sys/checksum: Add LSB-first (reflected) CRC8 Function

sys/checksum/crc8.c

@@ -39,4 +39,21 @@ uint8_t crc8(const uint8_t *data, size_t len, uint8_t g_polynom, uint8_t crc)
     return crc;
 }
 
+uint8_t crc8_lsb(const uint8_t *data, size_t len, uint8_t g_polynom, uint8_t crc)
+{
+    for (size_t i = 0; i < len; i++)
+    {
+        crc ^= data[i];
+
+        for (int i = 0; i < 8; i++)
+        {
+            bool xor = crc & 0x01;
+            crc = crc >> 1;
+            crc = xor ? crc ^ g_polynom : crc;
+        }
+    }
+
+    return crc;
+}
+
 /** @} */

sys/checksum/doc.md

@@ -0,0 +1,33 @@
+<!--
+Copyright 2016 Freie Universität Berlin
+Copyright 2016 Ludwig Knüpfer <[email protected]>
+
+This file is subject to the terms and conditions of the GNU Lesser
+General Public License v2.1. See the file LICENSE in the top level
+directory for more details.
+-->
+
+@defgroup    sys_checksum Checksum
+@ingroup     sys_hashes
+@brief       Checksum function libraries
+
+This module provides a number of checksum functions. Most notably is the
+@ref sys_checksum_crc16_ccitt and the @ref sys_checksum_ucrc16 modules which
+provide support for the CRC16 checksum.
+
+@ref sys_checksum_crc16_ccitt only provides an implementation of the CCITT
+flavor of CRC16 (polynomial @$ x^{16} + x^{12} + x^{5} + 1 @$) for big-endian
+numbers with starting seed `0x1d0f` (though others can be provided), while
+@ref sys_checksum_ucrc16 is more generalized, since it takes the
+hexadecimal representation of the polynomial as a parameter and provides
+functions and standardized polynomials for both big- and little-endian
+numbers.
+
+The caveat of @ref sys_checksum_ucrc16 is that it is significantly slower
+(approximately factor 8) than @ref sys_checksum_crc16_ccitt since the latter
+is able to calculate the checksum byte-wise, while the first calculates
+needs to calculate it bit-wise. @ref sys_checksum_crc16_ccitt achieves this
+advantage by using a look-up table that provides the checksum for every
+possible byte-value. It thus trades of memory against speed. If your
+platform is rather small equipped in memory you should prefer the
+@ref sys_checksum_ucrc16 version.

sys/include/checksum/crc8.h

@@ -29,20 +29,37 @@ extern "C" {
 #endif
 
 /**
- * @brief   Calculate CRC-8
+ * @brief   Calculate CRC-8 (MSB first)
+ *
+ * This CRC8 checksum type is usually more common.
  *
  * @param[in] data  Start of memory area to checksum
  * @param[in] len   Number of bytes in @p buf to calculate checksum for
  * @param[in] poly  The generator polynomial for the checksum
  * @param[in] seed  The seed (starting value) for the checksum
  *
- * @note Reflected inputs or outputs and final XOR must be realized
- *       by the caller if needed.
+ * @note A final XOR must be realized by the caller if needed.
  *
  * @return  Checksum of the specified memory area.
  */
 uint8_t crc8(const uint8_t *data, size_t len, uint8_t poly, uint8_t seed);
 
+/**
+ * @brief   Calculate a reflected CRC-8 (LSB first)
+ *
+ * This CRC8 checksum type is used for example by Onewire.
+ *
+ * @param[in] data  Start of memory area to checksum
+ * @param[in] len   Number of bytes in @p buf to calculate checksum for
+ * @param[in] poly  The generator polynomial for the checksum
+ * @param[in] seed  The seed (starting value) for the checksum
+ *
+ * @note A final XOR must be realized by the caller if needed.
+ *
+ * @return  Checksum of the specified memory area.
+ */
+uint8_t crc8_lsb(const uint8_t *data, size_t len, uint8_t poly, uint8_t seed);
+
 #ifdef __cplusplus
 }
 #endif

