fix(spline): validate interpolator initialization before interpolation

Author: fbriol

cxx/include/pyinterp/math/interpolate/bivariate.hpp

@@ -92,6 +92,12 @@ class BivariateBase {
     return z;
   }
 
+  /// @brief Check if the interpolator is valid (coefficients computed)
+  /// @return True if the interpolator is valid, false otherwise.
+  [[nodiscard]] constexpr auto is_valid() const noexcept -> bool {
+    return is_valid_;
+  }
+
  protected:
   /// @brief Compute coefficients from stored data
   /// @return true if successful, false otherwise.
@@ -109,12 +115,6 @@ class BivariateBase {
   /// @return Z-values of the data points.
   [[nodiscard]] constexpr auto za() const -> const Matrix<T>& { return za_; }
 
-  /// @brief Check if the interpolator is valid (coefficients computed)
-  /// @return True if the interpolator is valid, false otherwise.
-  [[nodiscard]] constexpr auto is_valid() const noexcept -> bool {
-    return is_valid_;
-  }
-
  private:
   /// Stored X-coordinates of the data points
   Vector<T> xa_;

cxx/include/pyinterp/math/interpolate/bivariate/spline.hpp

@@ -10,6 +10,7 @@
 #include <memory>
 #include <stdexcept>
 
+#include "pyinterp/math/fill.hpp"
 #include "pyinterp/math/interpolate/bivariate.hpp"
 #include "pyinterp/math/interpolate/univariate.hpp"
 
@@ -59,10 +60,11 @@ class Spline : public BivariateBase<T> {
     const auto& ya = this->ya();
     const auto& za = this->za();
 
-    // Validate grid dimensions
-    if (za.size() == 0) {
-      throw std::runtime_error("Cannot interpolate on empty grid");
+    // Check if interpolator was properly initialized
+    if (!this->is_valid()) [[unlikely]] {
+      return math::Fill<T>::value();
     }
+
     return interpolate_y_then_x(xa, ya, za, x, y);
   }
 

cxx/tests/math/interpolate/bivariate/spline.cpp

@@ -225,8 +225,9 @@ TEST(Spline, EmptyGrid) {
   Eigen::Matrix<double, Eigen::Dynamic, 1> ya(0);
 
   auto spline = Spline<double>(std::make_unique<univariate::Linear<double>>());
+  EXPECT_TRUE(std::isnan(spline(1.0, 1.0)));
   spline.prepare(xa, ya, za);
-  EXPECT_THROW(static_cast<void>(spline(1.0, 1.0)), std::runtime_error);
+  EXPECT_TRUE(std::isnan(spline(1.0, 1.0)));
 }
 
 // Test with larger grid to verify capacity management