add xor8 checksum

we are starting with 0xff like all known LEGO checksums of this style.

src/utils/math.test.ts

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: MIT
 // Copyright (c) 2020 The Pybricks Authors
 
-import { fmod, sumComplement32 } from './math';
+import { fmod, sumComplement32, xor8 } from './math';
 
 describe('fmod', () => {
     test('positive numbers', () => {
@@ -21,3 +21,15 @@ describe('sumComplement32', () => {
         expect(sumComplement32([0xffffffff, 0xffffffff])).toBe(-(-1 + -1));
     });
 });
+
+describe('xor8', () => {
+    test('basic', () => {
+        expect(xor8([0])).toBe(0xff);
+    });
+    test('basic2', () => {
+        expect(xor8([0xff, 0xff, 0xff])).toBe(0);
+    });
+    test('basic3', () => {
+        expect(xor8([0xc0, 0x0c])).toBe(0x33);
+    });
+});

src/utils/math.ts

@@ -28,3 +28,16 @@ export function sumComplement32(data: Iterable<number>): number {
     // checksum is two's complement of total
     return ~total + 1;
 }
+
+/**
+ * Calculates the 8-bit "xor" checksum
+ * @data an iterable of 8-bit integers
+ * @returns all of the values xored together along with 0xff
+ */
+export function xor8(data: Iterable<number>): number {
+    let checksum = 0xff;
+    for (const n of data) {
+        checksum ^= n & 0xff;
+    }
+    return checksum;
+}