import torch
import torch.nn as nn
from torch.nn import functional as F
import mmap
import random
import pickle
import os


device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
print(device)
block_size = 128
batch_size = 32
max_iters = 4000
learning_rate = 3e-4
eval_every = 500
n_embd = 384
n_head = 8
n_layer = 8
dropout = 0.2

if not os.path.exists("./openwebtext/vocab.txt") or not os.path.exists("./openwebtext/train_split.txt") or not os.path.exists("./openwebtext/val_split.txt"):
    raise Exception("Please run extract.py first")
chars = ""
with open("./openwebtext/vocab.txt", 'r', encoding='utf-8') as f:
    text = f.read()
    chars = sorted(list(set(text)))
        
vocab_size = len(chars)

string_to_int = {ch: i for i, ch in enumerate(chars)}
int_to_string = {i: ch for i, ch in enumerate(chars)}

encode = lambda s: [string_to_int[ch] for ch in s]
decode = lambda x: ''.join([int_to_string[i] for i in x])
# memory map for using small snippets of text from a single file of any size
def get_random_chunk(split):
    filename = "./openwebtext/train_split.txt" if split == 'train' else "./openwebtext/val_split.txt"
    with open(filename, 'rb') as f:
        with mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ) as mm:
            # Determine the file size and a random position to start reading
            file_size = len(mm)
            start_pos = random.randint(0, (file_size) - block_size*batch_size)

            # Seek to the random position and read the block of text
            mm.seek(start_pos)
            block = mm.read(block_size*batch_size-1)

            # Decode the block to a string, ignoring any invalid byte sequences
            decoded_block = block.decode('utf-8', errors='ignore').replace('\r', '')
            
            # Train and test splits
            data = torch.tensor(encode(decoded_block), dtype=torch.long)
            
    return data


def get_batch(split):
    data = get_random_chunk(split)
    ix = torch.randint(len(data) - block_size, (batch_size,))
    x = torch.stack([data[i:i+block_size] for i in ix])
    y = torch.stack([data[i+1:i+block_size+1] for i in ix])
    x, y = x.to(device), y.to(device)
    return x, y

@torch.no_grad()
def estimate_loss():
    out = {}
    model.eval()
    for split in ['train', 'val']:
        losses = torch.zeros(eval_every)
        for k in range(eval_every):
            X, Y = get_batch(split)
            logits, loss = model(X, Y)
            losses[k] = loss.item()
        out[split] = losses.mean()
    model.train()
    return out


class Head(nn.Module):
    """ one head of self-attention """

    def __init__(self, head_size):
        super().__init__()
        self.key = nn.Linear(n_embd, head_size, bias=False)
        self.query = nn.Linear(n_embd, head_size, bias=False)
        self.value = nn.Linear(n_embd, head_size, bias=False)
        self.register_buffer('tril', torch.tril(torch.ones(block_size, block_size)))

        self.dropout = nn.Dropout(dropout)

    def forward(self, x):
        # input of size (batch, time-step, channels)
        # output of size (batch, time-step, head size)
        B,T,C = x.shape
        k = self.key(x)   # (B,T,hs)
        q = self.query(x) # (B,T,hs)
        # compute attention scores ("affinities")
        wei = q @ k.transpose(-2,-1) * k.shape[-1]**-0.5 # (B, T, hs) @ (B, hs, T) -> (B, T, T)
        wei = wei.masked_fill(self.tril[:T, :T] == 0, float('-inf')) # (B, T, T)
        wei = F.softmax(wei, dim=-1) # (B, T, T)
        wei = self.dropout(wei)
        # perform the weighted aggregation of the values
        v = self.value(x) # (B,T,hs)
        out = wei @ v # (B, T, T) @ (B, T, hs) -> (B, T, hs)
        return out

# [1, 0, 0]
# [1, 0.6, 0]
# [1, 0.6, 0.4]
class MultiHeadAttention(nn.Module):
    """ multiple heads of self-attention in parallel """

    def __init__(self, num_heads, head_size):
        super().__init__()
        self.heads = nn.ModuleList([Head(head_size) for _ in range(num_heads)])
        self.proj = nn.Linear(head_size * num_heads, n_embd)
        self.dropout = nn.Dropout(dropout)

    def forward(self, x):
        out = torch.cat([h(x) for h in self.heads], dim=-1) # (B, T, F) -> (B, T, [h1, h1, h1, h1, h2, h2, h2, h2, h3, h3, h3, h3])
        out = self.dropout(self.proj(out))
        return out
    

