[[[

Simpler/faster LCS algorithm in libsvn_diff by elimination of idx.

* subversion/libsvn_diff/lcs.c

(svn_diff__snake_t): Removed idx parameter (always 0)

(svn_diff__lcs_t): Removed idx variable (always 0) , let d have either

sign, and moved the origo of fp by d. New version no longer chooses

the shorter file as the "original", and can thus give different LCS's

depending on the order of the input files even when the files have

different lengths. Speed is ~15% faster for core lcs algorithm.

]]]

The old version of the LCS algorithm started at k=0 and ended at

k=d, where d is the length difference of the two files. d was required

to be positive. It is more efficient to start at k=(-)d and end at 0 (since

ending the loops at 0 allows for faster comparisons), and this also

makes it easy to let d have either sign, which makes the idx variable

unnecessary, further increasing speed by eliminating a parameter

to the frequently called function svn_diff__snake_t. The sign of d

is handled in the initial values of the loops (which have minimal speed

impact).

The new version will more often give different (but equally good) results

depending on the order of the input files. I estimated the speed

difference by diffing all of the first 100 revisions of merge.c in

libsvn_client against each other (when diffing only against the previous

revision, the core LCS algorithm usually takes a minor part of the total

running time, so this is not suitable for gauging the LCS algorithm

speed).

Morten

Received on 2011-05-26 22:43:31 CEST