docs(clang-tidy): Add Windows build instructions and GeneralsTools mention

- Document Windows built-in approach (recommended)
- Document Mac/Linux plugin approach
- Add instructions for building checks into clang-tidy on Windows
- Mention pre-built binaries in GeneralsTools repository

Author: bobtista

scripts/clang-tidy-plugin/README.md

@@ -0,0 +1,250 @@
+# GeneralsGameCode Clang-Tidy Custom Checks
+
+Custom clang-tidy checks for the GeneralsGameCode codebase. **This is primarily designed for Windows**, where checks are built directly into clang-tidy. Mac/Linux users can optionally use the plugin version.
+
+## Quick Start
+
+### Windows
+
+**Option 1: Use Pre-built clang-tidy**
+- Precompiled binaries can be found at: https://github.com/TheSuperHackers/GeneralsTools
+
+**Option 2: Build It Yourself**
+
+Follow the manual steps in the [Windows: Building Checks Into clang-tidy](#windows-building-checks-into-clang-tidy) section below.
+
+**Usage**
+```powershell
+llvm-project\build\bin\clang-tidy.exe -p build/clang-tidy `
+  --checks='-*,generals-use-is-empty,generals-use-this-instead-of-singleton' `
+  file.cpp
+```
+
+### macOS/Linux (Plugin Version)
+
+If you prefer the plugin approach on macOS/Linux:
+
+```bash
+# Build the plugin
+cd scripts/clang-tidy-plugin
+mkdir build && cd build
+cmake .. -DLLVM_DIR=/path/to/llvm/lib/cmake/llvm
+cmake --build . --config Release
+
+# Use with --load flag
+clang-tidy -p build/clang-tidy \
+  --checks='-*,generals-use-is-empty' \
+  -load scripts/clang-tidy-plugin/build/lib/libGeneralsGameCodeClangTidyPlugin.so \
+  file.cpp
+```
+
+**Note:** On Windows, plugin loading via `--load` is **not supported** due to DLL static initialization limitations. Windows users must use the built-in version.
+
+## Checks
+
+### `generals-use-is-empty`
+
+Finds uses of `getLength() == 0`, `getLength() > 0`, `compare("") == 0`, or `compareNoCase("") == 0` on `AsciiString` and `UnicodeString`, and `Get_Length() == 0` on `StringClass` and `WideStringClass`, and suggests using `isEmpty()`/`Is_Empty()` or `!isEmpty()`/`!Is_Empty()` instead.
+
+**Examples:**
+
+```cpp
+// Before (AsciiString/UnicodeString)
+if (str.getLength() == 0) { ... }
+if (str.getLength() > 0) { ... }
+if (str.compare("") == 0) { ... }
+if (str.compareNoCase("") == 0) { ... }
+if (str.compare(AsciiString::TheEmptyString) == 0) { ... }
+
+// After (AsciiString/UnicodeString)
+if (str.isEmpty()) { ... }
+if (!str.isEmpty()) { ... }
+if (str.isEmpty()) { ... }
+if (str.isEmpty()) { ... }
+if (str.isEmpty()) { ... }
+
+// Before (StringClass/WideStringClass)
+if (str.Get_Length() == 0) { ... }
+if (str.Get_Length() > 0) { ... }
+
+// After (StringClass/WideStringClass)
+if (str.Is_Empty()) { ... }
+if (!str.Is_Empty()) { ... }
+```
+
+### `generals-use-this-instead-of-singleton`
+
+Finds uses of singleton global variables (like `TheGameLogic->method()` or `TheGlobalData->member`) inside member functions of the same class type, and suggests using the member directly (e.g., `method()` or `member`) instead of the singleton reference.
+
+**Examples:**
+
+```cpp
+// Before
+void GameLogic::update() {
+  UnsignedInt now = TheGameLogic->getFrame();
+  TheGameLogic->setFrame(now + 1);
+  TheGameLogic->m_frame = 10;
+}
+
+// After
+void GameLogic::update() {
+  UnsignedInt now = getFrame();
+  setFrame(now + 1);
+  m_frame = 10;
+}
+```
+
+## Prerequisites
+
+Before using clang-tidy, you need to generate a compile commands database:
+
+```bash
+cmake -B build/clang-tidy -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON -G Ninja
+```
+
+This creates `build/clang-tidy/compile_commands.json` which tells clang-tidy how to compile each file.
+
+## Windows: Building Checks Into clang-tidy
+
+Since plugin loading via `--load` doesn't work on Windows, the checks are integrated directly into the clang-tidy source tree. This is the recommended approach for Windows.
+
+### Step 1: Clone and Build LLVM
+
+### Step 2: Copy Plugin Files to LLVM Source Tree
+
+```powershell
+# Create the plugin directory in clang-tools-extra
