From a5d0e0fb6988dccc59cb1c58013bd1c554f394a0 Mon Sep 17 00:00:00 2001
From: Daniel Pfeifer <daniel@pfeifer-mail.de>
Date: Sun, 8 Mar 2026 21:45:14 +0100
Subject: [PATCH] cmEnvironment: Introduce class for a set of environment
 variables

---
 Source/CMakeLists.txt    |  2 ++
 Source/cmEnvironment.cxx | 66 ++++++++++++++++++++++++++++++++++++++++
 Source/cmEnvironment.h   | 55 +++++++++++++++++++++++++++++++++
 bootstrap                |  1 +
 4 files changed, 124 insertions(+)
 create mode 100644 Source/cmEnvironment.cxx
 create mode 100644 Source/cmEnvironment.h

diff --git a/Source/CMakeLists.txt b/Source/CMakeLists.txt
index 8d4a57aea7..f520816c9a 100644
--- a/Source/CMakeLists.txt
+++ b/Source/CMakeLists.txt
@@ -191,6 +191,8 @@ add_library(
   cmDyndepCollation.h
   cmELF.h
   cmELF.cxx
+  cmEnvironment.cxx
+  cmEnvironment.h
   cmEvaluatedTargetProperty.cxx
   cmEvaluatedTargetProperty.h
   cmExprParserHelper.cxx
diff --git a/Source/cmEnvironment.cxx b/Source/cmEnvironment.cxx
new file mode 100644
index 0000000000..9cc9bc7868
--- /dev/null
+++ b/Source/cmEnvironment.cxx
@@ -0,0 +1,66 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file LICENSE.rst or https://cmake.org/licensing for details.  */
+#include "cmEnvironment.h"
+
+#include <utility>
+
+#include <cm/string_view>
+#include <cm/vector>
+#include <cmext/algorithm>
+
+#if defined(_WIN32)
+#  include "cmsys/String.h"
+#endif
+
+bool cmEnvironment::EnvNameLess::operator()(std::string const& lhs,
+                                            std::string const& rhs) const
+{
+#if defined(_WIN32)
+  // Environment variable names are case-insensitive on Windows
+  return cmsysString_strcasecmp(lhs.c_str(), rhs.c_str()) < 0;
+#else
+  return lhs < rhs;
+#endif
+}
+
+cmEnvironment::cmEnvironment(std::vector<std::string> const& env)
+{
+  for (std::string const& var : env) {
+    this->PutEnv(var);
+  }
+}
+
+void cmEnvironment::PutEnv(std::string const& env)
+{
+  auto const pos = env.find('=');
+  if (pos != std::string::npos) {
+    this->Map[env.substr(0, pos)] = env.substr(pos + 1);
+  } else {
+    this->Map[env] = cm::nullopt;
+  }
+}
+
+void cmEnvironment::UnPutEnv(std::string const& env)
+{
+  this->Map[env] = cm::nullopt;
+}
+
+void cmEnvironment::Update(cmEnvironment&& other)
+{
+  for (auto& kv : other.Map) {
+    this->Map[kv.first] = std::move(kv.second);
+  }
+  other.Map.clear();
+}
+
+std::vector<std::string> cmEnvironment::GetVariables() const
+{
+  auto result = std::vector<std::string>{};
+  result.reserve(this->Map.size());
+  for (auto const& elem : this->Map) {
+    if (elem.second) {
+      result.push_back(elem.first + '=' + *elem.second);
+    }
+  }
+  return result;
+}
diff --git a/Source/cmEnvironment.h b/Source/cmEnvironment.h
new file mode 100644
index 0000000000..5ceaa857eb
--- /dev/null
+++ b/Source/cmEnvironment.h
@@ -0,0 +1,55 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file LICENSE.rst or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include <map>
+#include <string>
+#include <vector>
+
+#include <cm/optional>
+
+/**
+ * Helper class to represent an environment.
+ */
+class cmEnvironment
+{
+public:
+  /** Initialize from `NAME=VALUE` strings. */
+  cmEnvironment(std::vector<std::string> const& env = {});
+
+  /**
+   * Add a single variable (or remove if no = sign) to the current
+   * environment.
+   */
+  void PutEnv(std::string const& env);
+
+  /** Remove a single variable from the current environment. */
+  void UnPutEnv(std::string const& env);
+
+  /**
+   * Move the environment entries from `other` into this one, overwriting
+   * existing variables when a key is already present. */
+  void Update(cmEnvironment&& other);
+
+  /** Modify the value of a variable in-place using a transform function. */
+  template <typename F>
+  void Modify(std::string const& name, F&& transform)
+  {
+    cm::optional<std::string>& val = this->Map[name];
+    if (!val.has_value()) {
+      val = std::string{};
+    }
+    transform(*val);
+  }
+
+  std::vector<std::string> GetVariables() const;
+
+protected:
+  struct EnvNameLess
+  {
+    bool operator()(std::string const& lhs, std::string const& rhs) const;
+  };
+  std::map<std::string, cm::optional<std::string>, EnvNameLess> Map;
+};
diff --git a/bootstrap b/bootstrap
index 6041787a67..48c6491dbf 100755
--- a/bootstrap
+++ b/bootstrap
@@ -340,6 +340,7 @@ CMAKE_CXX_SOURCES="\
   cmELF \
   cmEnableLanguageCommand \
   cmEnableTestingCommand \
+  cmEnvironment \
   cmEvaluatedTargetProperty \
   cmExecProgramCommand \
   cmExecuteProcessCommand \