From d6f553a794efa8fda5b530e096fe89d223531477 Mon Sep 17 00:00:00 2001
From: Alexander McCord <alex@mccord.dev>
Date: Sun, 26 May 2024 13:11:38 -0700
Subject: [PATCH] Sketch out Language a little bit. Still WIP.

---
 EqSat/include/Luau/EGraph.h   |  2 +-
 EqSat/include/Luau/Language.h | 63 +++++++++++++++++++++++++++++++++--
 Sources.cmake                 |  2 ++
 tests/EqSat.language.test.cpp | 26 +++++++++++++++
 4 files changed, 90 insertions(+), 3 deletions(-)
 create mode 100644 tests/EqSat.language.test.cpp

diff --git a/EqSat/include/Luau/EGraph.h b/EqSat/include/Luau/EGraph.h
index 48f3a32a..21932f9d 100644
--- a/EqSat/include/Luau/EGraph.h
+++ b/EqSat/include/Luau/EGraph.h
@@ -57,7 +57,7 @@ private:
     std::unordered_map<Id, EClass<L, typename N::Data>> classes;
 
     /// The hashcons 𝐻 is a map from e-nodes to e-class ids.
-    std::unordered_map<L, Id> hashcons;
+    std::unordered_map<L, Id, typename L::Hash> hashcons;
 };
 
 } // namespace Luau::EqSat
diff --git a/EqSat/include/Luau/Language.h b/EqSat/include/Luau/Language.h
index e36b7611..bd4bd9d9 100644
--- a/EqSat/include/Luau/Language.h
+++ b/EqSat/include/Luau/Language.h
@@ -1,12 +1,71 @@
 // This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
 #pragma once
 
+#include <algorithm>
+#include <type_traits>
+#include <utility>
+
 namespace Luau::EqSat
 {
 
-struct Language
-{
+#define LUAU_EQSAT_ATOM(name, value) LUAU_EQSAT_ATOM_CUSTOM(name, #name, value)
+#define LUAU_EQSAT_ATOM_CUSTOM(name, custom, value) \
+    struct name : public ::Luau::EqSat::Atom<name, value> \
+    { \
+        static constexpr const char* tag = custom; \
+    }
 
+template<typename B, typename T>
+struct Atom
+{
+    T value;
+};
+
+// `Language` is very similar to `Luau::Variant` with enough differences warranting a different type altogether.
+//
+// Firstly, where `Luau::Variant` uses an `int` to decide which type the variant currently holds, we use
+// a `const char*` instead. We use the pointer address for tag checking, and the string buffer for stringification.
+//
+// Secondly, we need `Language` to have additional methods such as:
+// - `children()` to get child operands,
+// - `operator==` to decide equality, and
+// - `hash()` function.
+//
+// And finally, each `T` in `Ts` have additional requirements which `Luau::Variant` doesn't need.
+template<typename... Ts>
+class Language
+{
+    const char* tag;
+    char buffer[std::max({sizeof(Ts)...})];
+
+public:
+    template<typename T>
+    Language(T&& t)
+    {
+        using TT = std::decay_t<T>;
+        static_assert(std::disjunction_v<std::is_same<TT, Ts>...>);
+
+        tag = T::tag;
+        new (&buffer) TT(std::forward<T>(t));
+    }
+
+    template<typename T>
+    const T* get() const
+    {
+        static_assert(std::disjunction_v<std::is_same<std::decay_t<T>, Ts>...>);
+
+        return tag == T::tag ? reinterpret_cast<const T*>(&buffer) : nullptr;
+    }
+
+public:
+    struct Hash
+    {
+        size_t operator()(const Language<Ts...>& lang)
+        {
+            // TODO. Currently here so I can build and test this project.
+            return 0;
+        }
+    };
 };
 
 } // namespace Luau::EqSat
diff --git a/Sources.cmake b/Sources.cmake
index 8601fa28..89da27ca 100644
--- a/Sources.cmake
+++ b/Sources.cmake
@@ -429,6 +429,8 @@ if(TARGET Luau.UnitTest)
         tests/DiffAsserts.cpp
         tests/DiffAsserts.h
         tests/Differ.test.cpp
+        tests/EqSat.language.test.cpp
+        tests/EqSat.propositional.test.cpp
         tests/Error.test.cpp
         tests/Fixture.cpp
         tests/Fixture.h
diff --git a/tests/EqSat.language.test.cpp b/tests/EqSat.language.test.cpp
new file mode 100644
index 00000000..6977d50b
--- /dev/null
+++ b/tests/EqSat.language.test.cpp
@@ -0,0 +1,26 @@
+// This file is part of the Luau programming language and is licensed under MIT License; see LICENSE.txt for details
+#include <doctest.h>
+
+#include "Luau/Language.h"
+
+LUAU_EQSAT_ATOM(Atom1, bool);
+LUAU_EQSAT_ATOM(Atom2, bool);
+
+using namespace Luau;
+
+using Mini = EqSat::Language<Atom1, Atom2>;
+
+TEST_SUITE_BEGIN("EqSatLanguage");
+
+TEST_CASE("language_get_works")
+{
+    Mini m{Atom1{true}};
+
+    const Atom1* atom = m.get<Atom1>();
+    REQUIRE(atom);
+    CHECK(atom->value);
+
+    CHECK(!m.get<Atom2>());
+}
+
+TEST_SUITE_END();