include(CMakeParseArguments)

# macro set_ifnot(<var> <value>)
#       If variable var is not set, set its value to that provided
#
macro(set_ifnot _var _value)
  if(NOT ${_var})
    set(${_var} ${_value})
  endif()
endmacro()

# function enum_option(<option>
#                      VALUES <value1> ... <valueN>
#                      TYPE   <valuetype>
#                      DOC    <docstring>
#                      [DEFAULT <elem>]
#                      [CASE_INSENSITIVE])
#          Declare a cache variable <option> that can only take values
#          listed in VALUES. TYPE may be FILEPATH, PATH or STRING.
#          <docstring> should describe that option, and will appear in
#          the interactive CMake interfaces. If DEFAULT is provided,
#          <elem> will be taken as the zero-indexed element in VALUES
#          to which the value of <option> should default to if not
#          provided. Otherwise, the default is taken as the first
#          entry in VALUES. If CASE_INSENSITIVE is present, then
#          checks of the value of <option> against the allowed values
#          will ignore the case when performing string comparison.
#

function(enum_option _var)
  set(options CASE_INSENSITIVE)
  set(oneValueArgs DOC TYPE DEFAULT)
  set(multiValueArgs VALUES)
  cmake_parse_arguments(_ENUMOP "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})

  # - Validation as needed arguments
  if(NOT _ENUMOP_VALUES)
    message(FATAL_ERROR "enum_option must be called with non-empty VALUES\n(Called for enum_option '${_var}')")
  endif()

  # - Set argument defaults as needed
  if(_ENUMOP_CASE_INSENSITIVE)
    set(_ci_values )
    foreach(_elem ${_ENUMOP_VALUES})
      string(TOLOWER "${_elem}" _ci_elem)
      list(APPEND _ci_values "${_ci_elem}")
    endforeach()
    set(_ENUMOP_VALUES ${_ci_values})
  endif()

  set_ifnot(_ENUMOP_TYPE STRING)
  set_ifnot(_ENUMOP_DEFAULT 0)
  list(GET _ENUMOP_VALUES ${_ENUMOP_DEFAULT} _default)

  if(NOT DEFINED ${_var})
    set(${_var} ${_default} CACHE ${_ENUMOP_TYPE} "${_ENUMOP_DOC} (${_ENUMOP_VALUES})")
  else()
    set(_var_tmp ${${_var}})
    if(_ENUMOP_CASE_INSENSITIVE)
      string(TOLOWER ${_var_tmp} _var_tmp)
    endif()

    list(FIND _ENUMOP_VALUES ${_var_tmp} _elem)
    if(_elem LESS 0)
      message(FATAL_ERROR "Value '${${_var}}' for variable ${_var} is not allowed\nIt must be selected from the set: ${_ENUMOP_VALUES} (DEFAULT: ${_default})\n"
)
    else()
      # - convert to lowercase
      if(_ENUMOP_CASE_INSENSITIVE)
        set(${_var} ${_var_tmp} CACHE ${_ENUMOP_TYPE} "${_ENUMOP_DOC} (${_ENUMOP_VALUES})" FORCE)
      endif()
    endif()
  endif()
endfunction()
