feat(NODE-5909): optimize writing basic latin strings

src/utils/latin.ts

@@ -59,3 +59,42 @@ export function tryLatin(uint8array: Uint8Array, start: number, end: number): st
 
   return String.fromCharCode(...latinBytes);
 }
+
+/**
+ * This function is an optimization for writing small basic latin strings.
+ * @internal
+ * @remarks
+ * ### Important characteristics:
+ * - If the string length is 0 return 0, do not perform any work
+ * - If a string is longer than 25 code units return null
+ * - If any code unit exceeds 128 this function returns null
+ *
+ * @param destination - The uint8array to serialize the string to
+ * @param source - The string to turn into UTF-8 bytes if it fits in the basic latin range
+ * @param offset - The position in the destination to begin writing bytes to
+ * @returns the number of bytes written to destination if all code units are below 128, otherwise null
+ */
+export function tryWriteLatin(
+  destination: Uint8Array,
+  source: string,
+  offset: number
+): number | null {
+  if (source.length === 0) return 0;
+
+  if (source.length > 25) return null;
+
+  if (destination.length - offset < source.length) return null;
+
+  for (
+    let charOffset = 0, destinationOffset = offset;
+    charOffset < source.length;
+    charOffset++, destinationOffset++
+  ) {
+    const char = source.charCodeAt(charOffset);
+    if (char > 127) return null;
+
+    destination[destinationOffset] = char;
+  }
+
+  return source.length;
+}

src/utils/node_byte_utils.ts

@@ -1,6 +1,6 @@
 import { BSONError } from '../error';
 import { validateUtf8 } from '../validate_utf8';
-import { tryLatin } from './latin';
+import { tryLatin, tryWriteLatin } from './latin';
 
 type NodeJsEncoding = 'base64' | 'hex' | 'utf8' | 'binary';
 type NodeJsBuffer = ArrayBufferView &
@@ -149,6 +149,11 @@ export const nodeJsByteUtils = {
   },
 
   encodeUTF8Into(buffer: Uint8Array, source: string, byteOffset: number): number {
+    const latinBytesWritten = tryWriteLatin(buffer, source, byteOffset);
+    if (latinBytesWritten != null) {
+      return latinBytesWritten;
+    }
+
     return nodeJsByteUtils.toLocalBufferType(buffer).write(source, byteOffset, undefined, 'utf8');
   },
 

test/node/byte_utils.test.ts

@@ -365,33 +365,35 @@ const toISO88591Tests: ByteUtilTest<'toISO88591'>[] = [
     }
   }
 ];
-const fromUTF8Tests: ByteUtilTest<'fromUTF8'>[] = [
+const fromUTF8Tests: ByteUtilTest<'encodeUTF8Into'>[] = [
   {
-    name: 'should create buffer from utf8 input',
-    inputs: [Buffer.from('abc\u{1f913}', 'utf8').toString('utf8')],
+    name: 'should insert utf8 bytes into buffer',
+    inputs: [Buffer.alloc(7), 'abc\u{1f913}', 0],
     expectation({ output, error }) {
       expect(error).to.be.null;
-      expect(output).to.deep.equal(Buffer.from('abc\u{1f913}', 'utf8'));
+      expect(output).to.equal(7);
+      expect(this.inputs[0]).to.deep.equal(Buffer.from('abc\u{1f913}', 'utf8'));
     }
   },
   {
-    name: 'should return empty buffer for empty string input',
-    inputs: [''],
+    name: 'should return 0 and not modify input buffer',
+    inputs: [Uint8Array.from([2, 2]), '', 0],
     expectation({ output, error }) {
       expect(error).to.be.null;
-      expect(output).to.have.property('byteLength', 0);
+      expect(output).to.equal(0);
+      expect(this.inputs[0]).to.deep.equal(Uint8Array.from([2, 2]));
     }
   },
   {
-    name: 'should return bytes with replacement character if string is not encodable',
-    inputs: ['\u{1f913}'.slice(0, 1)],
+    name: 'should insert replacement character bytes if string is not encodable',
+    inputs: [Uint8Array.from({ length: 10 }, () => 2), '\u{1f913}'.slice(0, 1), 2],
     expectation({ output, error }) {
       expect(error).to.be.null;
-      expect(output).to.have.property('byteLength', 3);
-      expect(output).to.have.property('0', 0xef);
-      expect(output).to.have.property('1', 0xbf);
-      expect(output).to.have.property('2', 0xbd);
-      const backToString = Buffer.from(output!).toString('utf8');
+      expect(output).to.equal(3);
+      expect(this.inputs[0]).to.have.property('2', 0xef);
+      expect(this.inputs[0]).to.have.property('3', 0xbf);
+      expect(this.inputs[0]).to.have.property('4', 0xbd);
+      const backToString = Buffer.from(this.inputs[0].subarray(2, 5)).toString('utf8');
       const replacementCharacter = '\u{fffd}';
       expect(backToString).to.equal(replacementCharacter);
     }
@@ -507,7 +509,7 @@ const table = new Map<keyof ByteUtils, ByteUtilTest<keyof ByteUtils>[]>([
   ['toHex', toHexTests],
   ['fromISO88591', fromISO88591Tests],
   ['toISO88591', toISO88591Tests],
-  ['fromUTF8', fromUTF8Tests],
+  ['encodeUTF8Into', fromUTF8Tests],
   ['toUTF8', toUTF8Tests],
   ['utf8ByteLength', utf8ByteLengthTests],
   ['randomBytes', randomBytesTests]

