Faster 8.3 Converted Long Filenames in NTFS

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In Windows NT version 3.1, worst-case file manipulation of extremely large directories (500 to several thousand files) with similar long names was time consuming. This was evident when receiving screen output from a DIR command or reading a directory in File Manager.

In Windows NT 3.5(and later), NTFS has been enhanced to address these worst- case scenarios as well as other general improvements in 8.3 enumeration.

Under Windows NT 3.1 NTFS, longfile names are converted to 8.3 names to support DOS based clients. This conversion simply takes the first 6 characters of the long name, and uses a "~n" suffix (where "n" is number) to keep the name unique if needed. When the tenth filename is converted and the suffix exceeds 2 characters, only 5 characters of the name are used to accommodate the three characters in the suffix and so on as needed. For example:

   MyTermPaper.doc   becomes  MYTERM~1.DOC
   MyTermPaper2.doc  becomes  MYTERM~2.DOC
   MyTermPaper3.doc  becomes  MYTERM~3.DOC
   MyTermPaper4.doc  becomes  MYTERM~4.DOC
   MyTermPaper5.doc  becomes  MYTERM~5.DOC
   MyTermPaper6.doc  becomes  MYTERM~6.DOC
   MyTermPaper7.doc  becomes  MYTERM~7.DOC
   MyTermPaper8.doc  becomes  MYTERM~8.DOC
   MyTermPaper9.doc  becomes  MYTERM~9.DOC
   MyTermPaper10.doc becomes  MYTER~10.DOC
   MyTermPaper11.doc becomes  MYTER~11.DOC
				

That algorithm had been changed in Windows NT 3.5(and later) for enhanced performance. Since the occurrence of similar 8.3 names is generally minimal, only the first four attempts at generating the 8.3 name work like they did in Windows NT 3.1. The fifth attempt takes the first two characters of the long name, then the next four characters are generated by a hashing function of the entire name, and the last two characters are "~5". Only when the hashing of the middle four characters no longer produces a unique name will the "~5" be incremented to "~6" and so on. For example:

   MyTermPaper.doc   becomes  MYTERM~1.DOC
   MyTermPaper2.doc  becomes  MYTERM~2.DOC
   MyTermPaper3.doc  becomes  MYTERM~3.DOC
   MyTermPaper4.doc  becomes  MYTERM~4.DOC
   MyTermPaper5.doc  becomes  MY1E2E~5.DOC
   MyTermPaper6.doc  becomes  MY1EEE~5.DOC
   MyTermPaper7.doc  becomes  MY1EAE~5.DOC
				

Subsequent iterations are performed faster than under Windows NT 3.1.

NOTE: By avoiding using long names that are similar in the first 6 characters, enumeration are even more efficient.