Author: stefan2
Date: Sun May 29 10:49:58 2011
New Revision: 1128862
URL: http://svn.apache.org/viewvc?rev=1128862&view=rev
Log:
Describe the general idea behind the LCS algorithm in place.
* subversion/libsvn_diff/lcs.c
(svn_diff__lcs): add description
Patch by: Morten Kloster <morklo{_AT_}gmail.com>
Modified:
subversion/trunk/subversion/libsvn_diff/lcs.c
Modified: subversion/trunk/subversion/libsvn_diff/lcs.c
URL:
http://svn.apache.org/viewvc/subversion/trunk/subversion/libsvn_diff/lcs.c?rev=1128862&r1=1128861&r2=1128862&view=diff
==============================================================================
--- subversion/trunk/subversion/libsvn_diff/lcs.c (original)
+++ subversion/trunk/subversion/libsvn_diff/lcs.c Sun May 29 10:49:58 2011
@@ -191,6 +191,46 @@ svn_diff__lcs(svn_diff__position_t *posi
apr_off_t suffix_lines,
apr_pool_t *pool)
{
+ /*
+ * 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.
+ */
apr_off_t length[2];
svn_diff__snake_t *fp;
apr_off_t d;