test/node/utils/latin.test.ts

@@ -1,5 +1,5 @@
 import { expect } from 'chai';
-import { tryLatin } from '../../../src/utils/latin';
+import { tryLatin, tryWriteLatin } from '../../../src/utils/latin';
 import * as sinon from 'sinon';
 
 describe('tryLatin()', () => {
@@ -116,3 +116,62 @@ describe('tryLatin()', () => {
     });
   });
 });
+
+describe('tryWriteLatin()', () => {
+  context('when given a string of length 0', () => {
+    it('returns 0 and does not modify the destination', () => {
+      const input = Uint8Array.from({ length: 10 }, () => 1);
+      expect(tryWriteLatin(input, '', 2)).to.equal(0);
+      expect(input).to.deep.equal(Uint8Array.from({ length: 10 }, () => 1));
+    });
+  });
+
+  context('when given a string with a length larger than the buffer', () => {
+    it('returns null', () => {
+      const input = Uint8Array.from({ length: 10 }, () => 1);
+      expect(tryWriteLatin(input, 'a'.repeat(11), 0)).to.be.null;
+      expect(tryWriteLatin(input, 'a'.repeat(13), 2)).to.be.null;
+    });
+  });
+
+  let charCodeAtSpy;
+
+  beforeEach(() => {
+    charCodeAtSpy = sinon.spy(String.prototype, 'charCodeAt');
+  });
+
+  afterEach(() => {
+    sinon.restore();
+  });
+
+  for (let stringLength = 1; stringLength <= 25; stringLength++) {
+    context(`when there is ${stringLength} bytes`, () => {
+      context('that exceed 127', () => {
+        it('returns null', () => {
+          expect(
+            tryWriteLatin(
+              new Uint8Array(stringLength * 3),
+              'a'.repeat(stringLength - 1) + '\x80',
+              0
+            )
+          ).be.null;
+        });
+      });
+
+      it(`calls charCodeAt ${stringLength}`, () => {
+        tryWriteLatin(
+          new Uint8Array(stringLength * 3),
+          String.fromCharCode(127).repeat(stringLength),
+          stringLength
+        );
+        expect(charCodeAtSpy).to.have.callCount(stringLength);
+      });
+    });
+  }
+
+  context('when there is >25 characters', () => {
+    it('returns null', () => {
+      expect(tryWriteLatin(new Uint8Array(75), 'a'.repeat(26), 0)).be.null;
+    });
+  });
+});