Description

   The DCX$MAKE_MAP routine uses the statistical information
   gathered by DCX$ANALYZE_DATA to compute the compression/expansion
   function. In essence, this map is the algorithm used to shorten
   (or compress) the original data records as well as to expand the
   compressed records to their original form.

   The map must be available in memory when any data compression
   or expansion takes place; the address of the map is passed as an
   argument to the DCX$COMPRESS_INIT and DCX$EXPAND_INIT routines,
   which initialize the data compression and expansion procedures,
   respectively.

   The map is stored with the compressed data records, because the
   compressed data records are indecipherable without the map. When
   compressed data records have been expanded to their original
   state and no further compression is desired, you should delete
   the map using the LIB$FREE_VM routine.

   DCX requires that you submit data records for analysis and then
   call the DCX$MAKE_MAP routine. Upon receiving the DCX$_AGAIN
   status code, you must again submit data records for analysis
   (in the same order) and call DCX$MAKE_MAP again; on the second
   iteration, DCX$MAKE_MAP returns the DCX$_NORMAL status code.