import torch



class FlowMatchScheduler():

    def __init__(self, num_inference_steps=100, num_train_timesteps=1000, shift=3.0, sigma_max=1.0, sigma_min=0.003/1.002):
        self.num_train_timesteps = num_train_timesteps
        self.shift = shift
        self.sigma_max = sigma_max
        self.sigma_min = sigma_min
        self.set_timesteps(num_inference_steps)


    def set_timesteps(self, num_inference_steps=100, denoising_strength=1.0):
        sigma_start = self.sigma_min + (self.sigma_max - self.sigma_min) * denoising_strength
        self.sigmas = torch.linspace(sigma_start, self.sigma_min, num_inference_steps)
        self.sigmas = self.shift * self.sigmas / (1 + (self.shift - 1) * self.sigmas)
        self.timesteps = self.sigmas * self.num_train_timesteps


    def step(self, model_output, timestep, sample, to_final=False):
        if isinstance(timestep, torch.Tensor):
            timestep = timestep.cpu()
        timestep_id = torch.argmin((self.timesteps - timestep).abs())
        sigma = self.sigmas[timestep_id]
        if to_final or timestep_id + 1 >= len(self.timesteps):
            sigma_ = 0
        else:
            sigma_ = self.sigmas[timestep_id + 1]
        prev_sample = sample + model_output * (sigma_ - sigma)
        return prev_sample
    

    def return_to_timestep(self, timestep, sample, sample_stablized):
        # This scheduler doesn't support this function.
        pass
    
    
    def add_noise(self, original_samples, noise, timestep):
        if isinstance(timestep, torch.Tensor):
            timestep = timestep.cpu()
        timestep_id = torch.argmin((self.timesteps - timestep).abs())
        sigma = self.sigmas[timestep_id]
        sample = (1 - sigma) * original_samples + sigma * noise
        return sample
    

    def training_target(self, sample, noise, timestep):
        target = noise - sample
        return target