From da68ff5ad6ac3dbe187386b10c311b50be46dd10 Mon Sep 17 00:00:00 2001
From: rusefillc <sdfsdfqsf2334234234>
Date: Mon, 22 Nov 2021 14:03:52 -0500
Subject: [PATCH] j1850 SAE crc8

---
 firmware/util/math/crc.c       | 21 +++++++++++++++++++++
 firmware/util/math/crc.h       |  1 +
 unit_tests/tests/test_util.cpp |  6 ++++++
 3 files changed, 28 insertions(+)

diff --git a/firmware/util/math/crc.c b/firmware/util/math/crc.c
index 8236034ff8..c4c1e16f79 100644
--- a/firmware/util/math/crc.c
+++ b/firmware/util/math/crc.c
@@ -95,6 +95,27 @@ uint32_t crc32(const void *buf, uint32_t size) {
 	return crc32inc(buf, 0, size);
 }
 
+/**
+ * http://www.sunshine2k.de/coding/javascript/crc/crc_js.html
+ * https://stackoverflow.com/questions/38639423/understanding-results-of-crc8-sae-j1850-normal-vs-zero
+ * j1850 SAE crc8
+ */
+uint8_t crc8(const uint8_t *data, uint8_t len) {
+	uint8_t crc = 0;
+
+	if (data == 0)
+		return 0;
+	crc ^= 0xff;
+	while (len--) {
+		crc ^= *data++;
+		for (unsigned k = 0; k < 8; k++)
+			crc = crc & 0x80 ? (crc << 1) ^ 0x1d : crc << 1;
+	}
+	crc &= 0xff;
+	crc ^= 0xff;
+	return crc;
+}
+
 uint32_t crc32inc(const void *buf, uint32_t crc, uint32_t size) {
 	const uint8_t *p;
 
diff --git a/firmware/util/math/crc.h b/firmware/util/math/crc.h
index 7e8d647548..83d897c270 100644
--- a/firmware/util/math/crc.h
+++ b/firmware/util/math/crc.h
@@ -16,6 +16,7 @@ extern "C" {
 #endif /* __cplusplus */
 
 crc_t calc_crc(const crc_t message[], int nBytes);
+uint8_t crc8(const uint8_t * buf, uint8_t len);
 uint32_t crc32(const void *buf, uint32_t size);
 uint32_t crc32inc(const void *buf, uint32_t crc, uint32_t size);
 
diff --git a/unit_tests/tests/test_util.cpp b/unit_tests/tests/test_util.cpp
index f391895988..a0995a0ab2 100644
--- a/unit_tests/tests/test_util.cpp
+++ b/unit_tests/tests/test_util.cpp
@@ -29,6 +29,12 @@ TEST(util, negativeZero) {
 	ASSERT_FALSE(IS_NEGATIVE_ZERO(0.0));
 }
 
+TEST(util, crc8) {
+	const uint8_t crc8_tab[] = {0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38};
+
+	ASSERT_EQ(0xB, crc8(crc8_tab, 8));
+}
+
 TEST(util, crc) {
 	ASSERT_EQ(4, efiRound(4.4, 1));
 	ASSERT_FLOAT_EQ(1.2, efiRound(1.2345, 0.1));