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This code implements the basic 'bitap' algorithm for approximate
string matching, within "edit distance" (Levenshtein measure).  The
edit distance is defined as the total number of insertions, deletions,
and substitutions required to transfrom a string to (possibly a
substring of) another. "sale" is within 1 edit from "salve", "ale",
and "same", and within 2 edits from "fake".

The algorithm was developed by Udi Manber and Sun Wu, as described in
the "Fast text search allowing errors", Communications of the ACM, Vol
35, No. 10, October 1992.  An earlier version of the paper, "Fast Text
Searching with Errors", Technical Report TR-91-11, Department of
Computer Science, University of Arizona, Tucson, AZ, June 1991, is
available online [1].

The 'bitap' algorithm is best known as part of the agrep(1)
("approximate grep") implementation [2].  The implementation has been
explored in the "agrep - a fast approximate pattern matching tool",
Proceedings of the Winter 1992 USENIX Conference, USENIX Association,
Berkeley, CA. A version of that paper is available online [3].  Newer
version of agrep, by Udi Manber, Sun Wu, and Burra Gopal, comes with
the Glimpse [4][5] indexing tool, the search engine of Harvest [6].
The version number of agrep hasn't been updated from 2.04 but the
interface has been librarised and the code made more portable (for
example--the original release didn't handle ISO Latin, only pure
ASCII).  Note also that the 'bitap' algorithm is just one of the many
string searching algorithms agrep uses--see the agrep implementations
for more details.  'bitap' is not the fastest approximate matching
algorithm in agrep, it is the most versatile one.

[1]	ftp://ftp.cs.arizona.edu/agrep/agrep.ps.1.Z
[2]	ftp://ftp.cs.arizona.edu/agrep/agrep-2.04.tar.Z
[3]	ftp://ftp.cs.arizona.edu/agrep/agrep.ps.1.Z
[4]	http://glimpse.cs.arizona.edu/
[5]	ftp://ftp.cs.arizona.edu/glimpse/glimpse-4.1.src.tar.gz
[6]	http://harvest.transarc.com/

agrep itself is not in public domain, it is copyright by University
of Arizona.

I also used the book "String Searching Algorithms",
Graham A. Stephen, World Scientific 1994, ISBN 981-02-1829-X,
in which the 'bitap' ("Wu-Manber k-differences shift-add") and
many other string searching algorithms are nicely summarized.

This code doesn't implement the "partition-scan" improvement described
in the TR-91-11, so this could still be made to run faster.  Neither
does it implement all the described extensions (implemented are "sets
of characters" (any-character and caseignoring as special cases of
this) and "patterns with and without errors"; missing are: "wild
cards" (Kleene star), "unknown number of errors" (finding out the edit
distance when given two strings), "non-uniform costs", "set of
patterns", "long patterns", and "regular expressions"), so it can
still be made to run slower, too.  In place of "non-uniform costs"
feature I have an invention of mine where one can for example
completely disallow substitutions.  The feature is still largely
untested (as is the whole program, come to think of it).

Please read the COPYRIGHT.  This implementation shares no code with
agrep, none, it was made from scratch based on the Manber and Wu
papers.  Still, I have looked at the source code of agrep.  Therefore
I also include in this distribution the original agrep copyright, in
the file COPYRIGHT.agrep.  The inclusion of this file in no way
affects the copyright of my code or the applicability of it to any
purposes, even commercial ones.  The existence of COPYRIGHT.agrep only
serves the clause (2) in it, courtesy.  The clauses (1) and (4) don't
apply because this is not agrep.  In clause (3) our copyrights agree,
we guarantee nothing.  This interpretation has been kindly sanctioned
by Udi Manber.

If you have any questions, detailed bug reports, enhancement
suggestions, feature requests, or fan mail (I would like to know of
any uses you put this code into), please feel free to contact me at
<jhi@iki.fi>.

All bugs are mine, mine, all mine.

Jarkko Hietaniemi November 1998

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