import copy

from mmcv.runner.optimizer.builder import build_optimizer_constructor


def build_optimizer(model, optimizer_cfg):
    """Build optimizer from configs.

    *** Modifying `build_optimizer` in MMCV ***

    Args:
        model (:obj:`nn.Module`): The model with parameters to be optimized.
        optimizer_cfg (dict): The config dict of the optimizer.
            Positional fields are:
                - type: class name of the optimizer.
                - lr: base learning rate.
            Optional fields are:
                - any arguments of the corresponding optimizer type, e.g.,
                  weight_decay, momentum, etc.
                - paramwise_options: a dict with regular expression as keys
                  to match parameter names and a dict containing options as
                  values. Options include 6 fields: lr, lr_mult, momentum,
                  momentum_mult, weight_decay, weight_decay_mult.

    Returns:
        torch.optim.Optimizer: The initialized optimizer.

    Example:
        >>> model = torch.nn.modules.Conv1d(1, 1, 1)
        >>> paramwise_options = {
        >>>     '(bn|gn)(\d+)?.(weight|bias)': dict(weight_decay_mult=0.1),
        >>>     '\Ahead.': dict(lr_mult=10, momentum=0)}
        >>> optimizer_cfg = dict(type='SGD', lr=0.01, momentum=0.9,
        >>>                      weight_decay=0.0001,
        >>>                      paramwise_options=paramwise_options)
        >>> optimizer = build_optimizer(model, optimizer_cfg)
    """
    optimizer_cfg = copy.deepcopy(optimizer_cfg)
    constructor_type = optimizer_cfg.pop('constructor',
                                         'DefaultOptimizerConstructor')
    paramwise_cfg = optimizer_cfg.pop('paramwise_options', None)
    optim_constructor = build_optimizer_constructor(
        dict(
            type=constructor_type,
            optimizer_cfg=optimizer_cfg,
            paramwise_cfg=paramwise_cfg))
    optimizer = optim_constructor(model)
    return optimizer
