# TODO Validate the argument of "requires"
function(get_project_build_order VAR)
  # This file will describe the dependencies among projects
  set(DEPS_FILE "${CMAKE_CURRENT_BINARY_DIR}/compiler.deps")

  # Remove .deps file
  file(REMOVE "${DEPS_FILE}")

  # Let's create .deps file
  list_subdirectories(PROJECT_DIRS)

  foreach(PROJECT_DIR IN ITEMS ${PROJECT_DIRS})
    set(SUCC "${PROJECT_DIR}")
    set(REQUIRES_FILE "${CMAKE_CURRENT_SOURCE_DIR}/${PROJECT_DIR}/requires.cmake")

    macro(require PRED)
      file(APPEND "${DEPS_FILE}" "${PRED} ${SUCC}\n")
    endmacro(require)

    file(APPEND "${DEPS_FILE}" "${SUCC} ${SUCC}\n")
    if(EXISTS "${REQUIRES_FILE}")
      include(${REQUIRES_FILE})
    endif(EXISTS "${REQUIRES_FILE}")
  endforeach(PROJECT_DIR)

  # NOTE "tsort" is a part of the POSIX.1 standard.
  #
  # Reference: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/tsort.html
  execute_process(COMMAND tsort "${DEPS_FILE}"
                  OUTPUT_VARIABLE ORDER
                  OUTPUT_STRIP_TRAILING_WHITESPACE)

  # Remove newline characters
  # TODO Check which one (UNIX_COMMAND or WINDOWS_COMMAND) is correct
  separate_arguments(ORDER UNIX_COMMAND ${ORDER})

  set(${VAR} "${ORDER}" PARENT_SCOPE)
endfunction(get_project_build_order)

function(add_compiler_directory DIR)
  string(TOUPPER ${DIR} PREFIX)

  option(BUILD_COMPILER_${PREFIX} "Build compiler/${dir}" ON)

  if(BUILD_COMPILER_${PREFIX})
    add_subdirectory(${DIR})
  endif(BUILD_COMPILER_${PREFIX})
endfunction(add_compiler_directory)

function(add_compiler_directories)
  get_project_build_order(PROJECT_DIRS)

  foreach(PROJECT_DIR IN ITEMS ${PROJECT_DIRS})
    add_compiler_directory(${PROJECT_DIR})
  endforeach(PROJECT_DIR)
endfunction(add_compiler_directories)

add_compiler_directories()