FindBoost: Overhaul caching and search repeat behavior

Overhaul the implementation as follows:

(1) Do not cache result variables such as Boost_VERSION,
Boost_LIB_VERSION, Boost_LIBRARY_DIRS, Boost_${COMPONENT}_FOUND,
Boost_${COMPONENT}_LIBRARY, or Boost_LIB_DIAGNOSTIC_DEFINITIONS that are
derived uniquely from other search results.  The user should not edit
them anyway.

(2) Add cache value Boost_LIBRARY_DIR to hold the single directory
expected to contain all libraries.  Once one library is found, search
only that directory for other libraries.

(3) Use the find_library NAMES_PER_DIR option to consider all possible
library names at the same time.

(4) Collect all documented input and cache variables and detect when
they have been changed by the user.  Discard prior search results that
may have been influenced by the changes and search for them again.
Environment variables are not expected to be persistent so use them only
as hints and do not consider changes to them to be meaningful.

1 files changed, 208 insertions, 128 deletions

diff --git a/Modules/FindBoost.cmake b/Modules/FindBoost.cmake
index 77a8991..1a9dfbd 100644
--- a/Modules/FindBoost.cmake
+++ b/Modules/FindBoost.cmake
@@ -200,16 +200,16 @@
 #   BOOST_LIBRARYDIR             Set this to the lib directory of Boost, if the
 #                                module has problems finding the proper Boost installation
 #
+#   Boost_INCLUDE_DIR            CMake cache entries storing Boost include directory
+#   Boost_LIBRARY_DIR            and library directory.
+#
 # Variables defined by this module:
 #
 #   Boost_FOUND                         System has Boost, this means the include dir was
 #                                       found, as well as all the libraries specified in
 #                                       the COMPONENTS list.
 #
-#   Boost_INCLUDE_DIRS                  Boost include directories: not cached
-#
-#   Boost_INCLUDE_DIR                   This is almost the same as above, but this one is
-#                                       cached and may be modified by advanced users
+#   Boost_INCLUDE_DIRS                  Boost include directories
 #
 #   Boost_LIBRARIES                     Link to these to use the Boost libraries that you
 #                                       specified: not cached
@@ -242,7 +242,7 @@
 #                                       when needed).
 
 #=============================================================================
-# Copyright 2006-2009 Kitware, Inc.
+# Copyright 2006-2012 Kitware, Inc.
 # Copyright 2006-2008 Andreas Schneider <mail@cynapses.org>
 # Copyright 2007      Wengo
 # Copyright 2007      Mike Jackson
@@ -350,29 +350,54 @@ macro(_Boost_ADJUST_LIB_VARS basename)
     endif()
 
     if(Boost_${basename}_LIBRARY)
-      set(Boost_${basename}_LIBRARY ${Boost_${basename}_LIBRARY} CACHE FILEPATH "The Boost ${basename} library")
-
-      # Remove superfluous "debug" / "optimized" keywords from
-      # Boost_LIBRARY_DIRS
-      foreach(_boost_my_lib ${Boost_${basename}_LIBRARY})
-        get_filename_component(_boost_my_lib_path "${_boost_my_lib}" PATH)
-        list(APPEND Boost_LIBRARY_DIRS ${_boost_my_lib_path})
-      endforeach()
-      list(REMOVE_DUPLICATES Boost_LIBRARY_DIRS)
-
-      set(Boost_LIBRARY_DIRS ${Boost_LIBRARY_DIRS} CACHE FILEPATH "Boost library directory")
-      set(Boost_${basename}_FOUND ON CACHE INTERNAL "Whether the Boost ${basename} library found")
+      set(Boost_${basename}_FOUND ON)
     endif()
 
   endif()
   # Make variables changeble to the advanced user
   mark_as_advanced(
-      Boost_${basename}_LIBRARY
       Boost_${basename}_LIBRARY_RELEASE
       Boost_${basename}_LIBRARY_DEBUG
   )
 endmacro()
 
