new plugin: remove unused IDs (close #76)

.svgo.yml

@@ -150,6 +150,10 @@ plugins:
     active: true
     type: full
 
+  - name: removeUnusedIDs
+    active: true
+    type: full
+
 svg2js:
 
   strict: true

plugins/_collections.js

@@ -2072,6 +2072,19 @@ exports.editorNamespaces = [
     'http://ns.adobe.com/XPath/1.0/'
 ];
 
+// http://www.w3.org/TR/SVG/linking.html#processingIRI
+exports.referencesProps = [
+    'clip-path',
+    'color-profile',
+    'fill',
+    'filter',
+    'marker-start',
+    'marker-mid',
+    'marker-end',
+    'mask',
+    'stroke'
+];
+
 // http://www.w3.org/TR/SVG/styling.html#SVGStylingProperties
 exports.stylingProps = [
     'font',

plugins/removeUnusedIDs.js

@@ -0,0 +1,113 @@
+'use strict';
+
+var referencesProps = require('./_collections').referencesProps,
+    regReferencesUrl = /^url\(#(.+?)\)$/,
+    regReferencesHref = /^#(.+?)$/,
+    styleOrScript = ['style', 'script'],
+    hasStyleOrScript = false;
+
+/**
+ * Remove unused IDs
+ * (only if there are no any <style> or <script>).
+ *
+ * @param {Object} item current iteration item
+ * @return {Boolean} if false, item will be filtered out
+ *
+ * @author Kir Belevich
+ */
+exports.removeUnusedIDs = function(data) {
+
+    var IDs = {},
+        referencesIDs = [];
+
+    /**
+     * Bananas!
+     *
+     * @param {Array} items input items
+     * @return {Array} output items
+     */
+    function monkeys(items) {
+
+        var i = 0,
+            length = items.content.length;
+
+        while(i < length) {
+
+            var item = items.content[i],
+                match;
+
+            // check if <style> of <script> presents
+            if (item.isElem(styleOrScript)) {
+                hasStyleOrScript = true;
+            }
+
+            // …and don't remove any ID if yes
+            if (!hasStyleOrScript) {
+
+                if (item.isElem()) {
+
+                    item.eachAttr(function(attr) {
+                        // save IDs
+                        if (attr.name === 'id') {
+                            IDs[item.attr('id').value] = item;
+                        }
+
+                        // save IDs url() references
+                        else if (referencesProps.indexOf(attr.name) > -1) {
+                            match = attr.value.match(regReferencesUrl);
+
+                            if (match && referencesIDs.indexOf(match[1]) === -1) {
+                                referencesIDs.push(match[1]);
+                            }
+                        }
+
+                        // save IDs href references
+                        else if (attr.name === 'xlink:href') {
+                            match = attr.value.match(regReferencesHref);
+
+                            if (match && referencesIDs.indexOf(match[1]) === -1) {
+                                referencesIDs.push(match[1]);
+                            }
+                        }
+                    });
+
+                }
+
+                // go deeper
+                if (item.content) {
+                    monkeys(item);
+                }
+
+            }
+
+            i++;
+
+        }
+
+        return items;
+
+    }
+
+    data = monkeys(data);
+
+    if (!hasStyleOrScript) {
+
+        // don't remove referenced IDs
+        if (referencesIDs.length) {
+            referencesIDs.forEach(function(referencesID) {
+                delete IDs[referencesID];
+            });
+        }
+
+        // remove not referenced IDs from elements
+        if (Object.keys(IDs).length) {
+            for(var ID in IDs) {
+                IDs[ID].removeAttr('id');
+            }
+        }
+
+    }
+
+    return data;
+
+};