import torch
import torch.nn as nn
import torch.nn.functional as F
import math


class MultiHeadAttention(nn.Module):
    def __init__(self, config):
        super().__init__()
        assert config.n_embd % config.n_head == 0
        self.n_head = config.n_head
        self.n_embd = config.n_embd
        self.head_dim = config.n_embd // config.n_head
        self.dropout = config.dropout

        self.c_attn = nn.Linear(config.n_embd, 3 * config.n_embd, bias=config.bias)
        self.c_proj = nn.Linear(config.n_embd, config.n_embd, bias=config.bias)
        self.attn_dropout = nn.Dropout(config.dropout)
        self.resid_dropout = nn.Dropout(config.dropout)

        self.register_buffer(
            "mask",
            torch.tril(torch.ones(config.block_size, config.block_size)).view(
                1, 1, config.block_size, config.block_size
            ),
        )

    def forward(self, x):
        B, T, C = x.shape
        q, k, v = self.c_attn(x).split(self.n_embd, dim=2)

        q = q.view(B, T, self.n_head, self.head_dim).transpose(1, 2)
        k = k.view(B, T, self.n_head, self.head_dim).transpose(1, 2)
        v = v.view(B, T, self.n_head, self.head_dim).transpose(1, 2)

        scale = 1.0 / math.sqrt(self.head_dim)
        attn = (q @ k.transpose(-2, -1)) * scale
        attn = attn.masked_fill(self.mask[:, :, :T, :T] == 0, float("-inf"))
        attn = F.softmax(attn, dim=-1)
        attn = self.attn_dropout(attn)

        out = (attn @ v).transpose(1, 2).contiguous().view(B, T, C)
        return self.resid_dropout(self.c_proj(out))


class FeedForward(nn.Module):
    def __init__(self, config):
        super().__init__()
        self.net = nn.Sequential(
            nn.Linear(config.n_embd, 4 * config.n_embd, bias=config.bias),
            nn.GELU(),
            nn.Linear(4 * config.n_embd, config.n_embd, bias=config.bias),
            nn.Dropout(config.dropout),
        )

    def forward(self, x):
        return self.net(x)


class TransformerBlock(nn.Module):
    def __init__(self, config):
        super().__init__()
        self.ln1 = nn.LayerNorm(config.n_embd, bias=config.bias)
        self.attn = MultiHeadAttention(config)
        self.ln2 = nn.LayerNorm(config.n_embd, bias=config.bias)
        self.ff = FeedForward(config)

    def forward(self, x):
        x = x + self.attn(self.ln1(x))
        x = x + self.ff(self.ln2(x))
        return x


class GPTConfig:
    def __init__(
        self,
        vocab_size=65,
        block_size=256,
        n_layer=6,
        n_head=6,
        n_embd=384,
        dropout=0.2,
        bias=True,
    ):
        self.vocab_size = vocab_size
        self.block_size = block_size
        self.n_layer = n_layer
        self.n_head = n_head
        self.n_embd = n_embd
        self.dropout = dropout
        self.bias = bias


class GPT(nn.Module):
    def __init__(self, config):
        super().__init__()
        self.config = config

        self.transformer = nn.ModuleDict(
            {
                "wte": nn.Embedding(config.vocab_size, config.n_embd),
                "wpe": nn.Embedding(config.block_size, config.n_embd),
                "drop": nn.Dropout(config.dropout),
                "h": nn.ModuleList([TransformerBlock(config) for _ in range(config.n_layer)]),
                "ln_f": nn.LayerNorm(config.n_embd, bias=config.bias),
            }
        )
        self.lm_head = nn.Linear(config.n_embd, config.vocab_size, bias=False)
        self.transformer.wte.weight = self.lm_head.weight  # weight tying

        self.apply(self._init_weights)

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

    def forward(self, idx, targets=None):
        B, T = idx.shape
        assert T <= self.config.block_size

        pos = torch.arange(0, T, dtype=torch.long, device=idx.device)
        tok_emb = self.transformer.wte(idx)
        pos_emb = self.transformer.wpe(pos)
        x = self.transformer.drop(tok_emb + pos_emb)

        for block in self.transformer.h:
            x = block(x)
        x = self.transformer.ln_f(x)

        if targets is not None:
            logits = self.lm_head(x)
            loss = F.cross_entropy(logits.view(-1, logits.size(-1)), targets.view(-1), ignore_index=-1)
            return logits, loss

        logits = self.lm_head(x[:, [-1], :])
        return logits, None

    @torch.no_grad()
    def generate(self, idx, max_new_tokens, temperature=1.0, top_k=None):
        for _ in range(max_new_tokens):
            idx_cond = idx if idx.size(1) <= self.config.block_size else idx[:, -self.config.block_size:]
            logits, _ = self(idx_cond)
            logits = logits[:, -1, :] / temperature

            if top_k is not None:
                v, _ = torch.topk(logits, min(top_k, logits.size(-1)))
                logits[logits < v[:, [-1]]] = float("-inf")

            probs = F.softmax(logits, dim=-1)
            next_token = torch.multinomial(probs, num_samples=1)
            idx = torch.cat((idx, next_token), dim=1)
        return idx

    @torch.no_grad()
    def stream(self, idx, max_new_tokens, temperature=1.0, top_k=None):
        """Yield one token id at a time for real-time streaming."""
        for _ in range(max_new_tokens):
            idx_cond = idx if idx.size(1) <= self.config.block_size else idx[:, -self.config.block_size:]
            logits, _ = self(idx_cond)
            logits = logits[:, -1, :] / temperature

            if top_k is not None:
                v, _ = torch.topk(logits, min(top_k, logits.size(-1)))
                logits[logits < v[:, [-1]]] = float("-inf")

            probs = F.softmax(logits, dim=-1)
            next_token = torch.multinomial(probs, num_samples=1)
            idx = torch.cat((idx, next_token), dim=1)
            yield next_token.item()

    def num_params(self):
        return sum(p.numel() for p in self.parameters())