From 67553ee7c11be489f6511f1de162f0482612cf78 Mon Sep 17 00:00:00 2001
From: "github-actions[bot]"
 <41898282+github-actions[bot]@users.noreply.github.com>
Date: Sun, 7 Dec 2025 19:23:45 +0000
Subject: [PATCH] wip transform

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Change-Id: I5db39a2437f1e957d1444cffbc0aecd78e9ad0f5
---
 sel/BUILD.bazel                       | 10 +++
 sel/detail/BUILD.bazel                |  5 ++
 sel/detail/tuple_transform.hpp        | 25 ++++++++
 sel/detail/tuple_transform_reduce.hpp | 26 ++++----
 sel/test/BUILD.bazel                  | 15 +++++
 sel/test/transform_test.cpp           | 84 +++++++++++++++++++++++++
 sel/transform.hpp                     | 91 +++++++++++++++++++++++++++
 sel/tree.hpp                          | 15 ++++-
 8 files changed, 258 insertions(+), 13 deletions(-)
 create mode 100644 sel/detail/tuple_transform.hpp
 create mode 100644 sel/test/transform_test.cpp
 create mode 100644 sel/transform.hpp

diff --git a/sel/BUILD.bazel b/sel/BUILD.bazel
index f25b5ab..277c6bb 100644
--- a/sel/BUILD.bazel
+++ b/sel/BUILD.bazel
@@ -76,6 +76,16 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "transform",
+    hdrs = ["transform.hpp"],
+    deps = [
+        ":tree",
+        "//sel/detail:tuple_transform",
+    ],
+)
+
+
 cc_library(
     name = "tree",
     hdrs = ["tree.hpp"],
diff --git a/sel/detail/BUILD.bazel b/sel/detail/BUILD.bazel
index 9c2a16e..ccca8a3 100644
--- a/sel/detail/BUILD.bazel
+++ b/sel/detail/BUILD.bazel
@@ -18,6 +18,11 @@ cc_library(
     hdrs = ["tuple_ref.hpp"],
 )
 
+cc_library(
+    name = "tuple_transform",
+    hdrs = ["tuple_transform.hpp"],
+)
+
 cc_library(
     name = "tuple_transform_reduce",
     hdrs = ["tuple_transform_reduce.hpp"],
diff --git a/sel/detail/tuple_transform.hpp b/sel/detail/tuple_transform.hpp
new file mode 100644
index 0000000..57a8e9a
--- /dev/null
+++ b/sel/detail/tuple_transform.hpp
@@ -0,0 +1,25 @@
+#pragma once
+
+#include <cstddef>
+#include <tuple>
+#include <type_traits>
+#include <utility>
+
+namespace sel::detail {
+
+/// applies an invocable to each element of a tuple
+inline constexpr struct
+{
+  template <class Tuple>
+  [[nodiscard]]
+  static constexpr auto operator()(Tuple&& tup, auto transform)
+  {
+    using tuple_type = std::remove_cvref_t<Tuple>;
+
+    return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
+      return std::tuple{transform(std::get<Is>(std::forward<Tuple>(tup)))...};
+    }(std::make_index_sequence<std::tuple_size_v<tuple_type>>{});
+  }
+} tuple_transform_reduce{};
+
+}  // namespace sel::detail
diff --git a/sel/detail/tuple_transform_reduce.hpp b/sel/detail/tuple_transform_reduce.hpp
index 0bab68a..ee9aa11 100644
--- a/sel/detail/tuple_transform_reduce.hpp
+++ b/sel/detail/tuple_transform_reduce.hpp
@@ -2,6 +2,7 @@
 
