# Copyright (c) 2017-present, Facebook, Inc.
# All rights reserved.
#
# This source code is licensed under the license found in the LICENSE file in
# the root directory of this source tree. An additional grant of patent rights
# can be found in the PATENTS file in the same directory.


import torch.nn.functional as F
from torch import nn


class AdaptiveSoftmax(nn.Module):
    """
    This is an implementation of the efficient softmax approximation for
    graphical processing units (GPU), described in the paper "Efficient softmax
    approximation for GPUs" (http://arxiv.org/abs/1609.04309).
    """

    def __init__(self, vocab_size, input_dim, cutoff, dropout):
        super().__init__()

        if vocab_size > cutoff[-1]:
            cutoff = cutoff + [vocab_size]
        else:
            assert vocab_size == cutoff[
                -1], 'cannot specify cutoff smaller than vocab size'

        output_dim = cutoff[0] + len(cutoff) - 1

        self.vocab_size = vocab_size
        self.cutoff = cutoff
        self.dropout = dropout

        self.lsm = nn.LogSoftmax(dim=1)
        self.head = nn.Linear(input_dim, output_dim, bias=False)
        self.tail = nn.ModuleList()

        for i in range(len(cutoff) - 1):
            self.tail.append(
                nn.Sequential(
                    nn.Linear(input_dim, input_dim // 4 ** i, bias=False),
                    nn.Dropout(dropout),
                    nn.Linear(input_dim // 4 ** i, cutoff[i + 1] - cutoff[i], bias=False)
                )
            )

        def init_weights(m):
            if hasattr(m, 'weight'):
                nn.init.xavier_uniform_(m.weight)

        self.apply(init_weights)

    def adapt_target(self, target):
        """
        In order to be efficient, the AdaptiveSoftMax does not compute the
        scores for all the word of the vocabulary for all the examples. It is
        thus necessary to call the method adapt_target of the AdaptiveSoftMax
        layer inside each forward pass.
        """

        target = target.view(-1)
        new_target = [target.clone()]
        target_idxs = []

        for i in range(len(self.cutoff) - 1):
            mask = target.ge(self.cutoff[i]).mul(target.lt(self.cutoff[i + 1]))
            new_target[0][mask] = self.cutoff[0] + i - 1

            if mask.any():
                target_idxs.append(mask.nonzero().squeeze(1))
                new_target.append(target[mask].add(-self.cutoff[i]))
            else:
                target_idxs.append(None)
                new_target.append(None)

        return new_target, target_idxs

    def forward(self, input, target):
        """
        Args:
            input: (b x t x d)
            target: (b x t)
        Returns:
            2 lists: output for each cutoff section and new targets by cut off
        """

        input = input.contiguous().view(-1, input.size(-1))
        input = F.dropout(input, p=self.dropout, training=self.training)

        new_target, target_idxs = self.adapt_target(target)
        output = [self.head(input)]

        for i in range(len(target_idxs)):
            if target_idxs[i] is not None:
                output.append(self.tail[i](input.index_select(0, target_idxs[i])))
            else:
                output.append(None)

        return output, new_target

    def get_log_prob(self, input, target):
        """
        Computes the log probabilities for all the words of the vocabulary,
        given a 2D tensor of hidden vectors.
        """

        bsz, length, dim = input.size()
        input = input.contiguous().view(-1, dim)

        if target is not None:
            _, target_idxs = self.adapt_target(target)
        else:
            target_idxs = None

        head_y = self.head(input)
        log_probs = head_y.new_zeros(input.size(0), self.vocab_size)

        head_sz = self.cutoff[0] + len(self.tail)
        log_probs[:, :head_sz] = self.lsm(head_y)
        tail_priors = log_probs[:, self.cutoff[0] - 1: head_sz - 1].clone()

        for i in range(len(self.tail)):
            start = self.cutoff[i]
            end = self.cutoff[i + 1]

            if target_idxs is None:
                tail_out = log_probs[:, start:end]
                tail_out.copy_(self.tail[i](input))
                log_probs[:, start:end] = self.lsm(tail_out).add_(tail_priors[:, i, None])
            elif target_idxs[i] is not None:
                idxs = target_idxs[i]
                tail_out = log_probs[idxs, start:end]
                tail_out.copy_(self.tail[i](input[idxs]))
                log_probs[idxs, start:end] = self.lsm(tail_out).add_(tail_priors[idxs, i, None])

        log_probs = log_probs.view(bsz, length, -1)
        return log_probs
