch.ones(size), num_samples=size, replacement=True)
        return idx
    raise ValueError("Unknown sampling strategy")


class BootStrapper(Metric):
    r"""Using `Turn a Metric into a Bootstrapped`_

    That can automate the process of getting confidence intervals for metric values. This wrapper
    class basically keeps multiple copies of the same base metric in memory and whenever ``update`` or
    ``forward`` is called, all input tensors are resampled (with replacement) along the first dimension.

    Args:
        base_metric: base metric class to wrap
        num_bootstraps: number of copies to make of the base metric for bootstrapping
        mean: if ``True`` return the mean of the bootstraps
        std: if ``True`` return the standard diviation of the bootstraps
        quantile: if given, returns the quantile of the bootstraps. Can only be used with pytorch version 1.6 or higher
        raw: if ``True``, return all bootstrapped values
        sampling_strategy:
            Determines how to produce bootstrapped samplings. Either ``'poisson'`` or ``multinomial``.
            If ``'possion'`` is chosen, the number of times each sample will be included in the bootstrap
            will be given by :math:`n\sim Poisson(\lambda=1)`, which approximates the true bootstrap distribution
            when the number of samples is large. If ``'multinomial'`` is chosen, we will apply true bootstrapping
            at the batch level to approximate bootstrapping over the hole dataset.
        kwargs: Additional keyword arguments, see :ref:`Metric kwargs` for more info.

    Example::
        >>> from pprint import pprint
        >>> from torchmetrics import BootStrapper
        >>> from torchmetrics.classification import MulticlassAccuracy
        >>> _ = torch.manual_seed(123)
        >>> base_metric = MulticlassAccuracy(num_classes=5, average='micro')
        >>> bootstrap = BootStrapper(base_metric, num_bootstraps=20)
        >>> bootstrap.update(torch.randint(5, (20,)), torch.randint(5, (20,)))
        >>> output = bootstrap.compute()
        >>> pprint(output)
        {'mean': tensor(0.2205), 'std': tensor(0.0859)}
    """
    full_state_update: Optional[bool] = True

    def __init__(
        self,
        base_metric: Metric,
        num_bootstraps: int = 10,
        mean: bool = True,
        std: bool = True,
        quantile: Optional[Union[float, Tensor]] = None,
        raw: bool = False,
        sampling_strategy: str = "poisson",
        **kwargs: Any,
    ) -> None:
        super().__init__(**kwargs)
        if not isinstance(base_metric, Metric):
            raise ValueError(
                "Expected base metric to be an instance of torchmetrics.Metric" f" but received {base_metric}"
            )

        self.metrics = ModuleList([deepcopy(base_metric) for _ in range(num_bootstraps)])
        self.num_bootstraps = num_bootstraps

        self.mean = mean
        self.std = std
        self.quantile = quantile
        self.raw = raw

        allowed_sampling = ("poisson", "multinomial")
        if sampling_strategy not in allowed_sampling:
            raise ValueError(
                f"Expected argument ``sampling_strategy`` to be one of {allowed_sampling}"
                f" but recieved {sampling_strategy}"
            )
        self.sampling_strategy = sampling_strategy

    def update(self, *args: Any, **kwargs: Any) -> None:
        """Updates the state of the base metric.

        Any tensor passed in will be bootstrapped along dimension 0.
        """
        for idx in range(self.num_bootstraps):
            args_sizes = apply_to_collection(args, Tensor, len)
            kwargs_sizes = list(apply_to_collection(kwargs, Tensor, len))
            if len(args_sizes) > 0:
                size = args_sizes[0]
            elif len(kwargs_sizes) > 0:
                size = kwargs_sizes[0]
            else:
                raise ValueError("None of the input contained tensors, so could not determine the sampling size")
            sample_idx = _bootstrap_sampler(size, sampling_strategy=self.sampling_strategy).to(self.device)
            new_args = apply_to_collection(args, Tensor, torch.index_select, dim=0, index=sample_idx)
            new_kwargs = apply_to_collection(kwargs, Tensor, torch.index_select, dim=0, index=sample_idx)
            self.metrics[idx].update(*new_args, **new_kwargs)

    def compute(self) -> Dict[str, Tensor]:
        """Computes the bootstrapped metric values.

        Always returns a dict of tensors, which can contain the following keys: ``mean``, ``std``, ``quantile`` and
        ``raw`` depending on how the class was initialized.
        """
        computed_vals = torch.stack([m.compute() for m in self.metrics], dim=0)
        output_dict = {}
        if self.mean:
            output_dict["mean"] = computed_vals.mean(dim=0)
        if self.std:
            output_dict["std"] = computed_vals.std(dim=0)
        if self.quantile is not None:
            output_dict["quantile"] = torch.quantile(computed_vals, self.quantile)
        if self.raw:
            output_dict["raw"] = computed_vals
        return output_dict