#
# - This module provides the function
# itk_target_link_libraries_with_dynamic_lookup which can be used to
# "weakly" link loadable module.
#
# Link a library to a target such that the symbols are resolved at
# run-time not link-time. This should be used when compiling a
# loadable module when the symbols should be resolve from the run-time
# environment where the module is loaded, and not a specific system
# library.
#
# Specifically, for OSX it uses undefined dynamic_lookup. This is
# similar to using "-shared" on Linux where undefined symbols are
# ignored.
#
# Additionally, the linker is checked to see if it supports undefined
# symbols when linking a shared library. If it does then the library
# is not linked when specified with this function.
#
# https://blog.tim-smith.us/2015/09/python-extension-modules-os-x/
#

# Function: _itkCheckUndefinedSymbolsAllowed
#
# Check if the linker allows undefined symbols for shared libraries.
#
# ITK_UNDEFINED_SYMBOLS_ALLOWED - true if the linker will allow
#   undefined symbols for shared libraries
#

function(_itkCheckUndefinedSymbolsAllowed)

  set(VARIABLE "ITK_UNDEFINED_SYMBOLS_ALLOWED")
  set(cache_var "${VARIABLE}_hash")

  # hash the CMAKE_FLAGS passed and check cache to know if we need to rerun
  string(MD5 cmake_flags_hash "${CMAKE_SHARED_LINKER_FLAGS}")

  if(NOT DEFINED "${cache_var}")
    unset("${VARIABLE}" CACHE)
  elseif(
    NOT
    "${${cache_var}}"
    STREQUAL
    "${cmake_flags_hash}")
    unset("${VARIABLE}" CACHE)
  endif()

  if(NOT DEFINED "${VARIABLE}")
    set(test_project_dir "${PROJECT_BINARY_DIR}/CMakeTmp/${VARIABLE}")

    file(
      WRITE "${test_project_dir}/CMakeLists.txt"
      "
cmake_minimum_required(VERSION ${CMAKE_MINIMUM_REQUIRED_VERSION})
project(undefined C)
add_library(foo SHARED \"foo.c\")
")

    file(
      WRITE "${test_project_dir}/foo.c"
      "
extern int bar(void);
int foo(void) {return bar()+1;}
")
    # APPLE: the CMAKE_MACOSX_RPATH flag should be passed.
    if(APPLE)
      set(_rpath_arg "-DCMAKE_MACOSX_RPATH='${CMAKE_MACOSX_RPATH}'")
    else()
      set(_rpath_arg)
    endif()

    try_compile(
      ${VARIABLE} "${test_project_dir}"
      "${test_project_dir}" undefined
      CMAKE_FLAGS "-DCMAKE_SHARED_LINKER_FLAGS='${CMAKE_SHARED_LINKER_FLAGS}'" ${_rpath_arg}
      OUTPUT_VARIABLE output)

    set(${cache_var}
        "${cmake_flags_hash}"
        CACHE INTERNAL "hashed try_compile flags")

    if(${VARIABLE})
      message(STATUS "Performing Test ${VARIABLE} - Success")
    else()
      message(STATUS "Performing Test ${VARIABLE} - Failed")
      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
           "Performing Test ${VARIABLE} failed with the following output:\n" "${OUTPUT}\n")
    endif()
  endif()
endfunction()

_itkcheckundefinedsymbolsallowed()

macro(itk_target_link_libraries_with_dynamic_lookup target)
  if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
    set_target_properties(${target} PROPERTIES LINK_FLAGS "-undefined dynamic_lookup")
  elseif(ITK_UNDEFINED_SYMBOLS_ALLOWED)
    # linker allows undefined symbols, let's just not link
  else()
    target_link_libraries(${target} ${ARGN})
  endif()
endmacro()