From fee05a621d511da2caccae044856eeff07b2f509 Mon Sep 17 00:00:00 2001
From: Emily Boudreaux <thomas@boudreauxmail.com>
Date: Mon, 12 Oct 2026 09:37:03 -0400
Subject: [PATCH] feat(equations): Added EquationRegistry

This is a compile time pack of equations which can be used to do things like say "this model is described by this set of equations"
---
 src/include/serif/equations/registry.hpp | 72 ++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 src/include/serif/equations/registry.hpp

diff --git a/src/include/serif/equations/registry.hpp b/src/include/serif/equations/registry.hpp
new file mode 100644
index 00000000..35851d56
--- /dev/null
+++ b/src/include/serif/equations/registry.hpp
@@ -0,0 +1,72 @@
+#pragma once
+
+/**
+ * @file registry.hpp
+ * @author Emily M. Boudreaux
+ *
+ * @brief System for registering multiple equations. The idea here is that a model will be defined partly by an equation registry (e.g. all the
+ * equations that must be satisfied for a given model).
+ */
+
+#include <tuple>
+#include <utility>
+
+#include "serif/equations/concepts.hpp"
+
+namespace serif::equations {
+    /**
+     * @class EquationRegistry
+     *
+     * @brief A compile-time registry of equations.
+     * @tparam EquationTs Pack of equation types to register.
+     *
+     * @requires There must be at least one equation type in the pack.
+     * @requires Each residual row must be defined exactly once.
+     */
+    template <Equation... EquationTs>
+    class EquationRegistry final {
+    public:
+        static_assert(sizeof...(EquationTs) > 0, "An equation registry must contain at least one equation.");
+
+        using Equations = utils::types::TypeList<EquationTs...>; ///< The equation types in the registry.
+        using Rows = utils::types::TypeList<discretization::forms::Operand<typename EquationTs::Test>...>; ///< The residual row types in the registry
+
+        static_assert( ((OperandValues<Rows>::template index<typename EquationTs::Test>() < sizeof...(EquationTs)) && ...), "An equation registry must define each residual row exactly once.");
+
+        /**
+         * @brief Construct an equation registry with the given equations.
+         *
+         * @param equations The equations to register.
+         */
+        explicit EquationRegistry(EquationTs... equations) : m_equations(std::move(equations)...) {}
+
+        /**
+         * @brief Apply some function to every type in the registry.
+         *
+         * @tparam FunctionT The type of the function to apply. The function must have the signature `void(const EquationT&)` where `EquationT` is the type of the equation in the registry.
+         * @param function The function to apply to each equation in the registry.
+         */
+        template <typename FunctionT>
+        void for_each(FunctionT&& function) const {
+            std::apply([&](const auto&... equations) {
+                    (function(equations), ...);
+                },
+                m_equations
+            );
+        }
+
+        /**
+         * @brief Get the equation of a given type from the registry.
+         *
+         * @tparam EquationT The type of the equation to get.
+         * @return The equation of the given type.
+         */
+        template <Equation EquationT>
+        [[nodiscard]] const EquationT& get() const {
+            return std::get<EquationT>(m_equations);
+        }
+
+    private:
+        std::tuple<EquationTs...> m_equations; ///< The equations in the registry, stored as a tuple.
+    };
+} // namespace serif::equations