+macro(_Boost_CHANGE_DETECT changed_var)
+  set(${changed_var} 0)
+  foreach(v ${ARGN})
+    if(DEFINED _Boost_COMPONENTS_SEARCHED)
+      if(${v})
+        if(_${v}_LAST)
+          string(COMPARE NOTEQUAL "${${v}}" "${_${v}_LAST}" _${v}_CHANGED)
+        else()
+          set(_${v}_CHANGED 1)
+        endif()
+      elseif(_${v}_LAST)
+        set(_${v}_CHANGED 1)
+      endif()
+      if(_${v}_CHANGED)
+        set(${changed_var} 1)
+      endif()
+    else()
+      set(_${v}_CHANGED 0)
+    endif()
+  endforeach()
+endmacro()
+
+macro(_Boost_FIND_LIBRARY var)
+  find_library(${var} ${ARGN})
+
+  # If we found the first library save Boost_LIBRARY_DIR.
+  if(${var} AND NOT Boost_LIBRARY_DIR)
+    get_filename_component(_dir "${${var}}" PATH)
+    set(Boost_LIBRARY_DIR "${_dir}" CACHE PATH "Boost library directory" FORCE)
+  endif()
+
+  # If Boost_LIBRARY_DIR is known then search only there.
+  if(Boost_LIBRARY_DIR)
+    set(_boost_LIBRARY_SEARCH_DIRS ${Boost_LIBRARY_DIR} NO_DEFAULT_PATH)
+  endif()
+endmacro()
+
 #-------------------------------------------------------------------------------
 
 #
@@ -398,7 +423,7 @@ endfunction()
 function(_Boost_MARK_COMPONENTS_FOUND _yes_or_no)
   foreach(COMPONENT ${Boost_FIND_COMPONENTS})
     string(TOUPPER ${COMPONENT} UPPERCOMPONENT)
-    set(Boost_${UPPERCOMPONENT}_FOUND ${_yes_or_no} CACHE INTERNAL "Whether the Boost ${COMPONENT} library found" FORCE)
+    set(Boost_${UPPERCOMPONENT}_FOUND ${_yes_or_no})
   endforeach()
 endfunction()
 
@@ -596,48 +621,34 @@ if(WIN32)
   # then defining BOOST_LIB_DIAGNOSTIC will cause the auto-linking
   # code to emit a #pragma message each time a library is selected
   # for linking.
-  set(Boost_LIB_DIAGNOSTIC_DEFINITIONS
-    "-DBOOST_LIB_DIAGNOSTIC" CACHE STRING "Boost diagnostic define")
+  set(Boost_LIB_DIAGNOSTIC_DEFINITIONS "-DBOOST_LIB_DIAGNOSTIC")
 endif()
 
-set(_boost_INCLUDE_SEARCH_DIRS_SYSTEM
-  C:/boost/include
-  C:/boost
-  /sw/local/include
-)
-
 _Boost_CHECK_SPELLING(Boost_ROOT)
 _Boost_CHECK_SPELLING(Boost_LIBRARYDIR)
 _Boost_CHECK_SPELLING(Boost_INCLUDEDIR)
 
