"""
Step-by-step training script for nano GPT — SELF-CONTAINED.

Contains both the model architecture and training code so it can run
as a single file in an HF Job.
"""

import os
import math
import time
import torch
import torch.nn as nn
from torch.nn import functional as F
from dataclasses import dataclass

# =============================================================================
# PART 1: MODEL
# =============================================================================

@dataclass
class GPTConfig:
    block_size: int = 256
    vocab_size: int = 65
    n_layer: int = 4
    n_head: int = 4
    n_embd: int = 256
    dropout: float = 0.0

class CausalSelfAttention(nn.Module):
    def __init__(self, config: GPTConfig):
        super().__init__()
        assert config.n_embd % config.n_head == 0
        self.c_attn = nn.Linear(config.n_embd, 3 * config.n_embd)
        self.c_proj = nn.Linear(config.n_embd, config.n_embd)
        self.n_head = config.n_head
        self.n_embd = config.n_embd
        self.register_buffer(
            "bias",
            torch.tril(torch.ones(config.block_size, config.block_size))
                 .view(1, 1, config.block_size, config.block_size)
        )

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        B, T, C = x.size()
        qkv = self.c_attn(x)
        q, k, v = qkv.split(self.n_embd, dim=2)
        head_size = C // self.n_head
        q = q.view(B, T, self.n_head, head_size).transpose(1, 2)
        k = k.view(B, T, self.n_head, head_size).transpose(1, 2)
        v = v.view(B, T, self.n_head, head_size).transpose(1, 2)
        att = (q @ k.transpose(-2, -1)) * (1.0 / (head_size ** 0.5))
        att = att.masked_fill(self.bias[:, :, :T, :T] == 0, float("-inf"))
        att = F.softmax(att, dim=-1)
        y = att @ v
        y = y.transpose(1, 2).contiguous().view(B, T, C)
        y = self.c_proj(y)
        return y

class MLP(nn.Module):
    def __init__(self, config: GPTConfig):
        super().__init__()
        self.c_fc   = nn.Linear(config.n_embd, 4 * config.n_embd)
        self.gelu   = nn.GELU()
        self.c_proj = nn.Linear(4 * config.n_embd, config.n_embd)
        self.dropout = nn.Dropout(config.dropout)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        x = self.c_fc(x)
        x = self.gelu(x)
        x = self.c_proj(x)
        x = self.dropout(x)
        return x

class Block(nn.Module):
    def __init__(self, config: GPTConfig):
        super().__init__()
        self.ln_1 = nn.LayerNorm(config.n_embd)
        self.attn = CausalSelfAttention(config)
        self.ln_2 = nn.LayerNorm(config.n_embd)
        self.mlp  = MLP(config)

    def forward(self, x: torch.Tensor) -> torch.Tensor:
        x = x + self.attn(self.ln_1(x))
        x = x + self.mlp(self.ln_2(x))
        return x

class GPT(nn.Module):
    def __init__(self, config: GPTConfig):
        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),
            "h":   nn.ModuleList([Block(config) for _ in range(config.n_layer)]),
            "ln_f": nn.LayerNorm(config.n_embd),
        })
        self.lm_head = nn.Linear(config.n_embd, config.vocab_size, bias=False)
        self.transformer.wte.weight = self.lm_head.weight
        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, idx, targets=None):
        B, T = idx.size()
        assert T <= self.config.block_size
        pos = torch.arange(0, T, dtype=torch.long, device=idx.device)
        x = self.transformer.wte(idx) + self.transformer.wpe(pos)
        for block in self.transformer.h:
            x = block(x)
        x = self.transformer.ln_f(x)
        logits = self.lm_head(x)
        loss = None
        if targets is not None:
            loss = F.cross_entropy(logits.view(-1, logits.size(-1)), targets.view(-1), ignore_index=-1)
        return logits, loss

    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, :]
            if top_k is not None:
                v, _ = torch.topk(logits, top_k, dim=-1)
                logits[logits < v[:, [-1]]] = float("-inf")
            probs = F.softmax(logits / temperature, dim=-1)
            idx_next = torch.multinomial(probs, num_samples=1)
            idx = torch.cat((idx, idx_next), dim=1)
        return idx

# =============================================================================
# PART 2: TRAINING
# =============================================================================

BATCH_SIZE = 64
BLOCK_SIZE = 256
MAX_ITERS = 5000
LEARNING_RATE = 1e-3
WARMUP_ITERS = 200
LR_DECAY_ITERS = 5000
MIN_LR = 1e-4
EVAL_INTERVAL = 500
EVAL_ITERS = 200
GRAD_CLIP = 1.0

device = "cuda" if torch.cuda.is_available() else "cpu"
print(f"Using device: {device}")

