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Re: [PATCH] Simplification/speed-up of libsvn_diff by eliminating idx

From: Stefan Fuhrmann <eqfox_at_web.de>
Date: Sun, 29 May 2011 13:29:02 +0200

On 28.05.2011 12:25, Morten Kloster wrote:
> On Sat, May 28, 2011 at 10:18 AM, Johan Corveleyn<jcorvel_at_gmail.com> wrote:
> []
>> Actually, about the theory behind the algorithm: I think it would be
>> quite beneficial if lcs.c would contain more high level documentation
>> about how the algorithm works, and why it works. Right now it only
>> contains this reference to "the article", which is quite academic (not
>> to mention that there is still quite some distance between the
>> academic explanation, and the concrete way this is implemented;
>> especially after your series of patches :-)). It makes it very hard
>> for most developers to grok this piece of code (and I'm speaking for
>> myself here :-)), a lot of effort is required just to go and look up
>> the documentation/background etc...
>> Would you be interested in adding some high-level documentation to
>> lcs.c, explaining the algorithm at a high level, maybe with an
>> example, ...? You seem to have quite a good grip on this matter.
>> A high-level explanation, maybe combined with some technical comments
>> here and there in the code to document specifics of the concrete
>> implementation, would be highly beneficial IMHO to get more developers
>> interested in libsvn_diff, and hence increasing the chances to get
>> things reviewed and improved ...
>> Cheers,
>> --
>> Johan
I was going to ask for the same ;)
> How's this?
> [[[
> /*
> * Calculate the Longest Common Subsequence (LCS) between two datasources.
> * This function is what makes the diff code tick.
> *
> * The LCS algorithm implemented here is based on the approach described
> * by Sun Wu, Udi Manber and Gene Meyers in "An O(NP) Sequence Comparison
> * Algorithm", but has been modified for better performance.
> *
> * Let M and N be the lengths (number of tokens) of the two sources
> * ('files'). The goal is to reach the end of both sources (files) with the
> * minimum number of insertions + deletions. Since there is a known length
> * difference N-M between the files, that is equivalent to just the minimum
> * number of deletions, or equivalently the minimum number of insertions.
> * For symmetry, we use the lesser number - deletions if M<N, insertions if
> * M>N.
> *
> * Let 'k' be the difference in remaining length between the files, i.e.
> * if we're at the beginning of both files, k=N-M, whereas k=0 for the
> * 'end state', at the end of both files. An insertion will increase k by
> * one, while a deletion decreases k by one. If k<0, then insertions are
> * 'free' - we need those to reach the end state k=0 anyway - but deletions
> * are costly: Adding a deletion means that we will have to add an additional
> * insertion later to reach the end state, so it doesn't matter if we count
> * deletions or insertions. Similarly, deletions are free for k>0.
> *
> * Let a 'state' be a given position in each file {pos1, pos2}. An array
> * 'fp' keeps track of the best possible state (largest values of
> * {pos1, pos2}) that can be achieved for a given cost 'p' (# moves away
> * from k=0), as well as a linked list of what matches were used to reach
> * that state. For each new value of p, we find for each value of k the
> * best achievable state for that k - either by doing a costly operation
> * (deletion if k<0) from a state achieved at a lower p, or doing a free
> * operation (insertion if k<0) from a state achieved at the same p -
> * and in both cases advancing past any matching regions found. This is
> * handled by running loops over k in order of descending absolute value.
> *
> * A recent improvement of the algorithm is to ignore tokens that are unique
> * to one file or the other, as those are known from the start to be
> * impossible to match.
> */
> ]]]
Perfect. Committed as r1128862.

-- Stefan^2.
Received on 2011-05-29 13:29:39 CEST

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