[PATCH] Document levenshtein.c
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
On Wed, 19 Nov 2008, Junio C Hamano wrote:
> It is a sure sign that the original implementation was too
> scantily described, and that the fix was not explained well in the
> proposed commit log message (i.e. in what corner cases the original
> was bad in what way, and how the patch fixes it).
How about this?
levenshtein.c | 31 +++++++++++++++++++++++++++++++
1 files changed, 31 insertions(+), 0 deletions(-)
diff --git a/levenshtein.c b/levenshtein.c
index db52f2c..298907a 100644
--- a/levenshtein.c
+++ b/levenshtein.c
@@ -1,6 +1,39 @@
#include "cache.h"
#include "levenshtein.h"
+/*
+ * This function implements the Damerau-Levenshtein algorithm to
+ * calculate a distance between strings.
+ *
+ * The idea is to build a distance matrix for the substrings of both
+ * strings. To avoid a large space complexity, only the last three rows
+ * are kept in memory (if swaps had the same or higher cost as one deletion
+ * plus one insertion, only two rows would be needed).
+ *
+ * At any stage, "i + 1" denotes the length of the current substring of
+ * string1 that the distance is calculated for (likewise "j + 1" for
+ * string2).
+ *
+ * row2 holds the current row, row1 the previous row (i.e. for the substring
+ * of string1 of length "i"), and row0 the row before that.
+ *
+ * In other words, at the start of the big loop, row1[j + 1] contains the
+ * Damerau-Levenshtein distance between the substring of string1 of length
+ * "i" and the substring of string2 of length "j + 1".
+ *
+ * All the big loop does is determine the partial minimum-cost paths.
+ *
+ * It does so by calculating the costs of the path ending in characters
+ * i (in string1) and j (in string2), respectively, given that the last
+ * operation is a substition, a swap, a deletion, or an insertion.
+ *
+ * This implementation allows the costs to be weighted:
+ *
+ * - w (as in "sWap")
+ * - s (as in "Substition")
+ * - a (for insertion, AKA "Add")
+ * - d (as in "Deletion")
+ */
int levenshtein(const char *string1, const char *string2,
int w, int s, int a, int d)
{
--
1.6.0.2.763.g72663
Johannes Schindelin <johannes.schindelin@gmx.de>