from __future__ import annotations

from functools import lru_cache
from typing import Any

import torch
from transformers import AutoModelForCausalLM, AutoTokenizer


def torch_dtype(name: str) -> torch.dtype:
    aliases = {
        "float16": torch.float16,
        "fp16": torch.float16,
        "bfloat16": torch.bfloat16,
        "bf16": torch.bfloat16,
        "float32": torch.float32,
        "fp32": torch.float32,
    }
    try:
        return aliases[name.lower()]
    except KeyError as exc:
        raise ValueError(f"Unsupported torch dtype {name!r}") from exc


def resolve_device(requested: str) -> torch.device:
    if requested == "auto":
        return torch.device("cuda" if torch.cuda.is_available() else "cpu")
    if requested.startswith("cuda") and not torch.cuda.is_available():
        raise RuntimeError("CUDA was requested but torch.cuda.is_available() is false.")
    return torch.device(requested)


@lru_cache(maxsize=None)
def load_tokenizer(model_id: str) -> Any:
    tokenizer = AutoTokenizer.from_pretrained(model_id)
    ensure_pad_token(tokenizer)
    return tokenizer


def load_model_and_tokenizer(model_id: str, *, device: str, dtype: str) -> tuple[torch.nn.Module, Any]:
    torch_device = resolve_device(device)
    tokenizer = load_tokenizer(model_id)

    model_dtype = torch_dtype(dtype)
    if torch_device.type == "cpu" and model_dtype in {torch.float16, torch.bfloat16}:
        model_dtype = torch.float32

    model = AutoModelForCausalLM.from_pretrained(
        model_id,
        torch_dtype=model_dtype,
        low_cpu_mem_usage=True,
    )
    model.to(torch_device)
    model.eval()
    print(
        f"Loaded {model_id} on {torch_device} "
        f"(requested_device={device}, cuda_available={torch.cuda.is_available()})",
        flush=True,
    )
    return model, tokenizer


def ensure_pad_token(tokenizer: Any) -> None:
    if tokenizer.pad_token_id is None:
        if tokenizer.eos_token_id is None:
            raise ValueError("Tokenizer has neither pad_token_id nor eos_token_id.")
        tokenizer.pad_token = tokenizer.eos_token