import json
import re
from dataclasses import dataclass
from pathlib import Path

import torch
from tokenizers import Tokenizer

from model import GPT, GPTConfig


EOS_TOKEN = "<|endoftext|>"
DEFAULT_PROMPT = "Once upon a time"
DEFAULT_TARGET_TOKENS = 120
DEFAULT_EXTRA_TOKENS = 80
DEFAULT_TEMPERATURE = 0.8
DEFAULT_TOP_K = 40

MAX_PROMPT_TOKENS = 256
MAX_TARGET_TOKENS = 500
MAX_EXTRA_TOKENS = 200
MIN_TEMPERATURE = 0.1
MAX_TEMPERATURE = 2.0

END_PUNCTUATION = (".", "!", "?")
STORY_START_PATTERN = re.compile(
    r"\b(?:once upon a time|there was once|there once was)\b",
    re.IGNORECASE,
)


@dataclass(frozen=True)
class GenerationResult:
    story: str
    generated_tokens: int


def get_device() -> torch.device:
    if torch.backends.mps.is_available():
        return torch.device("mps")

    if torch.cuda.is_available():
        return torch.device("cuda")

    return torch.device("cpu")


def _build_config(config_data: object) -> GPTConfig:
    if isinstance(config_data, GPTConfig):
        return config_data

    if isinstance(config_data, dict):
        return GPTConfig(**config_data)

    if hasattr(config_data, "__dict__"):
        return GPTConfig(**vars(config_data))

    raise ValueError("Checkpoint contains an unsupported model configuration.")


def load_training_checkpoint(
    checkpoint_path: str | Path,
    device: torch.device,
) -> GPT:
    checkpoint = torch.load(
        Path(checkpoint_path),
        map_location="cpu",
        weights_only=False,
    )

    if not isinstance(checkpoint, dict):
        raise ValueError("Checkpoint must contain a dictionary.")

    if "model_state" not in checkpoint or "config" not in checkpoint:
        raise ValueError("Checkpoint is missing model_state or config.")

    model = GPT(_build_config(checkpoint["config"]))
    model.load_state_dict(checkpoint["model_state"])
    model.to(device)
    model.eval()
    return model


def load_exported_model(
    config_path: str | Path,
    weights_path: str | Path,
    device: torch.device,
) -> GPT:
    config_data = json.loads(Path(config_path).read_text(encoding="utf-8"))
    model = GPT(_build_config(config_data))
    state_dict = torch.load(
        Path(weights_path),
        map_location="cpu",
        weights_only=True,
    )

    if not isinstance(state_dict, dict):
        raise ValueError("Exported weights must contain a state dictionary.")

    model.load_state_dict(state_dict)
    model.to(device)
    model.eval()
    return model


def normalize_text(text: str) -> str:
    text = text.replace(EOS_TOKEN, "")
    text = re.sub(r"\s+", " ", text)
    text = re.sub(r"\s+([,.;:!?])", r"\1", text)
    return text.strip()


def ends_with_sentence(text: str) -> bool:
    text = normalize_text(text)
    return bool(re.search(r"""[.!?](?:["'\u2019\u201d])?$""", text))


def trim_repeated_story(text: str, prompt: str = "") -> str:
    text = normalize_text(text)
    normalized_prompt = normalize_text(prompt)
    prompt_boundary = (
        len(normalized_prompt) if text.startswith(normalized_prompt) else 0
    )

    for match in STORY_START_PATTERN.finditer(text):
        if match.start() < prompt_boundary or match.start() == 0:
            continue

        candidate = text[: match.start()].strip()
        if len(candidate.split()) >= 20:
            return candidate

    return text


def trim_to_last_sentence(text: str, prompt: str = "") -> str:
    text = normalize_text(text)
    normalized_prompt = normalize_text(prompt)
    last_position = max(text.rfind(mark) for mark in END_PUNCTUATION)

    if last_position == -1:
        return text

    if text.startswith(normalized_prompt) and last_position < len(normalized_prompt):
        return text

    return text[: last_position + 1].strip()


def clean_story(text: str, prompt: str = "") -> str:
    text = trim_repeated_story(text, prompt=prompt)
    text = trim_to_last_sentence(text, prompt=prompt)
    return normalize_text(text)


def validate_generation_inputs(
    tokenizer: Tokenizer,
    prompt: object,
    target_tokens: object,
    extra_tokens: object,
    temperature: object,
    top_k: object,
) -> tuple[str, list[int], int, int, float, int]:
    if not isinstance(prompt, str):
        raise ValueError("prompt must be a string.")

