Tweak to Levenshtein costs to prefer ins/del to sub/sub.

diff --git a/string-lerp.js b/string-lerp.js
index 40d9fd0..6e0b109 100644 (file)
--- a/string-lerp.js
+++ b/string-lerp.js
@@ -3,10 +3,10 @@
 
     var MAX_MATRIX_SIZE = 256 * 256;
 
-    function levenshteinMatrix(s, t) {
+    function levenshteinMatrix(s, t, ins, del, sub) {
         /** Calculate the Levenshtein edit distance matrix for two strings
 
-            The matrix is returned as a flat unsigned typed array.
+            The matrix is returned as a flat typed array.
             
             Following http://en.wikipedia.org/wiki/Levenshtein_distance
         */
@@ -23,9 +23,9 @@
                 if (s[i - 1] === t[j - 1])
                     d[n * i + j] = d[n * (i - 1) + j - 1];
                 else
-                    d[n * i + j] = 1 + Math.min(d[n * (i - 1) + j    ],
-                                                d[n *    i    + j - 1],
-                                                d[n * (i - 1) + j - 1]);
+                    d[n * i + j] = Math.min(del + d[n * (i - 1) + j    ],
+                                            ins + d[n *    i    + j - 1],
+                                            sub + d[n * (i - 1) + j - 1]);
         return d;
     }
 
@@ -56,7 +56,7 @@
 
     function diff(s, t) {
         /** Create a diff between string s and t */
-        return editPath(levenshteinMatrix(s, t), t);
+        return editPath(levenshteinMatrix(s, t, 2, 2, 3), t);
     }
 
     function patch(edits, s) {
@@ -103,7 +103,7 @@
     var GLYPH = /([\0-\u02FF\u0370-\u1DBF\u1E00-\u20CF\u2100-\uD7FF\uDC00-\uFE1F\uFE30-\uFFFF]|[\uD800-\uDBFF][\uDC00-\uDFFF]|[\uD800-\uDBFF])([\u0300-\u036F\u1DC0-\u1DFF\u20D0-\u20FF\uFE20-\uFE2F]*)/g;
 
     function diffLerp(a, b, p) {
-        /** Interpolate between two strings based on edit distance
+        /** Interpolate between two strings based on edit operations
 
             This interpolation algorithm applys a partial edit of one
             string into the other. This produces nice looking results,
@@ -159,10 +159,10 @@
             is clamped to an integer.
 
             For example, numericLerp("0.0", "100", 0.123) === "12.3"
-            because the "." in "0.0" is intepreted as a decimal point.
-            But numericLerp("0.", "100.", 0.123) === "12." because the
-            strings are interpreted as integers followed by a full
-            stop.
+            because the "." in "0.0" is interpreted as a decimal
+            point. But numericLerp("0.", "100.", 0.123) === "12."
+            because the strings are interpreted as integers followed
+            by a full stop.
 
             Calling this functions on strings that differ in more than
             numerals gives undefined results.
@@ -185,6 +185,10 @@
             front of one string with another. This approach is fast
             but does not look good when the strings are similar.
         */
+
+        // TODO: Consider fast-pathing this even more for very large
+        // strings, e.g. in the megabyte range. These are large enough
+        // that 
         if (a.match(MULTI) || b.match(MULTI)) {
             var ca = a.match(GLYPH) || [];
             var cb = b.match(GLYPH) || [];

diff --git a/tests/string-lerp.js b/tests/string-lerp.js
index 5728e7f..7cee7d1 100644 (file)
--- a/tests/string-lerp.js
+++ b/tests/string-lerp.js
@@ -114,6 +114,22 @@ JS.Test.describe('diff lerp', function () { with (this) {
             assert(r === "a" || r === UNUSUAL_Q);
         }
     }});
+
+    it("prefers ins/del to sub/sub", function () { with (this) {
+        // When the cost is uniform this string can be transformed by
+        // rewriting the whole thing for the same cost as deleting the
+        // front and adding to the back. But visually, we'd rather do
+        // the latter.
+        assertEqual("core", lerp("hard core", "core dump", 0.50));
+    }});
+
+    it("weights ins/del cheaper than sub", function () { with (this) {
+        // When the cost is uniform it is cheaper to rewrite the
+        // former into the latter. But we'd rather keep the "core" for
+        // visual reasons, so we need to make sure we have unequal
+        // costs.
+        assertEqual("core", lerp("apple core", "core dump", 0.51));
+    }});
 }});
 
 JS.Test.describe('numeric lerp', function () { with (this) {