+mkdir llvm-project\clang-tools-extra\clang-tidy\plugins\generalsgamecode
+
+# Copy the plugin source files
+cp -r scripts\clang-tidy-plugin\*.cpp llvm-project\clang-tools-extra\clang-tidy\plugins\generalsgamecode\
+cp -r scripts\clang-tidy-plugin\*.h llvm-project\clang-tools-extra\clang-tidy\plugins\generalsgamecode\
+cp -r scripts\clang-tidy-plugin\readability llvm-project\clang-tools-extra\clang-tidy\plugins\generalsgamecode\
+```
+
+**Important:** After copying, you need to update the include paths in the headers:
+- Change `#include "clang-tidy/ClangTidyCheck.h"` to `#include "../../../ClangTidyCheck.h"`
+- Change `#include "clang-tidy/ClangTidyModule.h"` to `#include "../../ClangTidyModule.h"`
+
+### Step 3: Create CMakeLists.txt for the Module
+
+Create `llvm-project/clang-tools-extra/clang-tidy/plugins/generalsgamecode/CMakeLists.txt`:
+
+```cmake
+add_clang_library(clangTidyGeneralsGameCodeModule STATIC
+  GeneralsGameCodeTidyModule.cpp
+  readability/UseIsEmptyCheck.cpp
+  readability/UseThisInsteadOfSingletonCheck.cpp
+
+  LINK_LIBS
+  clangTidy
+  clangTidyUtils
+  )
+
+clang_target_link_libraries(clangTidyGeneralsGameCodeModule
+  PRIVATE
+  clangAST
+  clangASTMatchers
+  clangBasic
+  clangLex
+  clangTooling
+  )
+```
+
+### Step 4: Register the Module
+
+**Modify `llvm-project/clang-tools-extra/clang-tidy/CMakeLists.txt`:**
+
+Add the subdirectory:
+```cmake
+add_subdirectory(plugins/generalsgamecode)
+```
+
+Add to `ALL_CLANG_TIDY_CHECKS`:
+```cmake
+set(ALL_CLANG_TIDY_CHECKS
+  ...
+  clangTidyGeneralsGameCodeModule
+  ...
+)
+```
+
+**Modify `llvm-project/clang-tools-extra/clang-tidy/ClangTidyForceLinker.h`:**
+
+Add the anchor to force linker inclusion:
+```cpp
+// This anchor is used to force the linker to link the GeneralsGameCodeModule.
+extern volatile int GeneralsGameCodeModuleAnchorSource;
+static int LLVM_ATTRIBUTE_UNUSED GeneralsGameCodeModuleAnchorDestination =
+    GeneralsGameCodeModuleAnchorSource;
+```
+
+**Modify `llvm-project/clang-tools-extra/clang-tidy/plugins/generalsgamecode/GeneralsGameCodeTidyModule.cpp`:**
+
+Ensure the anchor is defined:
+```cpp
+namespace clang::tidy {
+// ... module registration ...
+
+// Force linker to include this module
+volatile int GeneralsGameCodeModuleAnchorSource = 0;
+}
+```
+
+### Step 5: Rebuild clang-tidy
+
+```powershell
+cd llvm-project\build
+ninja clang-tidy
+```
+
+### Step 6: Use the Built-in Checks
+
+Once rebuilt, the checks are always available - no `--load` flag needed:
+
+```powershell
+llvm-project\build\bin\clang-tidy.exe -p build/clang-tidy `
+  --checks='-*,generals-use-is-empty,generals-use-this-instead-of-singleton' `
+  file.cpp
+```
+
+
+## macOS/Linux: Building the Plugin
+
+If you're on macOS or Linux and want to use the plugin version:
+
+### Prerequisites
+
+**macOS:**
+```bash
+brew install llvm@21
+```
+
+**Linux (Ubuntu/Debian):**
+```bash
+sudo apt-get install llvm-21-dev clang-21 libclang-21-dev
+```
+
+### Building the Plugin
+
+```bash
+cd scripts/clang-tidy-plugin
+mkdir build && cd build
+cmake .. -DLLVM_DIR=/path/to/llvm/lib/cmake/llvm -DClang_DIR=/path/to/clang/lib/cmake/clang
+cmake --build . --config Release
+```
+
+The plugin will be built as a shared library (`.so` on Linux, `.dylib` on macOS) in the `build/lib/` directory.
+
+### Using the Plugin
+
+```bash
+clang-tidy -p build/clang-tidy \
+  --checks='-*,generals-use-is-empty,generals-use-this-instead-of-singleton' \
+  -load scripts/clang-tidy-plugin/build/lib/libGeneralsGameCodeClangTidyPlugin.so \
+  file.cpp
+```
+
+**Important:** The plugin must be built with the same LLVM version as the `clang-tidy` executable in your PATH. The CMake build will display which LLVM version it found (e.g., `Found LLVM 21.1.7`). Verify this matches your `clang-tidy --version` output.
+
+- Windows plugins not working is a known limitation - see [GitHub issue #159710](https://github.com/llvm/llvm-project/issues/159710) and [LLVM Discourse discussion](https://discourse.llvm.org/t/clang-tidy-is-clang-tidy-out-of-tree-check-plugin-load-mechanism-guaranteed-to-work-with-msvc/84111)