import torch
import torch.nn as nn
import torch.optim as optim
import numpy as np

# Hyperparameters
sequence_length = 100
batch_size = 64
hidden_size = 128
num_layers = 1
learning_rate = 0.01
num_epochs = 100
device = torch.device('cpu')

# Sample text data
text = "Hello, this is a simple language model for text generation using PyTorch."
chars = sorted(list(set(text)))
vocab_size = len(chars)
char_to_idx = {ch: i for i, ch in enumerate(chars)}
idx_to_char = {i: ch for i, ch in enumerate(chars)}

# Prepare data
data = [char_to_idx[ch] for ch in text]
data = torch.tensor(data, dtype=torch.long).to(device)

# Create batches
def create_batches(data, batch_size, sequence_length):
    num_batches = len(data) // (batch_size * sequence_length)
    data = data[:num_batches * batch_size * sequence_length]
    data = data.view(batch_size, -1)
    for i in range(0, data.size(1), sequence_length):
        x = data[:, i:i+sequence_length]
        y = data[:, (i+1):(i+1)+sequence_length]
        yield x, y

# Define the model
class CharRNN(nn.Module):
    def __init__(self, vocab_size, hidden_size, num_layers):
        super(CharRNN, self).__init__()
        self.hidden_size = hidden_size
        self.num_layers = num_layers
        self.embedding = nn.Embedding(vocab_size, hidden_size)
        self.lstm = nn.LSTM(hidden_size, hidden_size, num_layers, batch_first=True)
        self.fc = nn.Linear(hidden_size, vocab_size)

    def forward(self, x, hidden):
        x = self.embedding(x)
        out, hidden = self.lstm(x, hidden)
        out = self.fc(out)
        return out, hidden

    def init_hidden(self, batch_size):
        return (torch.zeros(self.num_layers, batch_size, self.hidden_size).to(device),
                torch.zeros(self.num_layers, batch_size, self.hidden_size).to(device))

# Initialize model, loss, and optimizer
model = CharRNN(vocab_size, hidden_size, num_layers).to(device)
criterion = nn.CrossEntropyLoss()
optimizer = optim.Adam(model.parameters(), lr=learning_rate)

# Training loop
for epoch in range(num_epochs):
    hidden = model.init_hidden(batch_size)
    for i, (x, y) in enumerate(create_batches(data, batch_size, sequence_length)):
        x, y = x.to(device), y.to(device)
        hidden = tuple(h.detach() for h in hidden)  # Detach hidden state
        optimizer.zero_grad()
        output, hidden = model(x, hidden)
        loss = criterion(output.transpose(1, 2), y)
        loss.backward()
        optimizer.step()

        if (i+1) % 10 == 0:
            print(f'Epoch [{epoch+1}/{num_epochs}], Step [{i+1}], Loss: {loss.item():.4f}')

# Generate text
def generate_text(model, start_str, length=100):
    model.eval()
    chars = [char_to_idx[ch] for ch in start_str]
    hidden = model.init_hidden(1)
    for i in range(length):
        x = torch.tensor([chars[-1]], dtype=torch.long).unsqueeze(0).to(device)
        with torch.no_grad():
            output, hidden = model(x, hidden)
        prob = torch.softmax(output.squeeze(), dim=0).cpu().numpy()
        next_char = np.random.choice(vocab_size, p=prob)
        chars.append(next_char)
    return ''.join([idx_to_char[ch] for ch in chars])

# Test text generation
start_str = "Hello"
generated_text = generate_text(model, start_str, length=200)
print(generated_text)