Fixes svg/svgo#761. Sorting defs to improve compression.

plugins/sortDefsChildren.js

@@ -0,0 +1,47 @@
+'use strict';
+
+exports.type = 'perItem';
+
+exports.active = true;
+
+exports.description = 'Sorts children of <defs> to improve compression';
+
+/**
+ * Sorts children of defs in order to improve compression.
+ * Sorted first by frequency then by element name length then by element name (to ensure grouping).
+ *
+ * @param {Object} item current iteration item
+ * @return {Boolean} if false, item will be filtered out
+ *
+ * @author David Leston
+ */
+exports.fn = function(item) {
+
+    if (item.isElem('defs')) {
+
+        if (item.content) {
+            var frequency = item.content.reduce(function (frequency, child) {
+                if (child.elem in frequency) {
+                    frequency[child.elem]++;
+                } else {
+                    frequency[child.elem] = 1;
+                }
+                return frequency;
+            }, {});
+            item.content.sort(function (a, b) {
+                var frequencyComparison = frequency[b.elem] - frequency[a.elem];
+                if (frequencyComparison !== 0 ) {
+                    return frequencyComparison;
+                }
+                var lengthComparison = b.elem.length - a.elem.length;
+                if (lengthComparison !== 0) {
+                    return lengthComparison;
+                }
+                return a.elem > b.elem;
+            });
+        }
+
+        return true;
+    }
+
+};