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

tests/unit/fem/gauss_rule_test.cpp

@@ -0,0 +1,53 @@
+#include <fesa/fem/gauss_rule.hpp>
+
+#include <array>
+#include <limits>
+#include <stdexcept>
+
+#include <gtest/gtest.h>
+
+namespace {
+
+TEST(Quadrature, OnePointRuleIntegratesDegreeZeroAndOneExactly) {
+    const auto rule = fesa::gauss_rule_1d(1);
+
+    ASSERT_EQ(rule.size(), 1);
+    EXPECT_DOUBLE_EQ(rule[0].weight, 2.0);
+    EXPECT_DOUBLE_EQ(rule[0].weight * rule[0].xi, 0.0);
+}
+
+TEST(Quadrature, TwoPointRuleIntegratesThroughDegreeThreeWithinRoundoff) {
+    const auto rule = fesa::gauss_rule_1d(2);
+
+    ASSERT_EQ(rule.size(), 2);
+
+    double degree_zero = 0.0;
+    double degree_one = 0.0;
+    double degree_two = 0.0;
+    double degree_three = 0.0;
+    for (const auto& point : rule) {
+        const double xi_squared = point.xi * point.xi;
+        degree_zero += point.weight;
+        degree_one += point.weight * point.xi;
+        degree_two += point.weight * xi_squared;
+        degree_three += point.weight * xi_squared * point.xi;
+    }
+
+    // Sixteen ulps at unit scale cover only the rounding in these short sums.
+    constexpr double tolerance =
+        16.0 * std::numeric_limits<double>::epsilon();
+    EXPECT_NEAR(degree_zero, 2.0, tolerance);
+    EXPECT_NEAR(degree_one, 0.0, tolerance);
+    EXPECT_NEAR(degree_two, 2.0 / 3.0, tolerance);
+    EXPECT_NEAR(degree_three, 0.0, tolerance);
+}
+
+TEST(Quadrature, RejectsUnsupportedOrders) {
+    for (const int order : std::array{0, 3, -1}) {
+        EXPECT_THROW(
+            static_cast<void>(fesa::gauss_rule_1d(order)),
+            std::invalid_argument);
+    }
+}
+
+}  // namespace