perf(NODE-6126): improve Long.fromBigInt performance

Author: aditi-khare-mongoDB

src/long.ts

@@ -243,7 +243,20 @@ export class Long extends BSONValue {
    * @returns The corresponding Long value
    */
   static fromBigInt(value: bigint, unsigned?: boolean): Long {
-    return Long.fromString(value.toString(), unsigned);
+    const byteStr = value.toString(16);
+    const isNeg = byteStr[0] === '-';
+    const startIndex = isNeg ? 1 : 0;
+    const lowByteString =
+      byteStr.length - startIndex > 8
+        ? byteStr.substring(byteStr.length - 8, byteStr.length)
+        : isNeg
+          ? byteStr.slice(1)
+          : byteStr;
+    const highByteString =
+      byteStr.length - startIndex > 8 ? byteStr.substring(startIndex, byteStr.length - 8) : 0;
+    return isNeg
+      ? new Long(Number('0x' + lowByteString), Number('0x' + highByteString), unsigned).neg()
+      : new Long(Number('0x' + lowByteString), Number('0x' + highByteString), unsigned);
   }
 
   /**

test/node/long.test.ts

@@ -1,5 +1,6 @@
 import { expect } from 'chai';
 import { Long, BSONError, __noBigInt__ } from '../register-bson';
+import { BSON_INT32_MAX, BSON_INT32_MIN } from '../../src/constants';
 
 describe('Long', function () {
   it('accepts strings in the constructor', function () {
@@ -164,6 +165,41 @@ describe('Long', function () {
     });
   });
 
+  describe('static fromBigInt()', function () {
+    const inputs: [
+      name: string,
+      input: bigint,
+      unsigned: boolean | undefined,
+      expectedStr?: string
+    ][] = [
+      ['0', BigInt('0'), false, '0'],
+      ['-0 (bigint coerces this to 0)', BigInt('-0'), false, '0'],
+      [
+        'max unsigned input',
+        BigInt(Long.MAX_UNSIGNED_VALUE.toString(10)),
+        true,
+        Long.MAX_UNSIGNED_VALUE.toString(10)
+      ],
+      ['max signed input', BigInt(Long.MAX_VALUE.toString(10)), false, Long.MAX_VALUE.toString(10)],
+      ['min signed input', BigInt(Long.MIN_VALUE.toString(10)), false, Long.MIN_VALUE.toString(10)],
+      ['negative greater than 32 bits', BigInt(-9228915101), false, '-9228915101'],
+      ['less than 32 bits', BigInt(245666), false, '245666'],
+      ['unsigned less than 32 bits', BigInt(245666), true, '245666'],
+      ['negative less than 32 bits', BigInt(-245666), false, '-245666'],
+      ['max int32', BigInt(BSON_INT32_MAX), false, BSON_INT32_MAX.toString(10)],
+      ['max int32 unsigned', BigInt(BSON_INT32_MAX), true, BSON_INT32_MAX.toString(10)],
+      ['min int32', BigInt(BSON_INT32_MIN), false, BSON_INT32_MIN.toString(10)]
+    ];
+
+    for (const [testName, num, unsigned, expectedStr] of inputs) {
+      context(`when the input is ${testName}`, () => {
+        it(`should return a Long representation of the input`, () => {
+          expect(Long.fromBigInt(num, unsigned).toString(10)).to.equal(expectedStr);
+        });
+      });
+    }
+  });
+
   describe('static fromString()', function () {
     const successInputs: [
       name: string,