ViewDesignEngine/include/vde/sdf/sdf_optimize.h

#pragma once
#include "vde/sdf/sdf_tree.h"
#include "vde/core/point.h"
#include <vector>
#include <functional>

namespace vde::sdf {

using core::Point3D;
using core::Vector3D;

// ────────────────────────────────────────────────
//  Shape Optimization
// ────────────────────────────────────────────────

/// Optimization result
struct OptimizeResult {
    SdfNodePtr optimized_shape;
    double final_loss;
    int iterations;
    bool converged;
    std::vector<double> loss_history;
};

/// Loss function type: takes SDF evaluator + target, returns scalar loss
using LossFn = std::function<double(const std::function<double(const Point3D&)>&)>;

// ── Shape Fitting ───────────────────────────────

/// Fit a parameterized shape to a target point cloud by minimizing
/// distance of surface to target points.
/// @param initial Initial shape (sphere, box, cylinder, etc.)
/// @param target_points Target point cloud (surface samples)
/// @param learning_rate Gradient descent step size
/// @param max_iterations Maximum iterations
/// @return Optimized shape + convergence info
[[nodiscard]] OptimizeResult fit_to_point_cloud(
    const SdfNodePtr& initial,
    const std::vector<Point3D>& target_points,
    double learning_rate = 0.01,
    int max_iterations = 100);

/// Fit a shape to minimize SDF values at target points (drive surface to points)
/// Loss = mean(|sdf(p_i)|) for p_i in targets
[[nodiscard]] OptimizeResult fit_surface_to_points(
    const SdfNodePtr& initial,
    const std::vector<Point3D>& surface_points,
    double learning_rate = 0.01,
    int max_iterations = 100);

/// Fit a shape to match another SDF (shape matching)
/// Loss = mean((sdf_a(p_i) - sdf_b(p_i))^2) over sample grid
[[nodiscard]] OptimizeResult fit_to_sdf(
    const SdfNodePtr& source,
    const std::function<double(const Point3D&)>& target_sdf,
    const Point3D& bmin, const Point3D& bmax,
    int grid_resolution = 16,
    double learning_rate = 0.01,
    int max_iterations = 100);

// ── Collision Avoidance ─────────────────────────

/// Push shapes apart to resolve interpenetration.
/// Modifies shapes in-place by translating them.
/// @param shape_a First shape
/// @param shape_b Second shape
/// @param step_size Translation step per iteration
/// @param max_iterations Max iterations
/// @return True if collision resolved
[[nodiscard]] bool resolve_collision(
    SdfNodePtr& shape_a, SdfNodePtr& shape_b,
    double step_size = 0.1,
    int max_iterations = 50);

/// Find minimum translation distance to avoid collision
[[nodiscard]] double penetration_depth(
    const SdfNodePtr& shape_a,
    const SdfNodePtr& shape_b,
    const Point3D& bmin, const Point3D& bmax,
    int samples = 1000);

// ── Reachability / Accessibility ─────────────────

/// Compute accessibility score at point p on surface of shape.
/// Returns 0 (inaccessible) to 1 (fully accessible).
/// Uses hemisphere sampling + occlusion testing.
[[nodiscard]] double accessibility(
    const SdfNodePtr& shape,
    const Point3D& p,           // surface point
    const Vector3D& direction,  // approach direction
    int hemisphere_samples = 64);

/// Find point with maximum accessibility
[[nodiscard]] Point3D find_accessible_point(
    const SdfNodePtr& shape,
    const Vector3D& approach_dir,
    const Point3D& bmin, const Point3D& bmax,
    int grid_res = 32);

// ── Symmetry Detection ──────────────────────────

/// Detect if shape has reflective symmetry in given direction.
/// Returns symmetry score: 0 = asymmetric, 1 = perfectly symmetric.
[[nodiscard]] double symmetry_score(
    const SdfNodePtr& shape,
    const Point3D& bmin, const Point3D& bmax,
    const Vector3D& plane_normal,
    double plane_offset = 0.0,
    int samples = 1000);

/// Find the best symmetry plane
struct SymmetryResult {
    double score;
    Vector3D normal;
    double offset;
};
[[nodiscard]] SymmetryResult find_symmetry_plane(
    const SdfNodePtr& shape,
    const Point3D& bmin, const Point3D& bmax,
    int samples = 1000);

// ── SDF Volume Computation ──────────────────────

/// Approximate volume of implicit shape via Monte Carlo sampling
[[nodiscard]] double estimate_volume(
    const SdfNodePtr& shape,
    const Point3D& bmin, const Point3D& bmax,
    int samples = 10000);

/// Compute center of mass via Monte Carlo
[[nodiscard]] Point3D center_of_mass(
    const SdfNodePtr& shape,
    const Point3D& bmin, const Point3D& bmax,
    int samples = 10000);

// ── Parameter Collection ────────────────────────

/// A reference to a mutable double parameter inside an SDF tree
struct ParamRef {
    double* value;
    std::string name;
};

/// Collect all mutable numeric parameters from primitive leaf nodes
[[nodiscard]] std::vector<ParamRef> collect_params(SdfNodePtr& root);

/// Compute numerical gradient of a scalar loss w.r.t. collected parameters
[[nodiscard]] std::vector<double> numerical_gradient(
    const std::vector<ParamRef>& params,
    const std::function<double()>& loss_fn,
    double eps = 1e-6);

// ── Gradient Descent Utility ────────────────────

/// Simple gradient descent with fixed learning rate
class GradientDescent {
public:
    explicit GradientDescent(double lr) : lr_(lr) {}

    /// Step parameters toward minimizing loss.
    /// @param params Parameter vector (mutable)
    /// @param grad_fn Function that computes gradient for given params;
    ///                returns loss as scalar and fills gradient vector.
    /// @return Current loss
    double step(std::vector<double>& params,
                const std::function<double(const std::vector<double>&,
                                           std::vector<double>&)>& grad_fn);

    void set_learning_rate(double lr) { lr_ = lr; }
    [[nodiscard]] double learning_rate() const { return lr_; }
    [[nodiscard]] int iteration() const { return iteration_; }

private:
    double lr_;
    int iteration_ = 0;
};

} // namespace vde::sdf