From 1839385fa95f391f24fcb596c065d322d938b656 Mon Sep 17 00:00:00 2001
From: Paul Gessinger <paul.gessinger@cern.ch>
Date: Tue, 9 Jun 2026 10:51:37 +0200
Subject: [PATCH] build: Add optional in-build ACTS build

---
 CMakeLists.txt             |  2 ++
 cmake/MaybeBuildActs.cmake | 24 ++++++++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 cmake/MaybeBuildActs.cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index a0aa9d6..9ceda04 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -26,6 +26,8 @@ SET( ${PROJECT_NAME}_VERSION_PATCH 0 )
 
 SET( ${PROJECT_NAME}_VERSION  "${${PROJECT_NAME}_VERSION_MAJOR}.${${PROJECT_NAME}_VERSION_MINOR}.${${PROJECT_NAME}_VERSION_PATCH}" )
 
+include(cmake/MaybeBuildActs.cmake)
+
 find_package(EDM4HEP REQUIRED)
 find_package(k4FWCore REQUIRED)
 find_package(Acts REQUIRED COMPONENTS Core PluginDD4hep)
diff --git a/cmake/MaybeBuildActs.cmake b/cmake/MaybeBuildActs.cmake
new file mode 100644
index 0000000..a87136c
--- /dev/null
+++ b/cmake/MaybeBuildActs.cmake
@@ -0,0 +1,24 @@
+# The setup below allows optionally building ACTS directly with the k4ActsTracking build.
+# This avoids having to run separate build and install steps, and also allows getting a 
+# combined compilation database out.
+if(IS_DIRECTORY ${ACTS_SOURCE_DIR})
+  message(STATUS "Using ACTS from: ${ACTS_SOURCE_DIR}")
+
+  include(FetchContent)
+
+  # Build configuration needed for k4ActsTracking.
+  set(ACTS_USE_SYSTEM_NLOHMANN_JSON ON CACHE BOOL "Use system json")
+  set(ACTS_BUILD_PLUGIN_JSON ON CACHE BOOL "Build json plugin")
+  set(ACTS_BUILD_PLUGIN_ROOT ON CACHE BOOL "Build root plugin")
+  set(ACTS_BUILD_PLUGIN_DD4HEP ON CACHE BOOL "Build dd4hep plugin")
+
+
+  set(FETCHCONTENT_SOURCE_DIR_ACTS "${ACTS_SOURCE_DIR}" CACHE PATH "")
+
+  # This makes `find_package(Acts)` transparently defer to `FetchContent`
+  # to being the ACTS build in.
+  FetchContent_Declare(Acts
+    SYSTEM
+    OVERRIDE_FIND_PACKAGE
+  )
+endif()