question: public MVMap < K , V > openVersion ( long version ) { if ( readOnly ) { throw DataUtils . newUnsupportedOperationException ( "This map is read-only; need to call " + "the method on the writable map" ) ; } DataUtils . checkArgument ( version >= createVersion , "Unknown version {0}; this map was created in version is {1}" , version , createVersion ) ; Page newest = null ; Page r = root ; if ( version >= r . getVersion ( ) && ( version == writeVersion || r . getVersion ( ) >= 0 || version <= createVersion || store . getFileStore ( ) == null ) ) { newest = r ; } else { Page last = oldRoots . peekFirst ( ) ; if ( last == null || version < last . getVersion ( ) ) { return store . openMapVersion ( version , id , this ) ; } Iterator < Page > it = oldRoots . iterator ( ) ; while ( it . hasNext ( ) ) { Page p = it . next ( ) ; if ( p . getVersion ( ) > version ) { break ; } last = p ; } newest = last ; } MVMap < K , V > m = openReadOnly ( ) ; m . root = newest ; return m ; }
target:  Open an old version for the given map.