 #include <cstddef>
 #include <tuple>
+#include <type_traits>
 #include <utility>
 
 namespace sel::detail {
@@ -9,28 +10,31 @@ namespace sel::detail {
 /// applies an invocable to each element of two tuples, then reduces
 inline constexpr struct
 {
-  template <class... T1, class... T2, class T>
-    requires (sizeof...(T1) == sizeof...(T2))
+  template <class Tuple1, class Tuple2, class T>
+    requires (
+        std::tuple_size_v<std::remove_cvref_t<Tuple1>> ==
+        std::tuple_size_v<std::remove_cvref_t<Tuple2>>
+    )
   [[nodiscard]]
-  static constexpr auto operator()(
-      const std::tuple<T1...>& tup1,
-      const std::tuple<T2...>& tup2,
-      T init,
-      auto reduce,
-      auto transform
-  ) -> T
+  static constexpr auto
+  operator()(Tuple1&& tup1, Tuple2&& tup2, T init, auto reduce, auto transform)
+      -> T
   {
     return [&]<std::size_t... Is>(std::index_sequence<Is...>) {
       auto _ = {(
           init = reduce(
               std::move(init),  //
-              transform(std::get<Is>(tup1), std::get<Is>(tup2))
+              transform(
+                  std::get<Is>(std::forward<Tuple1>(tup1)),
+                  std::get<Is>(std::forward<Tuple2>(tup2))
+              )
           ),
           true
       )...};
 
       return init;
-    }(std::index_sequence_for<T1...>{});
+    }(std::make_index_sequence<
+               std::tuple_size_v<std::remove_cvref_t<Tuple1>>>{});
   }
 } tuple_transform_reduce{};
 
diff --git a/sel/test/BUILD.bazel b/sel/test/BUILD.bazel
index 0152419..ce64ffe 100644
--- a/sel/test/BUILD.bazel
+++ b/sel/test/BUILD.bazel
@@ -23,6 +23,21 @@ cc_test(
     ],
 )
 
+cc_test(
+    name = "transform_test",
+    size = "small",
+    srcs = ["transform_test.cpp"],
+    deps = [
+        "//sel:tree",
+        "//sel:transform",
+        "//sel:constant",
+        "//sel:variable",
+        "//sel:plus",
+        "//sel:multiplies",
+        "@skytest",
+    ],
+)
+
 cc_test(
     name = "tree_test",
     size = "small",
diff --git a/sel/test/transform_test.cpp b/sel/test/transform_test.cpp
new file mode 100644
index 0000000..46c4f95
--- /dev/null
+++ b/sel/test/transform_test.cpp
@@ -0,0 +1,84 @@
+#include "sel/constant.hpp"
+#include "sel/multiplies.hpp"
+#include "sel/operation.hpp"
+#include "sel/plus.hpp"
+#include "sel/transform.hpp"
+#include "sel/tree.hpp"
+#include "sel/variable.hpp"
+#include "skytest/skytest.hpp"
+
+#include <cstddef>
+#include <utility>
+
+auto main() -> int
+{
+  using namespace skytest::literals;
+  using namespace skytest;
+
+  static constexpr auto x = sel::variable{"x"};
+  static constexpr auto y = sel::variable{"y"};
+
+  static constexpr auto a = sel::constant{1};
+  static constexpr auto b = sel::constant{2};
+
+  "replaces variables with constants"_ctest = [] {
+    return expect(
+        eq(a + b,
+           sel::transform(
+               sel::tree{x + y},
+               sel::equal_to{x + y}.then([](const auto&) { return a + b; })
+           )) and  //
+        eq(a + b,
+           sel::transform(sel::tree{x + y}, sel::equal_to{x + y}.then(a + b)))
+    );
+  };
+
+  "replaces subexpressions"_ctest = [] {
+    static constexpr auto f = sel::multiplies{x, x};
+    static constexpr auto g = sel::multiplies{x, y};
+    return expect(eq(
+        a + a + g,
+        sel::transform(
+            f + f + g,  //
+            sel::equal_to{f}.then(a)
+        )
+    ));
+  };
+
+  //  "replaces operation types"_ctest = [] {
+  //    return expect(eq(
+  //        sel::plus{sel::plus{sel::plus{sel::constant{0}}}},
+  //        sel::transform(
+  //            sel::multiplies{sel::plus{sel::multiplies{sel::constant{0}}}},
+  //            {//
+  //             &sel::tree::is_node<sel::plus>,
+  //             [](this const auto& self, const sel::tree::node<sel::plus>&
+  //             node) {
+  //               return sel::tree
+  //               {
+  //                 sel::node_tag<sel::plus>,
+  //                 std::from_range,
+  //                 node.args | std::views::transform([](const ::sel::tree&