# Download data if needed
data_path = "data.pt"
if not os.path.exists(data_path):
    import urllib.request
    print("Downloading tiny Shakespeare...")
    url = "https://raw.githubusercontent.com/karpathy/char-rnn/master/data/tinyshakespeare/input.txt"
    urllib.request.urlretrieve(url, "input.txt")

    with open("input.txt", "r", encoding="utf-8") as f:
        text = f.read()

    chars = sorted(list(set(text)))
    vocab_size = len(chars)
    stoi = {ch: i for i, ch in enumerate(chars)}
    itos = {i: ch for i, ch in enumerate(chars)}
    encode = lambda s: [stoi[c] for c in s]
    data = torch.tensor(encode(text), dtype=torch.long)
    n = int(0.9 * len(data))
    train_data = data[:n]
    val_data = data[n:]
    torch.save({
        "train": train_data,
        "val": val_data,
        "vocab_size": vocab_size,
        "chars": chars,
        "stoi": stoi,
        "itos": itos,
    }, data_path)
    print("Data saved.")

data = torch.load(data_path, weights_only=False)
train_data = data["train"]
val_data   = data["val"]
vocab_size = data["vocab_size"]
chars      = data["chars"]
stoi       = data["stoi"]
itos       = data["itos"]

print(f"Vocab size : {vocab_size}")
print(f"Train tokens: {len(train_data):,}")
print(f"Val tokens  : {len(val_data):,}")

def get_batch(split: str):
    data_split = train_data if split == "train" else val_data
    ix = torch.randint(len(data_split) - BLOCK_SIZE, (BATCH_SIZE,))
    x = torch.stack([data_split[i : i + BLOCK_SIZE] for i in ix])
    y = torch.stack([data_split[i + 1 : i + BLOCK_SIZE + 1] for i in ix])
    x, y = x.to(device), y.to(device)
    return x, y

def get_lr(iteration: int) -> float:
    if iteration < WARMUP_ITERS:
        return LEARNING_RATE * (iteration + 1) / WARMUP_ITERS
    if iteration > LR_DECAY_ITERS:
        return MIN_LR
    decay_ratio = (iteration - WARMUP_ITERS) / (LR_DECAY_ITERS - WARMUP_ITERS)
    coeff = 0.5 * (1.0 + math.cos(math.pi * decay_ratio))
    return MIN_LR + coeff * (LEARNING_RATE - MIN_LR)

config = GPTConfig(
    block_size=BLOCK_SIZE,
    vocab_size=vocab_size,
    n_layer=6,
    n_head=6,
    n_embd=384,
    dropout=0.0,
)

model = GPT(config)
model.to(device)

param_count = sum(p.numel() for p in model.parameters())
print(f"\nModel config: {config}")
print(f"Total parameters: {param_count / 1e6:.2f} M")

decay_params = []
no_decay_params = []
for name, param in model.named_parameters():
    if param.dim() >= 2:
        decay_params.append(param)
    else:
        no_decay_params.append(param)

optim_groups = [
    {"params": decay_params, "weight_decay": 0.1},
    {"params": no_decay_params, "weight_decay": 0.0},
]

optimizer = torch.optim.AdamW(optim_groups, lr=LEARNING_RATE, betas=(0.9, 0.95), eps=1e-8)

@torch.no_grad()
def estimate_loss():
    out = {}
    model.eval()
    for split in ["train", "val"]:
        losses = torch.zeros(EVAL_ITERS)
        for k in range(EVAL_ITERS):
            xb, yb = get_batch(split)
            _, loss = model(xb, yb)
            losses[k] = loss.item()
        out[split] = losses.mean()
    model.train()
    return out

print("\n" + "=" * 60)
print("Starting training...")
print("=" * 60)

best_val_loss = float("inf")
start_time = time.time()

for iter_num in range(MAX_ITERS):
    lr = get_lr(iter_num)
    for param_group in optimizer.param_groups:
        param_group["lr"] = lr

    if iter_num % EVAL_INTERVAL == 0 or iter_num == MAX_ITERS - 1:
        losses = estimate_loss()
        elapsed = time.time() - start_time
        print(
            f"step {iter_num:5d} | "
            f"train loss {losses['train']:.4f} | "
            f"val loss {losses['val']:.4f} | "
            f"lr {lr:.2e} | "
            f"time {elapsed:.1f}s"
        )

        if losses["val"] < best_val_loss:
            best_val_loss = losses["val"]
            torch.save({
                "model_state_dict": model.state_dict(),
                "config": config,
                "vocab_size": vocab_size,
                "chars": chars,
                "stoi": stoi,
                "itos": itos,
            }, "best.pt")
            print(f"  -> Saved new best model (val_loss={best_val_loss:.4f})")

    xb, yb = get_batch("train")
    logits, loss = model(xb, yb)
    optimizer.zero_grad(set_to_none=True)
    loss.backward()
    torch.nn.utils.clip_grad_norm_(model.parameters(), GRAD_CLIP)
    optimizer.step()

losses = estimate_loss()
print(f"\nFinal -> train loss {losses['train']:.4f} | val loss {losses['val']:.4f}")

model.eval()
start_token = stoi["\n"]
context = torch.zeros((1, 1), dtype=torch.long, device=device)
context[0, 0] = start_token

with torch.no_grad():
    generated = model.generate(context, max_new_tokens=500, temperature=1.0, top_k=40)

decode = lambda l: "".join([itos[i] for i in l])

print("\n--- Generated text ---\n")
print(decode(generated[0].tolist()))
print("\n--- End ---")

print("\nTraining complete! Best checkpoint saved to: best.pt")