In levenshtein_internal(), describe algorithm a bit more clearly.

diff --git a/contrib/fuzzystrmatch/fuzzystrmatch.c b/contrib/fuzzystrmatch/fuzzystrmatch.c
index 63921d92717b3333d353ed1329fab42e9f8bcef1..01084da4072c1513192b12099a286e3b794abc48 100644 (file)
--- a/contrib/fuzzystrmatch/fuzzystrmatch.c
+++ b/contrib/fuzzystrmatch/fuzzystrmatch.c
@@ -277,15 +277,25 @@ levenshtein_internal(text *s, text *t,
    ++n;
 
    /*
-    * Instead of building an (m+1)x(n+1) array, we'll use two different
-    * arrays of size m+1 for storing accumulated values. At each step one
-    * represents the "previous" row and one is the "current" row of the
-    * notional large array.
+    * One way to compute Levenshtein distance is to incrementally construct
+    * an (m+1)x(n+1) matrix where cell (i, j) represents the minimum number
+    * of operations required to transform the first i characters of s into
+    * the first j characters of t.  The last column of the final row is the
+    * answer.
+    *
+    * We use that algorithm here with some modification.  In lieu of holding
+    * the entire array in memory at once, we'll just use two arrays of size
+    * m+1 for storing accumulated values. At each step one array represents
+    * the "previous" row and one is the "current" row of the notional large
+    * array.
     */
    prev = (int *) palloc(2 * m * sizeof(int));
    curr = prev + m;
 
-   /* Initialize the "previous" row to 0..cols */
+   /*
+    * To transform the first i characters of s into the first 0 characters
+    * of t, we must perform i deletions.
+    */
    for (i = 0; i < m; i++)
        prev[i] = i * del_c;
 
@@ -297,8 +307,8 @@ levenshtein_internal(text *s, text *t,
        int         y_char_len = n != t_bytes + 1 ? pg_mblen(y) : 1;
 
        /*
-        * First cell must increment sequentially, as we're on the j'th row of
-        * the (m+1)x(n+1) array.
+        * To transform the first 0 characters of s into the first j
+        * characters of t, we must perform j insertions.
         */
        curr[0] = j * ins_c;