-# If BOOST_ROOT was defined in the environment, use it.
-if (NOT BOOST_ROOT AND NOT $ENV{Boost_DIR} STREQUAL "")
-  set(BOOST_ROOT $ENV{Boost_DIR})
-endif()
-
-# If BOOST_ROOT was defined in the environment, use it.
-if (NOT BOOST_ROOT AND NOT $ENV{BOOST_ROOT} STREQUAL "")
-  set(BOOST_ROOT $ENV{BOOST_ROOT})
-endif()
-
-# If BOOSTROOT was defined in the environment, use it.
-if (NOT BOOST_ROOT AND NOT $ENV{BOOSTROOT} STREQUAL "")
-  set(BOOST_ROOT $ENV{BOOSTROOT})
-endif()
-
-# If BOOST_INCLUDEDIR was defined in the environment, use it.
-if( NOT $ENV{BOOST_INCLUDEDIR} STREQUAL "" )
-  set(BOOST_INCLUDEDIR $ENV{BOOST_INCLUDEDIR})
-endif()
-
-# If BOOST_LIBRARYDIR was defined in the environment, use it.
-if( NOT $ENV{BOOST_LIBRARYDIR} STREQUAL "" )
-  set(BOOST_LIBRARYDIR $ENV{BOOST_LIBRARYDIR})
+# Collect environment variable inputs as hints.  Do not consider changes.
+foreach(v BOOSTROOT BOOST_ROOT BOOST_INCLUDEDIR BOOST_LIBRARYDIR)
+  set(_env $ENV{${v}})
+  if(_env)
+    file(TO_CMAKE_PATH "${_env}" _ENV_${v})
+  else()
+    set(_ENV_${v} "")
+  endif()
+endforeach()
+if(NOT _ENV_BOOST_ROOT AND _ENV_BOOSTROOT)
+  set(_ENV_BOOST_ROOT "${_ENV_BOOSTROOT}")
 endif()
 
-if( BOOST_ROOT )
-  file(TO_CMAKE_PATH ${BOOST_ROOT} BOOST_ROOT)
+# Collect inputs and cached results.  Detect changes since the last run.
+if(NOT BOOST_ROOT AND BOOSTROOT)
+  set(BOOST_ROOT "${BOOSTROOT}")
 endif()
+set(_Boost_VARS_DIR
+  BOOST_ROOT
+  Boost_NO_SYSTEM_PATHS
+  )
 
 if(Boost_DEBUG)
   message(STATUS "[ ${CMAKE_CURRENT_LIST_FILE}:${CMAKE_CURRENT_LIST_LINE} ] "
@@ -652,32 +663,44 @@ if(Boost_DEBUG)
                  "_boost_TEST_VERSIONS = ${_boost_TEST_VERSIONS}")
 endif()
 
-if( Boost_NO_SYSTEM_PATHS)
-  set(_boost_FIND_OPTIONS NO_CMAKE_SYSTEM_PATH)
-else()
-  set(_boost_INCLUDE_SEARCH_DIRS ${_boost_INCLUDE_SEARCH_DIRS_SYSTEM})
-endif()
+# ------------------------------------------------------------------------
+#  Search for Boost include DIR
+# ------------------------------------------------------------------------
 
-if( BOOST_ROOT )
-  set(_boost_INCLUDE_SEARCH_DIRS
-    ${BOOST_ROOT}/include
-    ${BOOST_ROOT}
-    ${_boost_INCLUDE_SEARCH_DIRS})
+set(_Boost_VARS_INC BOOST_INCLUDEDIR Boost_INCLUDE_DIR Boost_ADDITIONAL_VERSIONS)
+_Boost_CHANGE_DETECT(_Boost_CHANGE_INCDIR ${_Boost_VARS_DIR} ${_Boost_VARS_INC})
+# Clear Boost_INCLUDE_DIR if it did not change but other input affecting the
+# location did.  We will find a new one based on the new inputs.
+if(_Boost_CHANGE_INCDIR AND NOT _Boost_INCLUDE_DIR_CHANGED)
+  unset(Boost_INCLUDE_DIR CACHE)
 endif()
 
