Optimize IdentifierIssuer existing Map.

- Change clones from always-copy to copy-on-write.
- Depending on shape of data, this can reduce `Map` copies by ~90%.
  However, in some data patterns, these will have very few entries, so
  overall performance may not be noticeably effected.

CHANGELOG.md

@@ -21,6 +21,11 @@
   - Improvement depends on number of digests performed.
   - Node.js using the improved browser algorithm can be ~4-9% faster overall.
   - Node.js native `Buffer` conversion can be ~5-12% faster overall.
+- Optimize the `IdentifierIssuer` `existing` `Map`:
+  - Change clones from always-copy to copy-on-write.
+  - Depending on shape of data, this can reduce `Map` copies by ~90%. However,
+    in some data patterns, these will have very few entries, so overall
+    performance may not be noticeably effected.
 
 ### Fixed
 - Disable native lib tests in a browser.

lib/IdentifierIssuer.js

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2021 Digital Bazaar, Inc. All rights reserved.
+ * Copyright (c) 2016-2023 Digital Bazaar, Inc. All rights reserved.
  */
 'use strict';
 
@@ -9,12 +9,14 @@ module.exports = class IdentifierIssuer {
    * identifiers, keeping track of any previously issued identifiers.
    *
    * @param prefix the prefix to use ('<prefix><counter>').
-   * @param existing an existing Map to use.
+   * @param existing an existing refs and Map object to use.
    * @param counter the counter to use.
    */
-  constructor(prefix, existing = new Map(), counter = 0) {
+  constructor(prefix, existing = {refs: 0, map: new Map()}, counter = 0) {
     this.prefix = prefix;
-    this._existing = existing;
+    this.existing = existing;
+    // add ref to shared map
+    this.existing.refs++;
     this.counter = counter;
   }
 
@@ -24,8 +26,8 @@ module.exports = class IdentifierIssuer {
    * @return a copy of this IdentifierIssuer.
    */
   clone() {
-    const {prefix, _existing, counter} = this;
-    return new IdentifierIssuer(prefix, new Map(_existing), counter);
+    const {prefix, existing, counter} = this;
+    return new IdentifierIssuer(prefix, existing, counter);
   }
 
   /**
@@ -38,7 +40,7 @@ module.exports = class IdentifierIssuer {
    */
   getId(old) {
     // return existing old identifier
-    const existing = old && this._existing.get(old);
+    const existing = old && this.existing.map.get(old);
     if(existing) {
       return existing;
     }
@@ -49,7 +51,26 @@ module.exports = class IdentifierIssuer {
 
     // save mapping
     if(old) {
-      this._existing.set(old, identifier);
+      if(this.existing.refs > 1) {
+        // copy-on-write shared map
+        // TODO: improve copy-on-write reference handling
+        //   - current code handles copying the 'existing' maps when it is
+        //     shared
+        //   - it will remove a reference when doing a copy
+        //   - a reference is NOT removed when a copy is no longer used
+        //   - need a `release()` call or similar to do this and add it
+        //     throughout the code as needed
+        //   - this won't result in errors, only extra copies if a child does
+        //     not do an update, is done, and a parent then does an update
+        // unref shared map
+        this.existing.refs--;
+        // copy to new map
+        this.existing = {
+          refs: 1,
+          map: new Map(this.existing.map)
+        };
+      }
+      this.existing.map.set(old, identifier);
     }
 
     return identifier;
@@ -65,7 +86,7 @@ module.exports = class IdentifierIssuer {
    *   false if not.
    */
   hasId(old) {
-    return this._existing.has(old);
+    return this.existing.map.has(old);
   }
 
   /**
@@ -75,6 +96,6 @@ module.exports = class IdentifierIssuer {
    * @return the list of old IDs that has been issued new IDs in order.
    */
   getOldIds() {
-    return [...this._existing.keys()];
+    return [...this.existing.map.keys()];
   }
 };