AP_Math: allow initial value to be specified for crc8_generic

some crc8's start with a non-zero value

Author: peterbarker

libraries/AP_Math/crc.cpp

@@ -93,9 +93,9 @@ uint8_t crc_crc8(const uint8_t *p, uint8_t len)
 }
 
 // CRC8 that does not use a lookup table: for generic polynomials
-uint8_t crc8_generic(const uint8_t *buf, const uint16_t buf_len, const uint8_t polynomial)
+uint8_t crc8_generic(const uint8_t *buf, const uint16_t buf_len, const uint8_t polynomial, uint8_t initial_value)
 {
-    uint8_t crc = 0;
+    uint8_t crc = initial_value;
     for (uint16_t i = 0; i < buf_len; i++) {
         crc = crc8_dvb(buf[i], crc, polynomial);
     }

libraries/AP_Math/crc.h

@@ -21,7 +21,7 @@
 
 uint16_t crc_crc4(uint16_t *data);
 uint8_t crc_crc8(const uint8_t *p, uint8_t len);
-uint8_t crc8_generic(const uint8_t *buf, const uint16_t buf_len, const uint8_t polynomial);     // CRC8 that does not use a lookup table for generic polynomials
+uint8_t crc8_generic(const uint8_t *buf, const uint16_t buf_len, const uint8_t polynomial, uint8_t initial_value=0);     // CRC8 that does not use a lookup table for generic polynomials
 uint8_t crc8_dvb_s2(uint8_t crc, uint8_t a);
 uint8_t crc8_dvb(uint8_t crc, uint8_t a, uint8_t seed);
 uint8_t crc8_dvb_s2_update(uint8_t crc, const void *data, uint32_t length);

libraries/AP_Math/tests/test_crc8.cpp

@@ -0,0 +1,14 @@
+#include <AP_gtest.h>
+
+#include <AP_Math/AP_Math.h>
+
+const AP_HAL::HAL& hal = AP_HAL::get_HAL();
+
+TEST(CRC8, crc8_dvb_buffer)
+{
+    const uint8_t buffer[2] { 0xBE, 0xEF };
+    EXPECT_EQ(0x92, crc8_generic(buffer, 2, 0x31, 0xff));
+}
+
+AP_GTEST_PANIC()
+AP_GTEST_MAIN()