fix: remove unused <use> elements when deleting empty symbols (#2051)

Author: johnkenny54

plugins/removeEmptyContainers.js

@@ -1,6 +1,7 @@
 import { elemsGroups } from './_collections.js';
 import { detachNodeFromParent } from '../lib/xast.js';
 import { collectStylesheet, computeStyle } from '../lib/style.js';
+import { findReferences } from '../lib/svgo/tools.js';
 
 export const name = 'removeEmptyContainers';
 export const description = 'removes empty container elements';
@@ -22,9 +23,33 @@ export const description = 'removes empty container elements';
  */
 export const fn = (root) => {
   const stylesheet = collectStylesheet(root);
+  const removedIds = new Set();
+  /**
+   * @type {Map<string, {
+   *   node: import('../lib/types.js').XastElement,
+   *   parent: import('../lib/types.js').XastParent,
+   * }[]>}
+   */
+  const usesById = new Map();
 
   return {
     element: {
+      enter: (node, parentNode) => {
+        if (node.name === 'use') {
+          // Record uses so those referencing empty containers can be removed.
+          for (const [name, value] of Object.entries(node.attributes)) {
+            const ids = findReferences(name, value);
+            for (const id of ids) {
+              let references = usesById.get(id);
+              if (references === undefined) {
+                references = [];
+                usesById.set(id, references);
+              }
+              references.push({ node: node, parent: parentNode });
+            }
+          }
+        }
+      },
       exit: (node, parentNode) => {
         // remove only empty non-svg containers
         if (
@@ -61,6 +86,22 @@ export const fn = (root) => {
         }
 
         detachNodeFromParent(node, parentNode);
+        if (node.attributes.id) {
+          removedIds.add(node.attributes.id);
+        }
+      },
+    },
+    root: {
+      exit: () => {
+        // Remove any <use> elements that referenced an empty container.
+        for (const id of removedIds) {
+          const uses = usesById.get(id);
+          if (uses) {
+            for (const use of uses) {
+              detachNodeFromParent(use.node, use.parent);
+            }
+          }
+        }
       },
     },
   };

test/plugins/removeEmptyContainers.08.svg.txt

@@ -0,0 +1,31 @@
+If a container with an id attribute is removed, remove any <use>s associated with the id.
+
+===
+
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 100 100">
+    <symbol id="a">
+        <path d="M 10 10 H 90" style="stroke:black;stroke-width:2"/>
+    </symbol>
+    <symbol id="b">
+        <path d="M 10 20 H 90"/>
+    </symbol>
+    <symbol id="c"/>
+    <symbol id="d"/>
+    <use xlink:href="#a"/>
+    <use href="#b" style="stroke:red;stroke-width:2"/>
+    <use xlink:href="#c"/>
+    <use href="#d" style="stroke:red;stroke-width:2"/>
+</svg>
+
+@@@
+
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 100 100">
+    <symbol id="a">
+        <path d="M 10 10 H 90" style="stroke:black;stroke-width:2"/>
+    </symbol>
+    <symbol id="b">
+        <path d="M 10 20 H 90"/>
+    </symbol>
+    <use xlink:href="#a"/>
+    <use href="#b" style="stroke:red;stroke-width:2"/>
+</svg>