+  //                 subtree) {
+  //                        return sel::transform(subtree,
+  //                        {&sel::tree::is_node<sel::plus>, self});
+  //                 })
+  //               };
+  //             }
+  //            }
+  //        )
+  //    ));
+  //  };
+
+  "expands an expression"_ctest = [] {
+    static constexpr auto f = x * x;
+
+    return expect(eq(
+        a + f,
+        sel::transform(
+            a + b,  //
+            sel::equal_to{b}.then(f)
+        )
+    ));
+  };
+}
diff --git a/sel/transform.hpp b/sel/transform.hpp
new file mode 100644
index 0000000..a4b0d49
--- /dev/null
+++ b/sel/transform.hpp
@@ -0,0 +1,91 @@
+#pragma once
+
+#include "sel/leaf.hpp"
+#include "sel/operation.hpp"
+#include "sel/term.hpp"
+#include "sel/tree.hpp"
+
+#include <concepts>
+#include <cstddef>
+#include <functional>
+#include <ranges>
+#include <tuple>
+
+namespace sel {
+
+template <class... Ts>
+struct overloads : Ts...
+{
+  using Ts::operator()...;
+};
+
+/// apply a transformation to a `tree`
+/// @param expr `tree` to transform
+/// @param func transformation function
+/// @return copy of `expr` with `func` applied to it
+///
+/// Applies transformation function `func` to `term`.
+///
+/// If `func(expr)` is a valid expression, returns `func(expr)`; otherwise if
+/// `expr` is an `operation`,returns `operation{func(args)...}` where `args...`
+/// is a pack introduced from `expr.args`.
+///
+inline constexpr struct
+{
+  [[nodiscard]]
+  constexpr auto
+  operator()(this const auto& self, const tree& expr, auto func) -> tree
+  {
+    if (func.pred(expr)) {
+      return func.transform(expr);
+    }
+
+    return expr.node().visit(overloads(
+        [](const leaf auto& node) -> tree { return node; },
+        [transform = std::views::transform(std::bind_back(self, func))]<
+            class OpNode>(const OpNode& op) -> tree {
+          return {
+              std::in_place_type<typename OpNode::op_tag_type>,
+              std::from_range,
+              op.args | transform
+          };
+        }
+    ));
+  }
+} transform{};
+
+template <class Pred, class Transform>
+struct conditional_transformer
+{
+  Pred pred;
+  Transform transform_;
+
+  [[nodiscard]]
+  constexpr auto transform(const tree& expr) const -> tree
+  {
+    if constexpr (std::convertible_to<Transform, tree>) {
+      return transform_;
+    } else {
+      return transform_(expr);
+    }
+  }
+};
+
+template <class T>
+struct equal_to
+{
+  T value;
+
+  template <class U>
+  [[nodiscard]]
+  constexpr auto then(const U& transform_) const
+  {
+    return conditional_transformer{
+        // TODO __cpp_lib_bind_back >= 202306L
+        std::bind_back(std::equal_to{}, value),  //
+        transform_
+    };
+  }
+};
+
+}  // namespace sel
diff --git a/sel/tree.hpp b/sel/tree.hpp
index 27ef06b..44c692d 100644
--- a/sel/tree.hpp
+++ b/sel/tree.hpp
@@ -30,6 +30,8 @@ class tree
   template <class OpTag>
   struct op_node
   {
+    using op_tag_type = OpTag;
+
     std::vector<tree> args;
 
     template <class... Args>
@@ -68,7 +70,7 @@ class tree
   /// construct from a `leaf`
   template <class T>
     requires leaf<std::remove_cvref_t<T>>
-  constexpr explicit tree(T&& value)
+  constexpr tree(T&& value)
       : value_{std::forward<T>(value)}
   {}
 
@@ -76,7 +78,7 @@ class tree
   template <class Op>
     requires (not std::same_as<tree, std::remove_cvref_t<Op>>) and
              operation<std::remove_cvref_t<Op>>
-  constexpr explicit tree(Op&& expr)
+  constexpr tree(Op&& expr)
       : value_{std::make_from_tuple<op_node<typename std::remove_cvref_t<
             Op>::op_tag>>(std::forward<Op>(expr).args)}
   {}
@@ -105,6 +107,15 @@ class tree
   {}
   /// @}
 
+  /// apply a visitor to the tree node
+  template <class Self>
+  [[nodiscard]]
+  constexpr auto
+  node(this Self&& self) -> decltype(std::forward<Self>(self).value_)
+  {
+    return std::forward<Self>(self).value_;
+  }
+
   /// compare a `tree` with a `leaf`
   [[nodiscard]]
   constexpr auto operator==(const leaf auto& other) const -> bool