// Mesh refinement algorithms

use crate::error::{CfdError, CfdResult};

/// Refinement strategy
#[derive(Debug, Clone, Copy)]
pub enum RefinementStrategy {
    /// Uniform refinement - refine all cells
    Uniform,
    /// Adaptive refinement based on error indicators
    Adaptive,
    /// Refine specific regions
    Regional,
}

/// Refinement criteria
#[derive(Debug, Clone)]
pub struct RefinementCriteria {
    /// Maximum allowed error per cell
    pub max_error: f64,
    /// Minimum cell size
    pub min_cell_size: f64,
    /// Maximum cell size
    pub max_cell_size: f64,
    /// Maximum refinement levels
    pub max_levels: usize,
}

impl Default for RefinementCriteria {
    fn default() -> Self {
        Self {
            max_error: 1e-3,
            min_cell_size: 1e-6,
            max_cell_size: 1e6,
            max_levels: 10,
        }
    }
}

/// Adaptive mesh refinement implementation
pub struct AdaptiveRefinement {
    criteria: RefinementCriteria,
    current_level: usize,
}

impl AdaptiveRefinement {
    /// Create new adaptive refinement
    #[must_use]
    pub fn new(criteria: RefinementCriteria) -> Self {
        Self {
            criteria,
            current_level: 0,
        }
    }

    /// Check if refinement is needed
    #[must_use]
    pub fn needs_refinement(&self, error: f64, cell_size: f64, current_level: usize) -> bool {
        error > self.criteria.max_error
            && cell_size > self.criteria.min_cell_size
            && current_level < self.criteria.max_levels
    }

    /// Mark cells for refinement based on error indicators
    pub fn mark_cells_for_refinement(
        &self,
        errors: &[f64],
        cell_sizes: &[f64],
        cell_levels: &[usize],
    ) -> CfdResult<Vec<usize>> {
        if errors.len() != cell_sizes.len() || errors.len() != cell_levels.len() {
            return Err(CfdError::mesh(
                "Error, cell size, and level arrays must have same length",
            ));
        }

        let mut marked_cells = Vec::new();

        for (i, (&error, (&size, &level))) in errors
            .iter()
            .zip(cell_sizes.iter().zip(cell_levels.iter()))
            .enumerate()
        {
            if self.needs_refinement(error, size, level) {
                marked_cells.push(i);
            }
        }

        Ok(marked_cells)
    }

    /// Compute gradient-based error indicator
    pub fn compute_gradient_error_indicator(
        &self,
        velocity_field: &[nalgebra::Vector3<f64>],
    ) -> CfdResult<Vec<f64>> {
        let mut errors = Vec::with_capacity(velocity_field.len());

        for velocity in velocity_field {
            // Use velocity magnitude as a simple gradient indicator
            // In practice, this would be computed from actual velocity gradients
            let error = velocity.magnitude();
            errors.push(error);
        }

        Ok(errors)
    }

    /// Compute residual-based error indicator
    pub fn compute_residual_error_indicator(
        &self,
        momentum_residuals: &[f64],
        continuity_residuals: &[f64],
    ) -> CfdResult<Vec<f64>> {
        if momentum_residuals.len() != continuity_residuals.len() {
            return Err(CfdError::mesh(
                "Momentum and continuity residual arrays must have same length",
            ));
        }

        let mut errors = Vec::with_capacity(momentum_residuals.len());

        for (&mom_res, &cont_res) in momentum_residuals.iter().zip(continuity_residuals.iter()) {
            // Combine momentum and continuity residuals using L2 norm
            let combined_error = (mom_res * mom_res + cont_res * cont_res).sqrt();
            errors.push(combined_error);
        }

        Ok(errors)
    }
}