class FeedFoward(nn.Module):
    """ a simple linear layer followed by a non-linearity """

    def __init__(self, n_embd):
        super().__init__()
        self.net = nn.Sequential(
            nn.Linear(n_embd, 4 * n_embd),
            nn.ReLU(),
            nn.Linear(4 * n_embd, n_embd),
            nn.Dropout(dropout),
        )

    def forward(self, x):
        return self.net(x)
    
class Block(nn.Module):
    """ Transformer block: communication followed by computation """

    def __init__(self, n_embd, n_head):
        # n_embd: embedding dimension, n_head: the number of heads we'd like
        super().__init__()
        head_size = n_embd // n_head
        self.sa = MultiHeadAttention(n_head, head_size)
        self.ffwd = FeedFoward(n_embd)
        self.ln1 = nn.LayerNorm(n_embd)
        self.ln2 = nn.LayerNorm(n_embd)

    def forward(self, x):
        y = self.sa(x)
        x = self.ln1(x + y)
        y = self.ffwd(x)
        x = self.ln2(x + y)
        return x
    
class GPTLanguageModel(nn.Module):
    def __init__(self, vocab_size):
        super().__init__()
        self.token_embedding_table = nn.Embedding(vocab_size, n_embd)
        self.position_embedding_table = nn.Embedding(block_size, n_embd)
        self.blocks = nn.Sequential(*[Block(n_embd, n_head=n_head) for _ in range(n_layer)])
        self.ln_f = nn.LayerNorm(n_embd) # final layer norm
        self.lm_head = nn.Linear(n_embd, vocab_size)
        
        
        self.apply(self._init_weights)

    def _init_weights(self, module):
        if isinstance(module, nn.Linear):
            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)
            if module.bias is not None:
                torch.nn.init.zeros_(module.bias)
        elif isinstance(module, nn.Embedding):
            torch.nn.init.normal_(module.weight, mean=0.0, std=0.02)

    def forward(self, index, targets=None):
        B, T = index.shape
        
        
        # idx and targets are both (B,T) tensor of integers
        tok_emb = self.token_embedding_table(index) # (B,T,C)
        pos_emb = self.position_embedding_table(torch.arange(T, device=device)) # (T,C)
        x = tok_emb + pos_emb # (B,T,C)
        x = self.blocks(x) # (B,T,C)
        x = self.ln_f(x) # (B,T,C)
        logits = self.lm_head(x) # (B,T,vocab_size)
        
        if targets is None:
            loss = None
        else:
            B, T, C = logits.shape
            logits = logits.view(B*T, C) # reshape to what torch.cross_entropy expects
            targets = targets.view(B*T)
            loss = F.cross_entropy(logits, targets) 
        return logits, loss
    
    def generate(self, index, max_new_tokens):
        # index is (B, T) array of indices in the current context
        for _ in range(max_new_tokens):
            # crop idx to the last block_size tokens
            index_cond = index[:, -block_size:]
            # get the predictions
            logits, loss = self.forward(index_cond)
            # focus only on the last time step
            logits = logits[:, -1, :] # becomes (B, C)
            # apply softmax to get probabilities
            probs = F.softmax(logits, dim=-1) # (B, C)
            # sample from the distribution
            index_next = torch.multinomial(probs, num_samples=1) # (B, 1)
            # append sampled index to the running sequence
            index = torch.cat((index, index_next), dim=1) # (B, T+1)
        return index

model = GPTLanguageModel(vocab_size).to(device)

model_pickle_path = './model.pkl'
if os.path.exists(model_pickle_path):
    print('loading model parameters...')
    with open(model_pickle_path, 'rb') as f:
        model = pickle.load(f)
    print('loaded successfully!')
# create a PyTorch optimizer
optimizer = torch.optim.AdamW(model.parameters(), lr=learning_rate)

for iter in range(max_iters):
    if iter % eval_every == 0:
        losses = estimate_loss()
        print(f"step: {iter}, train loss: {losses['train']:.3f}, val loss: {losses['val']:.3f}")

    # sample a batch of data
    xb, yb = get_batch('train')

    # evaluate the loss
    logits, loss = model.forward(xb, yb)
    optimizer.zero_grad(set_to_none=True)
    loss.backward()
    optimizer.step()
print(loss.item())

with open(model_pickle_path, 'wb') as f:
    pickle.dump(model, f)
print('model saved')