# Distributed under the MIT License.
# See LICENSE.txt for details.

# Support for precompiled headers (PCH)
# =====================================
#
# Precompiled headers are a way to speed up compilation by precompiling
# frequently used headers.
#
# We have the following requirements on the PCH:
# - it must be rebuilt whenever anything it includes is changed.
# - it must be removed when running `make clean`
# - it must be generated before any object files are built that
#   use it
# - it must be able to use flags from other targets (since in modern CMake
#   flags, etc. are propagated via targets)
# - the correct flags to include the PCH must be propagated to any targets
#   that use the PCH.
#
# We use CMake's PCH support to accomplish this. It is documented here:
# https://cmake.org/cmake/help/latest/command/target_precompile_headers.html.
#
# The PCH is generated by compiling a source file that includes all the headers
# we want to precompile. These precompiled headers are listed in
# `tools/SpectrePch.hpp`.
#
# Instructions for using the PCH:
# -------------------------------
#
# Targets can use the PCH generated for the `SpectrePch` library. They must also
# link the `SpectrePchFlags` library so they compile with the same flags as the
# PCH. Make sure to check the PCH exist before using them:
#
#     if(TARGET SpectrePch)
#       target_precompile_headers(${TARGET_NAME} REUSE_FROM SpectrePch)
#       target_link_libraries(${TARGET_NAME} PRIVATE SpectrePchFlags)
#     endif()

# This library tracks all compiler flags to build the PCH. Targets that use
# the PCH must link this library so they compile with the same flags as the
# PCH.
add_library(SpectrePchFlags INTERFACE)
target_link_libraries(
  SpectrePchFlags
  INTERFACE
  Blaze
  Brigand
  Charmxx::charmxx
  Charmxx::pup
  HDF5::HDF5
  SpectreFlags
  SpectreKokkos
  )

# Targets can reuse the PCH generated for this library. They must also link
# the `SpectrePchFlags` library. Notes:
# - We store the PCH header in ${CMAKE_SOURCE_DIR}/tools so that it is not
#   accidentally included anywhere.
# - We also store a source file that just includes the PCH header so the
#   library has something to compile.
# - This is an object library because we don't need to actually link it, we
#   only need to build it so it generates the PCH.
configure_file(
  ${CMAKE_SOURCE_DIR}/tools/SpectrePch.hpp
  ${CMAKE_BINARY_DIR}
  )
set(SPECTRE_PCH_HEADER ${CMAKE_BINARY_DIR}/SpectrePch.hpp)
file(GENERATE
  OUTPUT ${CMAKE_BINARY_DIR}/SpectrePch.cpp
  CONTENT "#include \"${SPECTRE_PCH_HEADER}\"\n")
add_library(
  SpectrePch
  OBJECT
  ${CMAKE_BINARY_DIR}/SpectrePch.cpp)
target_precompile_headers(
  SpectrePch
  PRIVATE
  ${SPECTRE_PCH_HEADER}
  )
target_link_libraries(
  SpectrePch
  PRIVATE
  SpectrePchFlags
  )