chore(foundation): faster toBufferBE via zero fast-path

yarn-project/foundation/src/bigint-buffer/bigint-buffer.test.ts

@@ -1,6 +1,25 @@
-import { fromHex, toHex } from './index.js';
+import { fromHex, toBufferBE, toHex } from './index.js';
 
 describe('bigint-buffer', () => {
+  describe('toBufferBE', () => {
+    it('serializes zero to a zeroed buffer of the requested width', () => {
+      expect(toBufferBE(0n, 32)).toEqual(Buffer.alloc(32));
+    });
+
+    it('big-endian pads small values on the left', () => {
+      expect(toBufferBE(1n, 4)).toEqual(Buffer.from([0, 0, 0, 1]));
+      expect(toBufferBE(0x0102n, 4)).toEqual(Buffer.from([0, 0, 1, 2]));
+    });
+
+    it('serializes a value that exactly fills the width', () => {
+      expect(toBufferBE(0xdeadbeefn, 4)).toEqual(Buffer.from('deadbeef', 'hex'));
+    });
+
+    it('throws on negative values', () => {
+      expect(() => toBufferBE(-1n, 32)).toThrow('negative');
+    });
+  });
+
   describe('toHex', () => {
     it('does not pad even length', () => {
       expect(toHex(16n)).toEqual('0x10');

yarn-project/foundation/src/bigint-buffer/index.ts

@@ -49,9 +49,14 @@ export function toBufferLE(num: bigint, width: number): Buffer {
  * @returns A big-endian buffer representation of num.
  */
 export function toBufferBE(num: bigint, width: number): Buffer {
-  if (num < BigInt(0)) {
+  if (num < 0n) {
     throw new Error(`Cannot convert negative bigint ${num.toString()} to buffer with toBufferBE.`);
   }
+  // The values serialized in the hot paths are overwhelmingly zero (field elements are mostly
+  // zero-padding in protocol structs), and the hex round-trip is wasteful for them.
+  if (num === 0n) {
+    return Buffer.alloc(width);
+  }
   const hex = num.toString(16);
   const buffer = Buffer.from(hex.padStart(width * 2, '0').slice(0, width * 2), 'hex');
   if (buffer.length > width) {