"""SGD optimizer factory."""

import torch.optim as optim
from taoTrain.core.base import BaseModel
from taoTrain.config import TrainingConfig
from .registry import register_optimizer


def _separate_parameters(model: BaseModel) -> tuple[list, list]:
    """
    Separate model parameters into decay and no-decay groups.
    
    Args:
        model: Model instance
    
    Returns:
        Tuple of (decay_params, no_decay_params)
    """
    decay_params = []
    no_decay_params = []
    
    for name, param in model.named_parameters():
        if not param.requires_grad:
            continue
        
        # Apply weight decay to all params except biases and layer norms
        if 'bias' in name or 'norm' in name:
            no_decay_params.append(param)
        else:
            decay_params.append(param)
    
    return decay_params, no_decay_params


@register_optimizer("sgd")
def create_sgd(model: BaseModel, config: TrainingConfig) -> optim.SGD:
    """
    Create SGD optimizer with weight decay applied selectively.
    
    Args:
        model: Model instance
        config: TrainingConfig
    
    Returns:
        SGD optimizer instance
    """
    optimizer_config = config.optimizer
    
    # Separate parameters for weight decay
    decay_params, no_decay_params = _separate_parameters(model)
    
    param_groups = [
        {"params": decay_params, "weight_decay": optimizer_config.weight_decay},
        {"params": no_decay_params, "weight_decay": 0.0},
    ]
    
    optimizer = optim.SGD(
        param_groups,
        lr=optimizer_config.learning_rate,
        momentum=optimizer_config.betas[0],  # Use first beta as momentum
    )
    
    return optimizer