-# prepend BOOST_INCLUDEDIR to search path if specified
-if( BOOST_INCLUDEDIR )
-  file(TO_CMAKE_PATH ${BOOST_INCLUDEDIR} BOOST_INCLUDEDIR)
-  set(_boost_INCLUDE_SEARCH_DIRS
-    ${BOOST_INCLUDEDIR} ${_boost_INCLUDE_SEARCH_DIRS})
-endif()
+if(NOT Boost_INCLUDE_DIR)
+  set(_boost_INCLUDE_SEARCH_DIRS "")
+  if(BOOST_INCLUDEDIR)
+    list(APPEND _boost_INCLUDE_SEARCH_DIRS ${BOOST_INCLUDEDIR})
+  elseif(_ENV_BOOST_INCLUDEDIR)
+    list(APPEND _boost_INCLUDE_SEARCH_DIRS ${_ENV_BOOST_INCLUDEDIR})
+  endif()
 
-# ------------------------------------------------------------------------
-#  Search for Boost include DIR
-# ------------------------------------------------------------------------
-# Try to find Boost by stepping backwards through the Boost versions
-# we know about.
-if( NOT Boost_INCLUDE_DIR )
+  if( BOOST_ROOT )
+    list(APPEND _boost_INCLUDE_SEARCH_DIRS ${BOOST_ROOT}/include ${BOOST_ROOT})
+  elseif( _ENV_BOOST_ROOT )
+    list(APPEND _boost_INCLUDE_SEARCH_DIRS ${_ENV_BOOST_ROOT}/include ${_ENV_BOOST_ROOT})
+  endif()
+
+  if( Boost_NO_SYSTEM_PATHS)
+    list(APPEND _boost_INCLUDE_SEARCH_DIRS NO_CMAKE_SYSTEM_PATH)
+  else()
+    list(APPEND _boost_INCLUDE_SEARCH_DIRS PATHS
+      C:/boost/include
+      C:/boost
+      /sw/local/include
+      )
+  endif()
+
+  # Try to find Boost by stepping backwards through the Boost versions
+  # we know about.
   # Build a list of path suffixes for each version.
   set(_boost_PATH_SUFFIXES)
   foreach(_boost_VER ${_boost_TEST_VERSIONS})
@@ -718,7 +741,6 @@ if( NOT Boost_INCLUDE_DIR )
     NAMES         boost/config.hpp
     HINTS         ${_boost_INCLUDE_SEARCH_DIRS}
     PATH_SUFFIXES ${_boost_PATH_SUFFIXES}
-    ${_boost_FIND_OPTIONS}
     )
 endif()
 
@@ -742,9 +764,6 @@ if(Boost_INCLUDE_DIR)
   string(REGEX REPLACE ".*#define BOOST_LIB_VERSION \"([0-9_]+)\".*" "\\1" Boost_LIB_VERSION "${_boost_VERSION_HPP_CONTENTS}")
   unset(_boost_VERSION_HPP_CONTENTS)
 
-  set(Boost_LIB_VERSION ${Boost_LIB_VERSION} CACHE INTERNAL "The library version string for boost libraries")
-  set(Boost_VERSION ${Boost_VERSION} CACHE INTERNAL "The version number for boost libraries")
-
   if(NOT "${Boost_VERSION}" STREQUAL "0")
     math(EXPR Boost_MAJOR_VERSION "${Boost_VERSION} / 100000")
     math(EXPR Boost_MINOR_VERSION "${Boost_VERSION} / 100 % 1000")
@@ -767,6 +786,18 @@ endif()
 #  Suffix initialization and compiler suffix detection.
 # ------------------------------------------------------------------------
 
