fix(NODE-5577): improve ObjectId serialization by around 10% (#614)

Co-authored-by: Warren James <warren.james@mongodb.com>

src/parser/serializer.ts

@@ -257,14 +257,18 @@ function serializeObjectId(buffer: Uint8Array, key: string, value: ObjectId, ind
   buffer[index++] = 0;
 
   // Write the objectId into the shared buffer
-  if (isUint8Array(value.id)) {
-    buffer.set(value.id.subarray(0, 12), index);
+  const idValue = value.id;
+
+  if (isUint8Array(idValue)) {
+    for (let i = 0; i < 12; i++) {
+      buffer[index++] = idValue[i];
+    }
   } else {
     throw new BSONError('object [' + JSON.stringify(value) + '] is not a valid ObjectId');
   }
 
   // Adjust index
-  return index + 12;
+  return index;
 }
 
 function serializeBuffer(buffer: Uint8Array, key: string, value: Uint8Array, index: number) {

test/bench/src/index.ts

@@ -1,4 +1,5 @@
 import { getStringDeserializationSuite } from './suites/string_deserialization';
+import { getObjectIdSerializationSuite } from './suites/objectid_serialization';
 import { type PerfSendData } from './util';
 import { writeFile } from 'fs';
 import { cpus, totalmem } from 'os';
@@ -17,7 +18,7 @@ console.log(
   ].join('\n')
 );
 
-for (const suite of [getStringDeserializationSuite()]) {
+for (const suite of [getStringDeserializationSuite(), getObjectIdSerializationSuite()]) {
   suite.run();
   results.push(suite.results);
 }

test/bench/src/suites/objectid_serialization.ts

@@ -0,0 +1,24 @@
+import { Suite } from '../suite';
+import * as BSON from '../../../../';
+
+export function getObjectIdSerializationSuite(): Suite {
+  const suite = new Suite('objectid serialization');
+  const data = {
+    docs: Array.from({ length: 1_000 }, () => new BSON.ObjectId())
+  };
+
+  suite.task({
+    name: 'ObjectId serialization',
+    data,
+    fn: objectIds => {
+      BSON.serialize(objectIds);
+    },
+    iterations: 10_000,
+    resultUnit: 'megabytes_per_second',
+    transform: (runtimeMS: number) => {
+      return BSON.calculateObjectSize(data) / 1024 ** 2 / (runtimeMS / 1000);
+    }
+  });
+
+  return suite;
+}