#!/usr/bin/python3

# https://pytorch.org/tutorials/intermediate/seq2seq_translation_tutorial.html

import sys
import torch
from torch import nn, optim

nb_of_char_codes = 128 + 2
SOS_token_id = 128 # start of sentence
EOS_token_id = 129 # end of sentence

hidden_size = 32
step = 200

device = torch.device('cpu')

f = open('eng-fra.txt')
def char_source():
    for line in f:
        s, t = line.rstrip('\n').split('\t')
        s_list = []
        t_list = []

        for c in s:
            c_code = ord(c)
            if c_code < nb_of_char_codes:
                s_list.append(ord(c))

        for c in t:
            c_code = ord(c)
            if c_code < nb_of_char_codes:
                t_list.append(ord(c))

        yield s_list, t_list

class EncoderRNN(nn.Module):
    def __init__(self, input_size, hidden_size):
        super(EncoderRNN, self).__init__()
        self.hidden_size = hidden_size

        self.embedding = nn.Embedding(input_size, hidden_size)
        self.gru = nn.GRU(hidden_size, hidden_size)

    def forward(self, input, hidden):
        embedded = self.embedding(input)
        output = embedded
        output, hidden = self.gru(output, hidden)
        return output, hidden

    def initHidden(self):
        return torch.zeros(1,1,  self.hidden_size, device=device)

class DecoderRNN(nn.Module):
    def __init__(self, hidden_size, output_size):
        super(DecoderRNN, self).__init__()
        self.hidden_size = hidden_size

        self.embedding = nn.Embedding(output_size, hidden_size)
        self.gru = nn.GRU(hidden_size, hidden_size)
        self.out = nn.Linear(hidden_size, output_size)
        self.softmax = nn.LogSoftmax(dim=1)

    def forward(self, input, hidden):
        output = self.embedding(input)
        output = torch.nn.functional.relu(output)
        output, hidden = self.gru(output, hidden)
        output = self.softmax(self.out(output[0]))
        return output, hidden


encoder = EncoderRNN(nb_of_char_codes, hidden_size).to(device)
decoder = DecoderRNN(hidden_size, nb_of_char_codes).to(device)
criterion = nn.NLLLoss().to(device)
optimizer = optim.Adam((list(encoder.parameters()) +  list(decoder.parameters())))

counter = 0
losses = []

for s,t in char_source():
    counter += 1
    encoder.zero_grad()
    decoder.zero_grad()
    x = torch.tensor(s, dtype=torch.long, device=device)
    encoder_hidden = encoder.initHidden()
    encoder_output = torch.zeros(hidden_size, hidden_size, device=device)
    for i in range(x.shape[0]):
        output, encoder_hidden = encoder(x[i].unsqueeze(0).unsqueeze(0), encoder_hidden)
        encoder_output[i] = output[0,0]

    decoder_hidden = encoder_hidden

    decoder_input = torch.tensor([[SOS_token_id]], device=device)

    t.append(EOS_token_id)
    y = torch.tensor(t, dtype=torch.long, device=device)
    loss = 0
    output_string = ''
    for di in range(y.shape[0]):
        decoder_output, decoder_hidden = decoder(
            decoder_input, decoder_hidden)
        topv, topi = decoder_output.topk(1)
        decoder_input = topi.detach()  # detach from history as input

        output_string += chr(topi)
        loss += criterion(decoder_output, y[di].unsqueeze(0))
        if chr(topi) == EOS_token_id:
            break

    losses.append(loss.item())
    if counter % step == 0:
        # print(counter, end='\t')
        avg_loss = sum(losses)/len(losses)
        print(f"{counter}: {avg_loss}")
        losses = []
        print('IN :\t', ''.join([chr(a) for a in s]))
        print('EXP:\t', ''.join([chr(a) for a in t]))
        print('OUT:\t', output_string)

    loss.backward()
    optimizer.step()