From: Joe Wreschnig <joe.wreschnig@gmail.com>
Date: Tue, 13 May 2014 15:12:44 +0000 (+0200)
Subject: Switch Levenshtein calculation to a flat, typed array.
X-Git-Tag: 1.0.0~29
X-Git-Url: https://git.yukkurigames.com/?a=commitdiff_plain;h=bbcf50caedbd6edea3fd502d7cb92ece795ec58f;p=string-lerp.git

Switch Levenshtein calculation to a flat, typed array.
---

diff --git a/string-lerp.js b/string-lerp.js
index 4a2c80f..84cc9b9 100644
--- a/string-lerp.js
+++ b/string-lerp.js
@@ -3,54 +3,48 @@
 
     var MAX_MATRIX_SIZE = 256 * 256;
 
-    function repeat(n, v) {
-        var r = [];
-        while (n--)
-            r.push(v);
-        return r;
-    }
-
     function createMatrix(i, j) {
-        var m = [];
-        while (i--)
-            m.push(repeat(j, 0));
-        return m;
+        return new Uint32Array(i * j);
     }
 
     function levenshteinMatrix(s, t) {
         /** Calculate the Levenshtein edit distance matrix for two strings
+
+            The matrix is returned as a flat unsigned typed array.
             
-        // Following http://en.wikipedia.org/wiki/Levenshtein_distance
+            Following http://en.wikipedia.org/wiki/Levenshtein_distance
         */
         var m = s.length + 1;
         var n = t.length + 1;
         var d = createMatrix(m, n);
         var i, j;
         for (i = 1; i < m; ++i)
-            d[i][0] = i;
+            d[n * i] = i;
         for (j = 1; j < n; ++j)
-            d[0][j] = j;
+            d[j] = j;
         for (j = 1; j < n; ++j)
             for (i = 1; i < m; ++i)
                 if (s[i - 1] === t[j - 1])
-                    d[i][j] = d[i - 1][j - 1];
+                    d[n * i + j] = d[n * (i - 1) + j - 1];
                 else
-                    d[i][j] = 1 + Math.min(d[i - 1][  j  ],
-                                           d[  i  ][j - 1],
-                                           d[i - 1][j - 1]);
+                    d[n * i + j] = 1 + Math.min(d[n * (i - 1) + j    ],
+                                                d[n *    i    + j - 1],
+                                                d[n * (i - 1) + j - 1]);
         return d;
     }
 
-    function editPath(m, s, t) {
+    function editPath(d, s, t) {
+        /** Given a Levenshtein result matrix, trace the shortest edit */
         var path = []
         var i = s.length;
         var j = t.length;
+        var n = j + 1;
         while (i || j) {
-            var sub = (i && j) ? m[i - 1][j - 1] : Infinity;
-            var del = i ? m[i - 1][j] : Infinity;
-            var ins = j ? m[i][j - 1] : Infinity;
+            var sub = (i && j) ? d[n * (i - 1) + j - 1] : Infinity;
+            var del = i ? d[n * (i - 1) + j] : Infinity;
+            var ins = j ? d[n * i + j - 1] : Infinity;
             if (sub <= ins && sub <= del) {
-                if (m[i][j] !== m[i - 1][j - 1])
+                if (d[n * i + j] !== d[n * (i - 1) + j - 1])
                     path.push(["sub", i - 1, t[j - 1]]);
                 --i; --j;
             } else if (ins <= del) {
@@ -64,7 +58,8 @@
         return path;
     }
 
-    function applyEdits(edits, s, t) {
+    function applyEdits(edits, s) {
+        /** Apply the list of edits to s */
         var i;
         for (i = 0; i < edits.length; ++i) {
             var edit = edits[i];
@@ -84,13 +79,26 @@
     }
 
     function slowLerp(a, b, p) {
-        var m = levenshteinMatrix(a, b);
-        var edits = editPath(m, a, b);
+        /** Interpolate between two strings slowly
+
+            This interpolation algorithm applys a partial edit of one
+            string into the other. This produces nice looking results,
+            but can take a significant amount of time and memory to
+            compute the edits. It is not recommended for strings
+            longer than a few hundred characters.
+         */
+        var edits = editPath(levenshteinMatrix(a, b), a, b);
         var partial = edits.slice(0, Math.round(p * edits.length));
-        return applyEdits(partial, a, b);
+        return applyEdits(partial, a);
     }
 
     function fastLerp(a, b, p) {
+        /** Interpolate between two strings very quickly
+
+            This interpolation algorithm progressively replaces the
+            front of one string with another. This approach is fast
+            but does not look good when the strings are similar.
+         */
         var alen = Math.round(a.length * p);
         var blen = Math.round(b.length * p);
         return b.substring(0, blen) + a.substring(alen, a.length);
@@ -100,6 +108,8 @@
         a = a.toString();
         b = b.toString();
         var n = a.length * b.length;
+        // TODO: if both strings are integers, do an integer lerp.
+        // TODO: if both strings are numbers, do a numeric lerp.
         return (n && n < MAX_MATRIX_SIZE)
             ? slowLerp(a, b, p)
             : fastLerp(a, b, p);