from __future__ import annotations

from typing import Any

from torch import nn
from torch.optim import Adam, AdamW, Optimizer
from torch.optim.lr_scheduler import (
    ConstantLR,
    CosineAnnealingLR,
    LinearLR,
    MultiStepLR,
    PolynomialLR,
    SequentialLR,
)


OPTIMIZER_REGISTRY = {
    "adam": Adam,
    "adamw": AdamW,
}


SCHEDULER_REGISTRY = {
    "linear": LinearLR,
    "cosine": CosineAnnealingLR,
    "poly": PolynomialLR,
    "multistep": MultiStepLR,
    "constant": ConstantLR,
}


DEFAULT_OPTIM_CONFIG = {
    "optimizer": {
        "name": "adamw",
        "lr": 5e-5,
        "weight_decay": 0.05,
        "betas": (0.9, 0.999),
    },
    "scheduler": {
        "name": "cosine",
        "warmup": {
            "name": "linear",
            "params": {
                "start_factor": 0.1,
                "total_iters": 10,
            },
        },
        "params": {
            "T_max": 90,
            "eta_min": 1e-6,
        },
    },
}


def _trainable_parameters(model: nn.Module) -> list[nn.Parameter]:
    return [param for param in model.parameters() if param.requires_grad]


def build_optimizer(model: nn.Module, config: dict[str, Any]) -> Optimizer:
    """Build a PyTorch optimizer from a yaml-style config dict."""

    if not isinstance(config, dict) or not config:
        raise ValueError("Optimizer config must be a non-empty dict.")

    name = config.get("name", "adamw")
    if not isinstance(name, str):
        raise ValueError("Optimizer config field 'name' must be a string.")

    optimizer_cls = OPTIMIZER_REGISTRY.get(name.lower())
    if optimizer_cls is None:
        raise ValueError(
            f"Unsupported optimizer '{name}'. Expected one of: {', '.join(OPTIMIZER_REGISTRY)}."
        )

    if "lr" not in config:
        raise ValueError("Optimizer config must provide 'lr'.")

    kwargs = {key: value for key, value in config.items() if key != "name"}
    return optimizer_cls(_trainable_parameters(model), **kwargs)


def _build_single_scheduler(optimizer: Optimizer, name: str, params: dict[str, Any] | None):
    scheduler_cls = SCHEDULER_REGISTRY.get(name.lower())
    if scheduler_cls is None:
        raise ValueError(
            f"Unsupported scheduler '{name}'. Expected one of: {', '.join(SCHEDULER_REGISTRY)}."
        )

    scheduler_params = {} if params is None else dict(params)
    if not isinstance(scheduler_params, dict):
        raise ValueError("Scheduler config field 'params' must be a dict if provided.")

    return scheduler_cls(optimizer, **scheduler_params)


def build_scheduler(optimizer: Optimizer, config: dict[str, Any] | None):
    """Build a PyTorch scheduler from a yaml-style config dict."""

    if config is None:
        return None
    if not isinstance(config, dict) or not config:
        raise ValueError("Scheduler config must be a non-empty dict.")

    name = config.get("name", "cosine")
    if not isinstance(name, str):
        raise ValueError("Scheduler config field 'name' must be a string.")

    main_scheduler = _build_single_scheduler(optimizer, name, config.get("params"))

    warmup = config.get("warmup")
    if warmup is None:
        return main_scheduler

    if not isinstance(warmup, dict) or not warmup:
        raise ValueError("Scheduler config field 'warmup' must be a non-empty dict if provided.")

    warmup_name = warmup.get("name", "linear")
    if not isinstance(warmup_name, str):
        raise ValueError("Warmup scheduler field 'name' must be a string.")

    warmup_params = warmup.get("params")
    if warmup_params is None:
        raise ValueError("Warmup scheduler must provide a 'params' dict.")
    if not isinstance(warmup_params, dict):
        raise ValueError("Warmup scheduler field 'params' must be a dict.")

    total_iters = warmup_params.get("total_iters")
    if not isinstance(total_iters, int) or total_iters <= 0:
        raise ValueError("Warmup scheduler requires integer 'total_iters' > 0.")

    warmup_scheduler = _build_single_scheduler(optimizer, warmup_name, warmup_params)
    return SequentialLR(
        optimizer,
        schedulers=[warmup_scheduler, main_scheduler],
        milestones=[total_iters],
    )


__all__ = [
    "DEFAULT_OPTIM_CONFIG",
    "OPTIMIZER_REGISTRY",
    "SCHEDULER_REGISTRY",
    "build_optimizer",
    "build_scheduler",
]