use crate::error::{DiffusionError, Result};
use crate::noise::NoiseGenerator;
use rtx_tensor::Tensor;

/// Different sampling schedulers for the reverse diffusion process
#[derive(Debug, Clone)]
pub enum SchedulerType {
    /// Denoising Diffusion Implicit Models (DDIM)
    DDIM { eta: f32 },
    /// DPM-Solver++
    DPMPlusPlus,
    /// Euler Ancestral
    EulerAncestral,
    /// DDPM (original)
    DDPM,
    /// UniPC (Unified Predictor-Corrector)
    UniPC {
        predictor_order: u8,
        corrector_order: u8,
        use_corrector: bool,
    },
}

/// Manages the reverse diffusion sampling process
#[derive(Debug, Clone)]
pub struct DiffusionScheduler {
    scheduler_type: SchedulerType,
    noise_generator: NoiseGenerator,
    num_inference_steps: u32,
    timesteps: Vec<u32>,
}

impl DiffusionScheduler {
    /// Create a new diffusion scheduler
    pub fn new(
        scheduler_type: SchedulerType,
        noise_generator: NoiseGenerator,
        num_inference_steps: u32,
    ) -> Result<Self> {
        if num_inference_steps == 0 {
            return Err(DiffusionError::Scheduler {
                message: "Number of inference steps must be greater than 0".to_string(),
            });
        }

        let timesteps = Self::compute_timesteps(&noise_generator, num_inference_steps)?;

        Ok(Self {
            scheduler_type,
            noise_generator,
            num_inference_steps,
            timesteps,
        })
    }

    /// Perform one step of the reverse diffusion process
    pub fn step(
        &self,
        model_output: &Tensor,
        timestep: u32,
        sample: &Tensor,
        generator: Option<&mut rand::rngs::StdRng>,
    ) -> Result<Tensor> {
        match &self.scheduler_type {
            SchedulerType::DDIM { eta } => self.ddim_step(model_output, timestep, sample, *eta),
            SchedulerType::DPMPlusPlus => self.dpm_plusplus_step(model_output, timestep, sample),
            SchedulerType::EulerAncestral => {
                self.euler_ancestral_step(model_output, timestep, sample, generator)
            }
            SchedulerType::DDPM => self.ddpm_step(model_output, timestep, sample, generator),
            SchedulerType::UniPC {
                predictor_order,
                corrector_order,
                use_corrector,
            } => self.unipc_step(
                model_output,
                timestep,
                sample,
                *predictor_order,
                *corrector_order,
                *use_corrector,
            ),
        }
    }

    /// Get the timesteps for inference
    pub fn timesteps(&self) -> &[u32] {
        &self.timesteps
    }

    /// Add noise to the initial sample (for training)
    pub fn add_noise(&self, original: &Tensor, noise: &Tensor, timestep: u32) -> Result<Tensor> {
        self.noise_generator.add_noise(original, noise, timestep)
    }

    /// Get variance at timestep
    pub fn get_variance(&self, timestep: u32) -> Result<f32> {
        self.noise_generator.get_variance(timestep)
    }

    /// Scale model input according to scheduler requirements
    pub fn scale_model_input(&self, sample: &Tensor, timestep: u32) -> Result<Tensor> {
        // For most schedulers, no scaling is needed
        // Some schedulers like UniPC may require scaling
        match &self.scheduler_type {
            SchedulerType::DDIM { .. }
            | SchedulerType::DDPM
            | SchedulerType::EulerAncestral
            | SchedulerType::DPMPlusPlus
            | SchedulerType::UniPC { .. } => Ok(sample.clone()),
        }
    }

    fn ddim_step(
        &self,
        model_output: &Tensor,
        timestep: u32,
        sample: &Tensor,
        eta: f32,
    ) -> Result<Tensor> {
        let (sqrt_alpha_cumprod, sqrt_one_minus_alpha_cumprod, alpha_cumprod, alpha_cumprod_prev) =
            self.noise_generator.get_schedule_params(timestep)?;

