From 28f63d5d855f52ec2d1e53415354296e58b24127 Mon Sep 17 00:00:00 2001
From: David Lechner <david@pybricks.com>
Date: Thu, 21 Jul 2022 14:27:27 -0500
Subject: [PATCH] utils/math: add crc32 function

This matches the STM32 hardware CRC.
---
 src/utils/math.test.ts | 16 ++++++++++++++--
 src/utils/math.ts      | 28 +++++++++++++++++++++++++++-
 2 files changed, 41 insertions(+), 3 deletions(-)

diff --git a/src/utils/math.test.ts b/src/utils/math.test.ts
index eaa60641..aa53d699 100644
--- a/src/utils/math.test.ts
+++ b/src/utils/math.test.ts
@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020 The Pybricks Authors
+// Copyright (c) 2020-2022 The Pybricks Authors
 
-import { fmod, sumComplement32, xor8 } from './math';
+import { crc32, fmod, sumComplement32, xor8 } from './math';
 
 describe('fmod', () => {
     test('positive numbers', () => {
@@ -22,6 +22,18 @@ describe('sumComplement32', () => {
     });
 });
 
+describe('crc32', () => {
+    test('trivial', () => {
+        expect(crc32([0])).toBe(0);
+    });
+    test('trivial2', () => {
+        expect(crc32([0xffffffff])).toBe(0);
+    });
+    test('basic', () => {
+        expect(crc32([1, 2, 3, 4, 5])).toBe(-2048796416);
+    });
+});
+
 describe('xor8', () => {
     test('basic', () => {
         expect(xor8([0])).toBe(0xff);
diff --git a/src/utils/math.ts b/src/utils/math.ts
index 0b2c7445..9af478e7 100644
--- a/src/utils/math.ts
+++ b/src/utils/math.ts
@@ -1,5 +1,5 @@
 // SPDX-License-Identifier: MIT
-// Copyright (c) 2020 The Pybricks Authors
+// Copyright (c) 2020-2022 The Pybricks Authors
 
 /**
  * Compute modulo using floored division
@@ -28,6 +28,32 @@ export function sumComplement32(data: Iterable<number>): number {
     // checksum is two's complement of total
     return ~total + 1;
 }
+// thanks https://stackoverflow.com/a/33152544/1976323
+
+const crc32Table: ReadonlyArray<number> = [
+    0x00000000, 0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b, 0x1a864db2,
+    0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6, 0x2b4bcb61, 0x350c9b64, 0x31cd86d3,
+    0x3c8ea00a, 0x384fbdbd,
+];
+
+/**
+ * Calculates the 32-bit CRC32 checksum.
+ * @data an iterable of 32-bit integers
+ * @returns the checksum
+ */
+export function crc32(data: Iterable<number>): number {
+    let crc = 0xffffffff;
+
+    for (const word of data) {
+        crc ^= word;
+
+        for (let i = 0; i < 8; i++) {
+            crc = (crc << 4) ^ crc32Table[crc >> 28];
+        }
+    }
+
+    return crc;
+}
 
 /**
  * Calculates the 8-bit "xor" checksum