# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.

import operator
import functools

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


class TiedLinear(nn.Module):
    def __init__(self, weight, transpose):
        super().__init__()
        self.weight = weight
        self.transpose = transpose

    def forward(self, input):
        return F.linear(input, self.weight.t() if self.transpose else self.weight)


class TiedHeadModule(nn.Module):
    def __init__(self, weights, input_dim, num_classes):
        super().__init__()
        tied_emb, _ = weights
        self.num_words, emb_dim = tied_emb.size()

        self.word_proj = TiedLinear(tied_emb, transpose=False)
        if input_dim != emb_dim:
            self.word_proj = nn.Sequential(
                nn.Linear(input_dim, emb_dim, bias=False),
                self.word_proj,
            )

        self.class_proj = nn.Linear(input_dim, num_classes, bias=False)
        self.out_dim = self.num_words + num_classes

        self.register_buffer('_float_tensor', torch.FloatTensor(1))

    def forward(self, input):
        inp_sz = functools.reduce(operator.mul, input.shape[:-1], 1)
        out = self._float_tensor.new(inp_sz, self.out_dim)
        out[:, :self.num_words] = self.word_proj(input.view(inp_sz, -1))
        out[:, self.num_words:] = self.class_proj(input.view(inp_sz, -1))
        return out


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, factor=4., adaptive_inputs=None, tie_proj=False):
        super().__init__()

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

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

        self.vocab_size = vocab_size
        self.cutoff = cutoff
        self.dropout = dropout
        self.input_dim = input_dim
        self.factor = factor

        self.lsm = nn.LogSoftmax(dim=1)

        if adaptive_inputs is not None:
            self.head = TiedHeadModule(adaptive_inputs.weights_for_band(0), input_dim, len(cutoff) - 1)
        else:
            self.head = nn.Linear(input_dim, output_dim, bias=False)

        self._make_tail(adaptive_inputs, tie_proj)

        def init_weights(m):
            if hasattr(m, 'weight') and not isinstance(m, TiedLinear) and not isinstance(m, TiedHeadModule):
                nn.init.xavier_uniform_(m.weight)

        self.apply(init_weights)

        self.register_buffer('version', torch.LongTensor([1]))

    def _make_tail(self, adaptive_inputs=None, tie_proj=False):
        self.tail = nn.ModuleList()
        for i in range(len(self.cutoff) - 1):
            dim = int(self.input_dim // self.factor ** (i + 1))

            tied_emb, tied_proj = adaptive_inputs.weights_for_band(i + 1) \
                if adaptive_inputs is not None else (None, None)

            if tied_proj is not None:
                if tie_proj:
                    proj = TiedLinear(tied_proj, transpose=True)
                else:
                    proj = nn.Linear(tied_proj.size(0), tied_proj.size(1), bias=False)
            else:
                proj = nn.Linear(self.input_dim, dim, bias=False)

            m = nn.Sequential(
                proj,
                nn.Dropout(self.dropout),
                nn.Linear(
                    dim, self.cutoff[i + 1] - self.cutoff[i], bias=False,
                ) if tied_emb is None else TiedLinear(tied_emb, transpose=False),
            )

            self.tail.append(m)

    def upgrade_state_dict_named(self, state_dict, name):
        version_name = name + '.version'
        if version_name not in state_dict:
            raise Exception('This version of the model is no longer supported')

    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

            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]: head_sz].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