        // Predict x0 from model output (assuming model predicts noise)
        // x0 = (x_t - sqrt(1-alpha_cumprod) * noise) / sqrt(alpha_cumprod)
        let scaled_noise = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod)?;
        let x_minus_noise = sample.subtract(&scaled_noise)?;
        let pred_x0 = x_minus_noise.scalar_mul(1.0 / sqrt_alpha_cumprod)?;

        // Compute variance
        let variance = if eta == 0.0 {
            0.0
        } else {
            let beta_t = 1.0 - alpha_cumprod / alpha_cumprod_prev;
            eta * eta * beta_t
        };

        let std_dev = variance.sqrt();

        // Compute direction pointing to x_t
        let sqrt_one_minus_alpha_cumprod_prev = (1.0 - alpha_cumprod_prev).sqrt();
        let direction = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod_prev - std_dev)?;

        // Compute x_{t-1}
        let sqrt_alpha_cumprod_prev = alpha_cumprod_prev.sqrt();
        let prev_sample = pred_x0.scalar_mul(sqrt_alpha_cumprod_prev)?;
        let prev_sample = prev_sample.add(&direction)?;

        if std_dev > 0.0 {
            // Add noise if eta > 0
            let noise_shape = sample.shape();
            // For now, we'll assume zero noise - in practice, you'd generate random noise
            // This is a simplification for the initial implementation
        }

        Ok(prev_sample)
    }

    fn dpm_plusplus_step(
        &self,
        model_output: &Tensor,
        timestep: u32,
        sample: &Tensor,
    ) -> Result<Tensor> {
        // Simplified DPM++ implementation
        // In practice, this would involve more complex integration schemes
        let (sqrt_alpha_cumprod, sqrt_one_minus_alpha_cumprod, alpha_cumprod, alpha_cumprod_prev) =
            self.noise_generator.get_schedule_params(timestep)?;

        // Predict x0
        let scaled_noise = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod)?;
        let x_minus_noise = sample.subtract(&scaled_noise)?;
        let pred_x0 = x_minus_noise.scalar_mul(1.0 / sqrt_alpha_cumprod)?;

        // DPM++ uses a different integration scheme
        let sqrt_alpha_cumprod_prev = alpha_cumprod_prev.sqrt();
        let sqrt_one_minus_alpha_cumprod_prev = (1.0 - alpha_cumprod_prev).sqrt();

        let prev_sample = pred_x0.scalar_mul(sqrt_alpha_cumprod_prev)?;
        let noise_component = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod_prev)?;

        prev_sample
            .add(&noise_component)
            .map_err(DiffusionError::Tensor)
    }

    fn euler_ancestral_step(
        &self,
        model_output: &Tensor,
        timestep: u32,
        sample: &Tensor,
        _generator: Option<&mut rand::rngs::StdRng>,
    ) -> Result<Tensor> {
        let (sqrt_alpha_cumprod, sqrt_one_minus_alpha_cumprod, alpha_cumprod, alpha_cumprod_prev) =
            self.noise_generator.get_schedule_params(timestep)?;

        // Euler ancestral method
        let sigma = sqrt_one_minus_alpha_cumprod / sqrt_alpha_cumprod;
        let sigma_prev = ((1.0 - alpha_cumprod_prev) / alpha_cumprod_prev).sqrt();

        // Compute derivative
        let derivative = sample.add(&model_output.scalar_mul(sigma)?)?;
        let derivative = derivative.scalar_mul(-1.0 / sigma)?;

        // Euler step
        let dt = sigma_prev - sigma;
        let prev_sample = sample.add(&derivative.scalar_mul(dt)?)?;