+set(_Boost_VARS_NAME
+  Boost_COMPILER
+  Boost_THREADAPI
+  Boost_USE_DEBUG_PYTHON
+  Boost_USE_MULTITHREADED
+  Boost_USE_STATIC_LIBS
+  Boost_USE_STATIC_RUNTIME
+  Boost_USE_STLPORT
+  Boost_USE_STLPORT_DEPRECATED_NATIVE_IOSTREAMS
+  )
+_Boost_CHANGE_DETECT(_Boost_CHANGE_LIBNAME ${_Boost_VARS_NAME})
+
 # Setting some more suffixes for the library
 set(Boost_LIB_PREFIX "")
 if ( WIN32 AND Boost_USE_STATIC_LIBS AND NOT CYGWIN)
@@ -848,35 +879,44 @@ endif()
 # ------------------------------------------------------------------------
 #  Begin finding boost libraries
 # ------------------------------------------------------------------------
-
-if(BOOST_ROOT)
-  set(_boost_LIBRARY_SEARCH_DIRS_ALWAYS
-    ${BOOST_ROOT}/lib
-    ${BOOST_ROOT}/stage/lib)
+set(_Boost_VARS_LIB BOOST_LIBRARYDIR Boost_LIBRARY_DIR)
+_Boost_CHANGE_DETECT(_Boost_CHANGE_LIBDIR ${_Boost_VARS_DIR} ${_Boost_VARS_LIB} Boost_INCLUDE_DIR)
+# Clear Boost_LIBRARY_DIR if it did not change but other input affecting the
+# location did.  We will find a new one based on the new inputs.
+if(_Boost_CHANGE_LIBDIR AND NOT _Boost_LIBRARY_DIR_CHANGED)
+  unset(Boost_LIBRARY_DIR CACHE)
 endif()
-set(_boost_LIBRARY_SEARCH_DIRS_ALWAYS
-  ${_boost_LIBRARY_SEARCH_DIRS_ALWAYS}
-  ${Boost_INCLUDE_DIR}/lib
-  ${Boost_INCLUDE_DIR}/../lib
-  ${Boost_INCLUDE_DIR}/stage/lib
-)
-set(_boost_LIBRARY_SEARCH_DIRS_SYSTEM
-  C:/boost/lib
-  C:/boost
-  /sw/local/lib
-)
-set(_boost_LIBRARY_SEARCH_DIRS ${_boost_LIBRARY_SEARCH_DIRS_ALWAYS})
-if( Boost_NO_SYSTEM_PATHS )
-  set(_boost_FIND_OPTIONS NO_CMAKE_SYSTEM_PATH)
+
+if(Boost_LIBRARY_DIR)
+  set(_boost_LIBRARY_SEARCH_DIRS ${Boost_LIBRARY_DIR} NO_DEFAULT_PATH)
 else()
-  list(APPEND _boost_LIBRARY_SEARCH_DIRS ${_boost_LIBRARY_SEARCH_DIRS_SYSTEM})
-endif()
+  set(_boost_LIBRARY_SEARCH_DIRS "")
+  if(BOOST_LIBRARYDIR)
+    list(APPEND _boost_LIBRARY_SEARCH_DIRS ${BOOST_LIBRARYDIR})
+  elseif(_ENV_BOOST_LIBRARYDIR)
+    list(APPEND _boost_LIBRARY_SEARCH_DIRS ${_ENV_BOOST_LIBRARYDIR})
+  endif()
+
+  if(BOOST_ROOT)
+    list(APPEND _boost_LIBRARY_SEARCH_DIRS ${BOOST_ROOT}/lib ${BOOST_ROOT}/stage/lib)
+  elseif(_ENV_BOOST_ROOT)
+    list(APPEND _boost_LIBRARY_SEARCH_DIRS ${_ENV_BOOST_ROOT}/lib ${_ENV_BOOST_ROOT}/stage/lib)
+  endif()
 
-# prepend BOOST_LIBRARYDIR to search path if specified
-if( BOOST_LIBRARYDIR )
-  file(TO_CMAKE_PATH ${BOOST_LIBRARYDIR} BOOST_LIBRARYDIR)
-  set(_boost_LIBRARY_SEARCH_DIRS
-    ${BOOST_LIBRARYDIR} ${_boost_LIBRARY_SEARCH_DIRS})
+  list(APPEND _boost_LIBRARY_SEARCH_DIRS
+    ${Boost_INCLUDE_DIR}/lib
+    ${Boost_INCLUDE_DIR}/../lib
+    ${Boost_INCLUDE_DIR}/stage/lib
+    )
+  if( Boost_NO_SYSTEM_PATHS )
+    list(APPEND _boost_LIBRARY_SEARCH_DIRS NO_CMAKE_SYSTEM_PATH)
+  else()
+    list(APPEND _boost_LIBRARY_SEARCH_DIRS PATHS
+      C:/boost/lib
+      C:/boost
+      /sw/local/lib
+      )
+  endif()
 endif()
 
 if(Boost_DEBUG)
@@ -928,11 +968,21 @@ if(Boost_VERSION AND Boost_FIND_COMPONENTS)
    endif()
 endif()
 
