[Haskell-cafe] Re: Memoizing longest-common-subsequence

Bayley, Alistair Alistair_Bayley at invescoperpetual.co.uk
Tue Aug 1 11:20:17 EDT 2006

> From: haskell-cafe-bounces at haskell.org 
> [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of 
> Mark.Carroll at Aetion.com (Mark T.B. Carroll) writes:
> > I wanted a longest common subsequence function and a bit of Googling
> > failed to turn up a functional one, except for in a scary 
> bit of darcs.
> The code in darcs is a translation of the example code in the 
> Eugene Myers
> paper mentioned in the comments and the C code in GNU diff. I 
> didn't try
> much to simplify it because I first wanted to see how close I 
> could come
> to the perfomance of the C code. 
> [..]
> > Take this as your cue to point out the much better LCS 
> algorithm that
> > already exists in the standard libraries, that I couldn't find. (-:
> There is a nice implementation of the Hunt-Szymanski algorithm by
> Ian Lynagh at http://urchin.earth.li/darcs/ian/lcs/.
> Benedikt

I also had a go at this a while ago, using this Eugene Myers' paper as
the specification:

which claims improvement over the one you mention above:

and also ended up with an STArray based implementation. I can send the
code if you're interested. I have no idea how well it performs compared
to Ian's, or the one in darcs (which uses PackedStrings).

There's some kind of summary/comparison of LCS algorithms (including
Hunt-Szymanski) here:

also by Eugene Myers.

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