        Ok(prev_sample)
    }

    fn ddpm_step(
        &self,
        model_output: &Tensor,
        timestep: u32,
        sample: &Tensor,
        _generator: Option<&mut rand::rngs::StdRng>,
    ) -> Result<Tensor> {
        let (sqrt_alpha_cumprod, sqrt_one_minus_alpha_cumprod, alpha_cumprod, alpha_cumprod_prev) =
            self.noise_generator.get_schedule_params(timestep)?;

        // DDPM step: predict x0 and sample from posterior
        let scaled_noise = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod)?;
        let x_minus_noise = sample.subtract(&scaled_noise)?;
        let pred_x0 = x_minus_noise.scalar_mul(1.0 / sqrt_alpha_cumprod)?;

        // Posterior mean
        let coeff1 = (alpha_cumprod_prev.sqrt() * (1.0 - alpha_cumprod / alpha_cumprod_prev))
            / (1.0 - alpha_cumprod);
        let coeff2 = ((alpha_cumprod / alpha_cumprod_prev).sqrt() * (1.0 - alpha_cumprod_prev))
            / (1.0 - alpha_cumprod);

        let mean = pred_x0.scalar_mul(coeff1)?;
        let mean = mean.add(&sample.scalar_mul(coeff2)?)?;

        // For now, return mean without noise (deterministic)
        // In practice, you'd add scaled random noise
        Ok(mean)
    }

    fn unipc_step(
        &self,
        model_output: &Tensor,
        timestep: u32,
        sample: &Tensor,
        predictor_order: u8,
        corrector_order: u8,
        use_corrector: bool,
    ) -> Result<Tensor> {
        // Simplified UniPC implementation for scheduler integration
        // This delegates to a basic predictor-corrector approach

        // First-order predictor step (similar to DDIM)
        let (sqrt_alpha_cumprod, sqrt_one_minus_alpha_cumprod, alpha_cumprod, alpha_cumprod_prev) =
            self.noise_generator.get_schedule_params(timestep)?;

        // Predict x0 from model output (assuming epsilon prediction)
        let scaled_noise = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod)?;
        let x_minus_noise = sample.subtract(&scaled_noise)?;
        let pred_x0 = x_minus_noise.scalar_mul(1.0 / sqrt_alpha_cumprod)?;

        // Compute predictor step
        let sqrt_alpha_cumprod_prev = alpha_cumprod_prev.sqrt();
        let sqrt_one_minus_alpha_cumprod_prev = (1.0 - alpha_cumprod_prev).sqrt();

        let predictor_result = pred_x0.scalar_mul(sqrt_alpha_cumprod_prev)?;
        let direction = model_output.scalar_mul(sqrt_one_minus_alpha_cumprod_prev)?;
        let predictor_result = predictor_result.add(&direction)?;

        // Apply corrector step if enabled
        if use_corrector {
            // Simple corrector: blend with original prediction
            let correction_weight = match corrector_order {
                1 => 0.1,
                2 => 0.05,
                _ => 0.02,
            };

            let corrected = predictor_result.scalar_mul(1.0 - correction_weight)?;
            let correction = model_output.scalar_mul(correction_weight)?;
            corrected.add(&correction).map_err(DiffusionError::Tensor)
        } else {
            Ok(predictor_result)
        }
    }

    fn compute_timesteps(
        noise_generator: &NoiseGenerator,
        num_inference_steps: u32,
    ) -> Result<Vec<u32>> {
        let num_train_timesteps = noise_generator.num_timesteps();

        if num_inference_steps > num_train_timesteps {
            return Err(DiffusionError::Scheduler {
                message: format!(
                    "Inference steps ({}) cannot exceed training timesteps ({})",
                    num_inference_steps, num_train_timesteps
                ),
            });
        }

        let step_size = num_train_timesteps / num_inference_steps;
        let mut timesteps = Vec::with_capacity(num_inference_steps as usize);

        for i in 0..num_inference_steps {
            let timestep = num_train_timesteps - 1 - (i * step_size);
            timesteps.push(timestep);
        }

        timesteps.reverse(); // Start from highest timestep and go to lowest
        Ok(timesteps)
    }
}