    if type(target_tokens) is not int:
        raise ValueError("tokens must be an integer.")

    if type(extra_tokens) is not int:
        raise ValueError("extra_tokens must be an integer.")

    if isinstance(temperature, bool) or not isinstance(temperature, (int, float)):
        raise ValueError("temperature must be a number.")

    if type(top_k) is not int:
        raise ValueError("top_k must be an integer.")

    if not 1 <= target_tokens <= MAX_TARGET_TOKENS:
        raise ValueError(f"tokens must be between 1 and {MAX_TARGET_TOKENS}.")

    if not 0 <= extra_tokens <= MAX_EXTRA_TOKENS:
        raise ValueError(
            f"extra_tokens must be between 0 and {MAX_EXTRA_TOKENS}."
        )

    temperature = float(temperature)
    if not MIN_TEMPERATURE <= temperature <= MAX_TEMPERATURE:
        raise ValueError(
            f"temperature must be between {MIN_TEMPERATURE} and {MAX_TEMPERATURE}."
        )

    vocab_size = tokenizer.get_vocab_size()
    if not 1 <= top_k <= vocab_size:
        raise ValueError(f"top_k must be between 1 and {vocab_size}.")

    prompt = prompt.strip() or DEFAULT_PROMPT
    prompt_ids = tokenizer.encode(prompt).ids

    if not prompt_ids:
        raise ValueError("prompt must contain text.")

    if len(prompt_ids) > MAX_PROMPT_TOKENS:
        raise ValueError(
            f"prompt must not exceed {MAX_PROMPT_TOKENS} encoded tokens."
        )

    return (
        prompt,
        prompt_ids,
        target_tokens,
        extra_tokens,
        temperature,
        top_k,
    )


def sample_next_token(
    model: GPT,
    input_ids: torch.Tensor,
    temperature: float,
    top_k: int,
) -> torch.Tensor:
    idx_cond = input_ids[:, -model.config.block_size :]
    logits, _ = model(idx_cond)
    logits = logits[:, -1, :] / temperature

    values, _ = torch.topk(logits, min(top_k, logits.size(-1)))
    logits = logits.masked_fill(logits < values[:, [-1]], float("-inf"))

    probabilities = torch.softmax(logits, dim=-1)
    return torch.multinomial(probabilities, num_samples=1)


@torch.no_grad()
def generate_story(
    model: GPT,
    tokenizer: Tokenizer,
    prompt: object = DEFAULT_PROMPT,
    target_tokens: object = DEFAULT_TARGET_TOKENS,
    extra_tokens: object = DEFAULT_EXTRA_TOKENS,
    temperature: object = DEFAULT_TEMPERATURE,
    top_k: object = DEFAULT_TOP_K,
    device: torch.device | None = None,
) -> GenerationResult:
    (
        prompt,
        prompt_ids,
        target_tokens,
        extra_tokens,
        temperature,
        top_k,
    ) = validate_generation_inputs(
        tokenizer=tokenizer,
        prompt=prompt,
        target_tokens=target_tokens,
        extra_tokens=extra_tokens,
        temperature=temperature,
        top_k=top_k,
    )

    if device is None:
        device = next(model.parameters()).device

    input_ids = torch.tensor([prompt_ids], dtype=torch.long, device=device)
    eos_token_id = tokenizer.token_to_id(EOS_TOKEN)
    generated_tokens = 0

    for generated_tokens in range(1, target_tokens + extra_tokens + 1):
        next_id = sample_next_token(
            model=model,
            input_ids=input_ids,
            temperature=temperature,
            top_k=top_k,
        )
        input_ids = torch.cat((input_ids, next_id), dim=1)

        if eos_token_id is not None and next_id.item() == eos_token_id:
            break

        if generated_tokens >= target_tokens:
            current_text = tokenizer.decode(
                input_ids[0].tolist(),
                skip_special_tokens=False,
            )
            if ends_with_sentence(current_text):
                break

    generated_ids = input_ids[0].tolist()
    if eos_token_id is not None and eos_token_id in generated_ids:
        generated_ids = generated_ids[: generated_ids.index(eos_token_id)]

    text = tokenizer.decode(generated_ids, skip_special_tokens=False)
    story = clean_story(text, prompt=prompt)

    if not story:
        raise RuntimeError("The model generated an empty result.")

    return GenerationResult(
        story=story,
        generated_tokens=generated_tokens,
    )