From 62ea6e891c9d57d60afaa25dbca807a7d169a926 Mon Sep 17 00:00:00 2001
From: Nick Gerleman <ngerlem@meta.com>
Date: Tue, 4 Feb 2025 21:51:21 -0800
Subject: [PATCH] `react/utils/toLower` (#49187)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49187

`tolower` is not `constexpr`. Share some quick utilities for char to lowercase, and case insensitive comparision that does not create new string.

Changelog: [Internal]

Reviewed By: lenaic

Differential Revision: D69134770

fbshipit-source-id: 57a84f2d1a441e5a4c07c0db96cb6c133770fb51
---
 .../react/renderer/css/CSSHexColor.h          |  9 ++---
 .../ReactCommon/react/utils/fnv1a.h           |  7 ++--
 .../ReactCommon/react/utils/iequals.h         | 34 +++++++++++++++++++
 .../ReactCommon/react/utils/toLower.h         | 22 ++++++++++++
 4 files changed, 61 insertions(+), 11 deletions(-)
 create mode 100644 packages/react-native/ReactCommon/react/utils/iequals.h
 create mode 100644 packages/react-native/ReactCommon/react/utils/toLower.h

diff --git a/packages/react-native/ReactCommon/react/renderer/css/CSSHexColor.h b/packages/react-native/ReactCommon/react/renderer/css/CSSHexColor.h
index 58607f67d4a..2a1268c4fe5 100644
--- a/packages/react-native/ReactCommon/react/renderer/css/CSSHexColor.h
+++ b/packages/react-native/ReactCommon/react/renderer/css/CSSHexColor.h
@@ -10,6 +10,8 @@
 #include <optional>
 #include <string_view>
 
+#include <react/utils/toLower.h>
+
 namespace facebook::react {
 
 namespace detail {
@@ -18,13 +20,6 @@ enum class HexColorType {
   Short,
 };
 
-constexpr char toLower(char c) {
-  if (c >= 'A' && c <= 'Z') {
-    return static_cast<char>(c + 32);
-  }
-  return c;
-}
-
 constexpr uint8_t hexToNumeric(std::string_view hex, HexColorType hexType) {
   int result = 0;
   for (char c : hex) {
diff --git a/packages/react-native/ReactCommon/react/utils/fnv1a.h b/packages/react-native/ReactCommon/react/utils/fnv1a.h
index 2acdeb3113b..4a945d968f7 100644
--- a/packages/react-native/ReactCommon/react/utils/fnv1a.h
+++ b/packages/react-native/ReactCommon/react/utils/fnv1a.h
@@ -11,6 +11,8 @@
 #include <functional>
 #include <string_view>
 
+#include <react/utils/toLower.h>
+
 namespace facebook::react {
 
 /**
@@ -41,10 +43,7 @@ constexpr uint32_t fnv1a(std::string_view string) noexcept {
 constexpr uint32_t fnv1aLowercase(std::string_view string) {
   struct LowerCaseTransform {
     constexpr char operator()(char c) const {
-      if (c >= 'A' && c <= 'Z') {
-        return c + static_cast<char>('a' - 'A');
-      }
-      return c;
+      return toLower(c);
     }
   };
 
diff --git a/packages/react-native/ReactCommon/react/utils/iequals.h b/packages/react-native/ReactCommon/react/utils/iequals.h
new file mode 100644
index 00000000000..b30bd5b9e9d
--- /dev/null
+++ b/packages/react-native/ReactCommon/react/utils/iequals.h
@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <cstdint>
+#include <string_view>
+
+#include <react/utils/toLower.h>
+
+namespace facebook::react {
+
+/**
+ * constexpr check for case insensitive equality of two strings.
+ */
+constexpr bool iequals(std::string_view a, std::string_view b) {
+  if (a.size() != b.size()) {
+    return false;
+  }
+
+  for (size_t i = 0; i < a.size(); i++) {
+    if (toLower(a[i]) != toLower(b[i])) {
+      return false;
+    }
+  }
+
+  return true;
+}
+
+} // namespace facebook::react
diff --git a/packages/react-native/ReactCommon/react/utils/toLower.h b/packages/react-native/ReactCommon/react/utils/toLower.h
new file mode 100644
index 00000000000..27212d006dc
--- /dev/null
+++ b/packages/react-native/ReactCommon/react/utils/toLower.h
@@ -0,0 +1,22 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+namespace facebook::react {
+
+/**
+ * constexpr version of tolower
+ */
+constexpr char toLower(char c) {
+  if (c >= 'A' && c <= 'Z') {
+    return static_cast<char>(c + 32);
+  }
+  return c;
+}
+
+} // namespace facebook::react