# EfficientViT: Multi-Scale Linear Attention for High-Resolution Dense Prediction
# Han Cai, Junyan Li, Muyan Hu, Chuang Gan, Song Han
# International Conference on Computer Vision (ICCV), 2023

import copy
import math

import torch
import torch.nn as nn

from efficientvit.models.utils import is_parallel

__all__ = ["EMA"]


def update_ema(
    ema: nn.Module, new_state_dict: dict[str, torch.Tensor], decay: float
) -> None:
    for k, v in ema.state_dict().items():
        if v.dtype.is_floating_point:
            v -= (1.0 - decay) * (v - new_state_dict[k].detach())


class EMA:
    def __init__(self, model: nn.Module, decay: float, warmup_steps=2000):
        self.shadows = copy.deepcopy(
            model.module if is_parallel(model) else model
        ).eval()
        self.decay = decay
        self.warmup_steps = warmup_steps

        for p in self.shadows.parameters():
            p.requires_grad = False

    def step(self, model: nn.Module, global_step: int) -> None:
        with torch.no_grad():
            msd = (model.module if is_parallel(model) else model).state_dict()
            update_ema(
                self.shadows,
                msd,
                self.decay * (1 - math.exp(-global_step / self.warmup_steps)),
            )

    def state_dict(self) -> dict[float, dict[str, torch.Tensor]]:
        return {self.decay: self.shadows.state_dict()}

    def load_state_dict(self, state_dict: dict[float, dict[str, torch.Tensor]]) -> None:
        for decay in state_dict:
            if decay == self.decay:
                self.shadows.load_state_dict(state_dict[decay])