tests/unittests/tests-checksum/tests-checksum-crc8-lsb.c

@@ -0,0 +1,91 @@
+/*
+ * Copyright 2019 Benjamin Valentin <[email protected]>
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+#include <stdint.h>
+
+#include "embUnit/embUnit.h"
+
+#include "checksum/crc8.h"
+
+#include "tests-checksum.h"
+
+#define CRC8_POLY   0x31
+#define CRC8_INIT   0xff
+
+static void test_checksum_crc8_lsb_sequence_empty(void)
+{
+    unsigned char buf[] = "";
+    uint8_t expect = 0xFF;
+
+    TEST_ASSERT_EQUAL_INT(expect, crc8_lsb(buf, sizeof(buf) - 1, CRC8_POLY, CRC8_INIT));
+}
+
+static void test_checksum_crc8_lsb_sequence_1a(void)
+{
+    unsigned char buf[] = "A";
+    uint8_t expect = 0x0F;
+
+    TEST_ASSERT_EQUAL_INT(expect, crc8_lsb(buf, sizeof(buf) - 1, CRC8_POLY, CRC8_INIT));
+}
+
+static void test_checksum_crc8_lsb_sequence_256a(void)
+{
+    unsigned char buf[] = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
+                          "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
+    uint8_t expect = 0x0F;
+
+    TEST_ASSERT_EQUAL_INT(expect, crc8_lsb(buf, sizeof(buf) - 1, CRC8_POLY, CRC8_INIT));
+}
+
+static void test_checksum_crc8_lsb_sequence_1to9(void)
+{
+    unsigned char buf[] = "123456789";
+    uint8_t expect = 0x20;
+
+    TEST_ASSERT_EQUAL_INT(expect, crc8_lsb(buf, sizeof(buf) - 1, CRC8_POLY, CRC8_INIT));
+}
+
+static void test_checksum_crc8_lsb_sequence_4bytes(void)
+{
+    unsigned char buf[] = { 0x12, 0x34, 0x56, 0x78 };
+    uint8_t expect = 0x1C;
+
+    TEST_ASSERT_EQUAL_INT(expect, crc8_lsb(buf, sizeof(buf), CRC8_POLY, CRC8_INIT));
+}
+
+static void test_checksum_crc8_lsb_onewire_params(void)
+{
+    unsigned char buf[] = { 0x2A };
+    uint8_t expect = 0x5D;
+
+    TEST_ASSERT_EQUAL_INT(expect, crc8_lsb(buf, sizeof(buf), 0x8C, 0x00));
+}
+
+Test *tests_checksum_crc8_lsb_tests(void)
+{
+    EMB_UNIT_TESTFIXTURES(fixtures) {
+        /* Reference values according to
+         * http://srecord.sourceforge.net/crc16-ccitt.html */
+        new_TestFixture(test_checksum_crc8_lsb_sequence_empty),
+        new_TestFixture(test_checksum_crc8_lsb_sequence_1a),
+        new_TestFixture(test_checksum_crc8_lsb_sequence_256a),
+        new_TestFixture(test_checksum_crc8_lsb_sequence_1to9),
+        new_TestFixture(test_checksum_crc8_lsb_sequence_4bytes),
+        new_TestFixture(test_checksum_crc8_lsb_onewire_params),
+    };
+
+    EMB_UNIT_TESTCALLER(checksum_crc8_lsb_tests, NULL, NULL, fixtures);
+
+    return (Test *)&checksum_crc8_lsb_tests;
+}

tests/unittests/tests-checksum/tests-checksum.c

@@ -11,6 +11,7 @@
 void tests_checksum(void)
 {
     TESTS_RUN(tests_checksum_crc8_tests());
+    TESTS_RUN(tests_checksum_crc8_lsb_tests());
     TESTS_RUN(tests_checksum_crc16_ccitt_kermit_tests());
     TESTS_RUN(tests_checksum_crc16_ccitt_mcrf4xx_tests());
     TESTS_RUN(tests_checksum_crc16_ccitt_aug_tests());

tests/unittests/tests-checksum/tests-checksum.h

@@ -36,6 +36,13 @@ void tests_checksum(void);
  */
 Test *tests_checksum_crc8_tests(void);
 
+/**
+ * @brief   Generates tests for checksum/crc8_lsb.h
+ *
+ * @return  embUnit tests if successful, NULL if not.
+ */
+Test *tests_checksum_crc8_lsb_tests(void);
+
 /**
  * @brief   Generates tests for crc16-ccitt-kermit from checksum/crc16_ccitt.h
  *