+# If the user changed any of our control inputs flush previous results.
+if(_Boost_CHANGE_LIBDIR OR _Boost_CHANGE_LIBNAME)
+  foreach(COMPONENT ${_Boost_COMPONENTS_SEARCHED})
+    string(TOUPPER ${COMPONENT} UPPERCOMPONENT)
+    foreach(c DEBUG RELEASE)
+      set(_var Boost_${UPPERCOMPONENT}_LIBRARY_${c})
+      unset(${_var} CACHE)
+      set(${_var} "${_var}-NOTFOUND")
+    endforeach()
+  endforeach()
+  set(_Boost_COMPONENTS_SEARCHED "")
+endif()
+
 foreach(COMPONENT ${Boost_FIND_COMPONENTS})
   string(TOUPPER ${COMPONENT} UPPERCOMPONENT)
-  set( Boost_${UPPERCOMPONENT}_LIBRARY "Boost_${UPPERCOMPONENT}_LIBRARY-NOTFOUND" )
-  set( Boost_${UPPERCOMPONENT}_LIBRARY_RELEASE "Boost_${UPPERCOMPONENT}_LIBRARY_RELEASE-NOTFOUND" )
-  set( Boost_${UPPERCOMPONENT}_LIBRARY_DEBUG "Boost_${UPPERCOMPONENT}_LIBRARY_DEBUG-NOTFOUND")
 
   set( _boost_docstring_release "Boost ${COMPONENT} library (release)")
   set( _boost_docstring_debug   "Boost ${COMPONENT} library (debug)")
@@ -961,12 +1011,12 @@ foreach(COMPONENT ${Boost_FIND_COMPONENTS})
     message(STATUS "[ ${CMAKE_CURRENT_LIST_FILE}:${CMAKE_CURRENT_LIST_LINE} ] "
                    "Searching for ${UPPERCOMPONENT}_LIBRARY_RELEASE: ${_boost_RELEASE_NAMES}")
   endif()
-  find_library(Boost_${UPPERCOMPONENT}_LIBRARY_RELEASE
-      NAMES ${_boost_RELEASE_NAMES}
-      HINTS ${_boost_LIBRARY_SEARCH_DIRS}
-      ${_boost_FIND_OPTIONS}
-      DOC "${_boost_docstring_release}"
-  )
+  _Boost_FIND_LIBRARY(Boost_${UPPERCOMPONENT}_LIBRARY_RELEASE
+    NAMES ${_boost_RELEASE_NAMES}
+    HINTS ${_boost_LIBRARY_SEARCH_DIRS}
+    NAMES_PER_DIR
+    DOC "${_boost_docstring_release}"
+    )
 
   #
   # Find DEBUG libraries
@@ -993,12 +1043,12 @@ foreach(COMPONENT ${Boost_FIND_COMPONENTS})
     message(STATUS "[ ${CMAKE_CURRENT_LIST_FILE}:${CMAKE_CURRENT_LIST_LINE} ] "
                    "Searching for ${UPPERCOMPONENT}_LIBRARY_DEBUG: ${_boost_DEBUG_NAMES}")
   endif()
-  find_library(Boost_${UPPERCOMPONENT}_LIBRARY_DEBUG
-      NAMES ${_boost_DEBUG_NAMES}
-      HINTS ${_boost_LIBRARY_SEARCH_DIRS}
-      ${_boost_FIND_OPTIONS}
-      DOC "${_boost_docstring_debug}"
-  )
+  _Boost_FIND_LIBRARY(Boost_${UPPERCOMPONENT}_LIBRARY_DEBUG
+    NAMES ${_boost_DEBUG_NAMES}
+    HINTS ${_boost_LIBRARY_SEARCH_DIRS}
+    NAMES_PER_DIR
+    DOC "${_boost_docstring_debug}"
+    )
 
