import torch
import torch.nn as nn
from transformers import GPT2Config, GPT2LMHeadModel


class GPTLMModel(nn.Module):

    def __init__(self,
                 hidden_size=768,
                 num_layers=12,
                 num_attention_heads=12,
                 max_seq_len=1024,
                 vocab_size=50257,
                 checkpoint=False):
        super().__init__()
        self.checkpoint = checkpoint
        self.model = GPT2LMHeadModel(
            GPT2Config(n_embd=hidden_size,
                       n_layer=num_layers,
                       n_head=num_attention_heads,
                       n_positions=max_seq_len,
                       n_ctx=max_seq_len,
                       vocab_size=vocab_size))
        if checkpoint:
            self.model.gradient_checkpointing_enable()

    def forward(self, input_ids, attention_mask):
        # Only return lm_logits
        return self.model(input_ids=input_ids, attention_mask=attention_mask, use_cache=not self.checkpoint)[0]


class GPTLMLoss(nn.Module):

    def __init__(self):
        super().__init__()
        self.loss_fn = nn.CrossEntropyLoss()

    def forward(self, logits, labels):
        shift_logits = logits[..., :-1, :].contiguous()
        shift_labels = labels[..., 1:].contiguous()
        # Flatten the tokens
        return self.loss_fn(shift_logits.view(-1, shift_logits.size(-1)), shift_labels.view(-1))


def gpt2_medium(checkpoint=False):
    return GPTLMModel(hidden_size=1024, num_layers=24, num_attention_heads=16, checkpoint=checkpoint)


def gpt2_xl(checkpoint=False):
    return GPTLMModel(hidden_size=1600, num_layers=48, num_attention_heads=32, checkpoint=checkpoint)
