feat(fem-and-beam-kernel): step 0 — quadrature-and-shape-functions

tests/unit/fem/line2_shape_test.cpp

@@ -0,0 +1,71 @@
+#include <fesa/fem/line2_shape.hpp>
+
+#include <array>
+#include <limits>
+#include <stdexcept>
+
+#include <gtest/gtest.h>
+
+namespace {
+
+TEST(ShapeFunction, InterpolatesBothEndpoints) {
+    const auto first_endpoint = fesa::line2_shape(-1.0);
+    const auto second_endpoint = fesa::line2_shape(1.0);
+
+    EXPECT_DOUBLE_EQ(first_endpoint[0], 1.0);
+    EXPECT_DOUBLE_EQ(first_endpoint[1], 0.0);
+    EXPECT_DOUBLE_EQ(second_endpoint[0], 0.0);
+    EXPECT_DOUBLE_EQ(second_endpoint[1], 1.0);
+}
+
+TEST(ShapeFunction, FormsPartitionOfUnityWithinRoundoff) {
+    for (const double xi : std::array{-1.0, -0.25, 0.0, 0.4, 1.0}) {
+        const auto shape = fesa::line2_shape(xi);
+
+        // Four ulps at unit scale cover the two affine evaluations and sum.
+        constexpr double tolerance =
+            4.0 * std::numeric_limits<double>::epsilon();
+        EXPECT_NEAR(shape[0] + shape[1], 1.0, tolerance) << "xi=" << xi;
+    }
+}
+
+TEST(ShapeFunction, DerivativesSumToZeroAtEveryNaturalCoordinate) {
+    for (const double xi : std::array{-1.0, -0.25, 0.0, 0.4, 1.0}) {
+        const auto derivative = fesa::line2_shape_derivative(xi);
+
+        EXPECT_DOUBLE_EQ(derivative[0], -0.5);
+        EXPECT_DOUBLE_EQ(derivative[1], 0.5);
+        EXPECT_DOUBLE_EQ(derivative[0] + derivative[1], 0.0);
+    }
+}
+
+TEST(Jacobian, ReturnsHalfThePhysicalLength) {
+    EXPECT_DOUBLE_EQ(fesa::line2_jacobian(4.0), 2.0);
+    EXPECT_DOUBLE_EQ(fesa::line2_jacobian(0.25), 0.125);
+}
+
+TEST(Jacobian, DoesNotAdjustSmallPositiveFiniteLengths) {
+    constexpr double length = 1.0e-300;
+
+    EXPECT_DOUBLE_EQ(fesa::line2_jacobian(length), length / 2.0);
+}
+
+TEST(Jacobian, RejectsNonpositiveAndNonfiniteLengths) {
+    const double infinity = std::numeric_limits<double>::infinity();
+    const double nan = std::numeric_limits<double>::quiet_NaN();
+
+    for (const double length : std::array{0.0, -1.0, infinity, nan}) {
+        EXPECT_THROW(
+            static_cast<void>(fesa::line2_jacobian(length)),
+            std::invalid_argument);
+    }
+}
+
+TEST(Jacobian, RejectsLengthWhoseHalfUnderflowsToZero) {
+    EXPECT_THROW(
+        static_cast<void>(fesa::line2_jacobian(
+            std::numeric_limits<double>::denorm_min())),
+        std::invalid_argument);
+}
+
+}  // namespace