   if(Boost_REALPATH)
     _Boost_SWAP_WITH_REALPATH(Boost_${UPPERCOMPONENT}_LIBRARY_RELEASE "${_boost_docstring_release}")
@@ -1025,9 +1075,8 @@ endif()
 #        for libraries...
 # ------------------------------------------------------------------------
 
-set(Boost_INCLUDE_DIRS
-  ${Boost_INCLUDE_DIR}
-)
+set(Boost_INCLUDE_DIRS ${Boost_INCLUDE_DIR})
+set(Boost_LIBRARY_DIRS ${Boost_LIBRARY_DIR})
 
 set(Boost_FOUND FALSE)
 if(Boost_INCLUDE_DIR)
@@ -1157,7 +1206,7 @@ if(Boost_INCLUDE_DIR)
     endif()
 
     if(_boost_LIB_DIR AND EXISTS "${_boost_LIB_DIR}")
-      set(Boost_LIBRARY_DIRS ${_boost_LIB_DIR} CACHE FILEPATH "Boost library directory")
+      set(Boost_LIBRARY_DIRS ${_boost_LIB_DIR})
     endif()
 
   endif()
@@ -1204,8 +1253,39 @@ else()
   endif()
 endif()
 
-# show the Boost_INCLUDE_DIRS AND Boost_LIBRARIES variables only in the advanced view
-mark_as_advanced(Boost_INCLUDE_DIR
-    Boost_INCLUDE_DIRS
-    Boost_LIBRARY_DIRS
-)
+# Configure display of cache entries in GUI.
+foreach(v BOOSTROOT BOOST_ROOT ${_Boost_VARS_INC} ${_Boost_VARS_LIB})
+  get_property(_type CACHE ${v} PROPERTY TYPE)
+  if(_type)
+    set_property(CACHE ${v} PROPERTY ADVANCED 1)
+    if("x${_type}" STREQUAL "xUNINITIALIZED")
+      if("x${v}" STREQUAL "xBoost_ADDITIONAL_VERSIONS")
+        set_property(CACHE ${v} PROPERTY TYPE STRING)
+      else()
+        set_property(CACHE ${v} PROPERTY TYPE PATH)
+      endif()
+    endif()
+  endif()
+endforeach()
+
+# Record last used values of input variables so we can
+# detect on the next run if the user changed them.
+foreach(v
+    ${_Boost_VARS_INC} ${_Boost_VARS_LIB}
+    ${_Boost_VARS_DIR} ${_Boost_VARS_NAME}
+    )
+  if(DEFINED ${v})
+    set(_${v}_LAST "${${v}}" CACHE INTERNAL "Last used ${v} value.")
+  else()
+    unset(_${v}_LAST CACHE)
+  endif()
+endforeach()
+
+# Maintain a persistent list of components requested anywhere since
+# the last flush.
+set(_Boost_COMPONENTS_SEARCHED "${_Boost_COMPONENTS_SEARCHED}")
+list(APPEND _Boost_COMPONENTS_SEARCHED ${Boost_FIND_COMPONENTS})
+list(REMOVE_DUPLICATES _Boost_COMPONENTS_SEARCHED)
+list(SORT _Boost_COMPONENTS_SEARCHED)
+set(_Boost_COMPONENTS_SEARCHED "${_Boost_COMPONENTS_SEARCHED}"
+  CACHE INTERNAL "Components requested for this build tree.")