feat(surface): major work on implementing surface constraints in a presciption agnostic manner

libmeanfield/interface/surface/constant.cppm

@@ -0,0 +1,65 @@
+module;
+
+#include <cmath>
+#include <format>
+#include <stdexcept>
+#include <type_traits>
+
+export module mean_field:surface.constant;
+
+export import :eos.quantities;
+
+export namespace mean_field::surface {
+    struct PressureSurfaceDescriptor final {
+        double targetPressure;
+    };
+
+    /*
+     * The only physical surface prescription currently supported by
+     * MeanField. It says nothing about which thermodynamic variable appears
+     * in a nonlinear state vector; resolving pressure into that representation
+     * is an EOS responsibility.
+     */
+    class ConstantPressureSurface final {
+    public:
+        using PhysicalQuantity = eos::quantity::Pressure;
+        using TargetValue      = eos::PressureValue;
+
+        explicit ConstantPressureSurface(const TargetValue targetPressure) : m_targetPressure(targetPressure) {
+            if (!std::isfinite(targetPressure.value())) {
+                throw std::invalid_argument(
+                    std::format(
+                        "The target surface pressure must be finite. Instead P = {} was provided.",
+                        targetPressure.value()
+                    )
+                );
+            }
+            if (targetPressure.value() < 0.0) {
+                throw std::invalid_argument(
+                    std::format(
+                        "The target surface pressure must be non-negative. Instead P = {} was provided.",
+                        targetPressure.value()
+                    )
+                );
+            }
+        }
+
+        [[nodiscard]] TargetValue targetPressure() const noexcept {
+            return m_targetPressure;
+        }
+
+        [[nodiscard]] PressureSurfaceDescriptor descriptor() const noexcept {
+            return PressureSurfaceDescriptor{.targetPressure = m_targetPressure.value()};
+        }
+
+    private:
+        TargetValue m_targetPressure;
+    };
+
+    template <typename Candidate>
+    concept ConstantPressureSurfaceType = std::same_as<std::remove_cvref_t<Candidate>, ConstantPressureSurface>;
+
+    // Familiar physical terminology retained as a synonym, not as a second
+    // surface-condition type.
+    using Isobaric                      = ConstantPressureSurface;
+} // namespace mean_field::surface