build: generate a BuildInfo translation unit with version, commit, branch, dirty flag and build kind

cmake/BuildInfo.cmake runs on every build and rewrites the generated
BuildInfo.cpp only when something changed, so a new commit recompiles one
file and relinks. The build kind comes from DLU_BUILD_KIND
(release/ci/local) or CI=true, defaulting to local.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dCommon/BuildInfo.cpp.in

@@ -0,0 +1,13 @@
+// Generated by cmake/BuildInfo.cmake on every build. Do not edit.
+#include "BuildInfo.h"
+
+namespace BuildInfo {
+	const uint8_t versionMajor = @VERSION_MAJOR@;
+	const uint8_t versionMinor = @VERSION_MINOR@;
+	const uint8_t versionPatch = @VERSION_PATCH@;
+	const eBuildKind buildKind = eBuildKind::@BUILD_KIND@;
+	const bool dirty = @BUILD_DIRTY@;
+	const std::string_view commit = "@BUILD_COMMIT@";
+	const std::string_view branch = "@BUILD_BRANCH@";
+	const std::string_view buildString = "@BUILD_STRING@";
+}

dCommon/BuildInfo.h

@@ -0,0 +1,51 @@
+#ifndef BUILDINFO_H
+#define BUILDINFO_H
+
+#include <cstdint>
+#include <string_view>
+
+// What this binary was built from. The values live in BuildInfo.cpp, which cmake/BuildInfo.cmake regenerates on
+// every build (only rewriting it when something changed), so a new commit recompiles that one file and relinks.
+namespace BuildInfo {
+	// Stored in 2 bits of Flags().
+	enum class eBuildKind : uint8_t {
+		UNKNOWN = 0,
+		LOCAL = 1,
+		CI = 2,
+		RELEASE = 3,
+	};
+
+	constexpr uint8_t BUILD_KIND_MASK = 0b011;
+	constexpr uint8_t DIRTY_FLAG = 0b100;
+
+	extern const uint8_t versionMajor;
+	extern const uint8_t versionMinor;
+	extern const uint8_t versionPatch;
+	extern const eBuildKind buildKind;
+	extern const bool dirty; // The build had uncommitted changes to tracked files
+	extern const std::string_view commit; // Full hex commit hash, empty without git
+	extern const std::string_view branch; // Empty when detached or without git
+	extern const std::string_view buildString; // e.g. 3.0.0-experimental+g1a2b3c4d-dirty
+
+	// The build kind in bits 0-1 and the dirty flag in bit 2.
+	inline uint8_t Flags() {
+		return (static_cast<uint8_t>(buildKind) & BUILD_KIND_MASK) | (dirty ? DIRTY_FLAG : 0);
+	}
+
+	// The first 32 bits (8 hex digits) of the commit hash, 0 without git.
+	inline uint32_t CommitPrefix() {
+		uint32_t value = 0;
+		for (size_t i = 0; i < 8 && i < commit.size(); i++) {
+			const char c = commit[i];
+			uint32_t digit = 0;
+			if (c >= '0' && c <= '9') digit = c - '0';
+			else if (c >= 'a' && c <= 'f') digit = c - 'a' + 10;
+			else if (c >= 'A' && c <= 'F') digit = c - 'A' + 10;
+			else return 0;
+			value = (value << 4) | digit;
+		}
+		return commit.size() >= 8 ? value : 0;
+	}
+}
+
+#endif // BUILDINFO_H

dCommon/CMakeLists.txt

@@ -88,7 +88,27 @@ else ()
 	)
 endif ()
 
+# Build identifier (BuildInfo.h). The generator runs on every build but only rewrites BuildInfo.cpp when the
+# commit, branch, dirty state or build kind changed, so only that one file recompiles.
+set(BUILDINFO_SOURCE "${CMAKE_CURRENT_BINARY_DIR}/generated/BuildInfo.cpp")
+add_custom_target(BuildInfoGenerate
+	COMMAND "${CMAKE_COMMAND}"
+		"-DSOURCE_DIR=${PROJECT_SOURCE_DIR}"
+		"-DTEMPLATE=${CMAKE_CURRENT_SOURCE_DIR}/BuildInfo.cpp.in"
+		"-DOUTPUT=${BUILDINFO_SOURCE}"
+		"-DVERSION_MAJOR=${PROJECT_VERSION_MAJOR}"
+		"-DVERSION_MINOR=${PROJECT_VERSION_MINOR}"
+		"-DVERSION_PATCH=${PROJECT_VERSION_PATCH}"
+		-P "${PROJECT_SOURCE_DIR}/cmake/BuildInfo.cmake"
+	BYPRODUCTS "${BUILDINFO_SOURCE}"
+	COMMENT "Checking build identifier"
+	VERBATIM
+)
+add_library(dBuildInfo STATIC "${BUILDINFO_SOURCE}")
+target_include_directories(dBuildInfo PUBLIC "${CMAKE_CURRENT_SOURCE_DIR}")
+add_dependencies(dBuildInfo BuildInfoGenerate)
+
 target_link_libraries(dCommon
-	PUBLIC glm::glm
+	PUBLIC glm::glm dBuildInfo
 	PRIVATE ZLIB::ZLIB bcrypt tinyxml2
 	INTERFACE dDatabase)