Adds CRC8 software implementations (#546)

* Adds implementation for computing and checking CRC-8 according to the Dallas/Maxim specification.

* Fix style

* Adds documentation to explain where the test vector comes from.

src/utils/Crc.cxx

@@ -169,3 +169,51 @@ void crc3_crc16_ibm(const void* data, size_t length_bytes, uint16_t* checksum) {
   checksum[1] = crc_16_ibm_finish(state2);
   checksum[2] = crc_16_ibm_finish(state3);
 }
+
+// static
+uint8_t Crc8DallasMaxim::table256[256] = {
+    0x00, 0x5e, 0xbc, 0xe2, 0x61, 0x3f, 0xdd, 0x83,
+    0xc2, 0x9c, 0x7e, 0x20, 0xa3, 0xfd, 0x1f, 0x41,
+    0x9d, 0xc3, 0x21, 0x7f, 0xfc, 0xa2, 0x40, 0x1e,
+    0x5f, 0x01, 0xe3, 0xbd, 0x3e, 0x60, 0x82, 0xdc,
+    0x23, 0x7d, 0x9f, 0xc1, 0x42, 0x1c, 0xfe, 0xa0,
+    0xe1, 0xbf, 0x5d, 0x03, 0x80, 0xde, 0x3c, 0x62,
+    0xbe, 0xe0, 0x02, 0x5c, 0xdf, 0x81, 0x63, 0x3d,
+    0x7c, 0x22, 0xc0, 0x9e, 0x1d, 0x43, 0xa1, 0xff,
+    0x46, 0x18, 0xfa, 0xa4, 0x27, 0x79, 0x9b, 0xc5,
+    0x84, 0xda, 0x38, 0x66, 0xe5, 0xbb, 0x59, 0x07,
+    0xdb, 0x85, 0x67, 0x39, 0xba, 0xe4, 0x06, 0x58,
+    0x19, 0x47, 0xa5, 0xfb, 0x78, 0x26, 0xc4, 0x9a,
+    0x65, 0x3b, 0xd9, 0x87, 0x04, 0x5a, 0xb8, 0xe6,
+    0xa7, 0xf9, 0x1b, 0x45, 0xc6, 0x98, 0x7a, 0x24,
+    0xf8, 0xa6, 0x44, 0x1a, 0x99, 0xc7, 0x25, 0x7b,
+    0x3a, 0x64, 0x86, 0xd8, 0x5b, 0x05, 0xe7, 0xb9,
+    0x8c, 0xd2, 0x30, 0x6e, 0xed, 0xb3, 0x51, 0x0f,
+    0x4e, 0x10, 0xf2, 0xac, 0x2f, 0x71, 0x93, 0xcd,
+    0x11, 0x4f, 0xad, 0xf3, 0x70, 0x2e, 0xcc, 0x92,
+    0xd3, 0x8d, 0x6f, 0x31, 0xb2, 0xec, 0x0e, 0x50,
+    0xaf, 0xf1, 0x13, 0x4d, 0xce, 0x90, 0x72, 0x2c,
+    0x6d, 0x33, 0xd1, 0x8f, 0x0c, 0x52, 0xb0, 0xee,
+    0x32, 0x6c, 0x8e, 0xd0, 0x53, 0x0d, 0xef, 0xb1,
+    0xf0, 0xae, 0x4c, 0x12, 0x91, 0xcf, 0x2d, 0x73,
+    0xca, 0x94, 0x76, 0x28, 0xab, 0xf5, 0x17, 0x49,
+    0x08, 0x56, 0xb4, 0xea, 0x69, 0x37, 0xd5, 0x8b,
+    0x57, 0x09, 0xeb, 0xb5, 0x36, 0x68, 0x8a, 0xd4,
+    0x95, 0xcb, 0x29, 0x77, 0xf4, 0xaa, 0x48, 0x16,
+    0xe9, 0xb7, 0x55, 0x0b, 0x88, 0xd6, 0x34, 0x6a,
+    0x2b, 0x75, 0x97, 0xc9, 0x4a, 0x14, 0xf6, 0xa8,
+    0x74, 0x2a, 0xc8, 0x96, 0x15, 0x4b, 0xa9, 0xf7,
+    0xb6, 0xe8, 0x0a, 0x54, 0xd7, 0x89, 0x6b, 0x35,
+};
+
+// static
+uint8_t Crc8DallasMaxim::tableLo16[16] = {
+    0x00, 0x5e, 0xbc, 0xe2, 0x61, 0x3f, 0xdd, 0x83,
+    0xc2, 0x9c, 0x7e, 0x20, 0xa3, 0xfd, 0x1f, 0x41,
+};
+
+// static
+uint8_t Crc8DallasMaxim::tableHi16[16] = {
+    0x00, 0x9d, 0x23, 0xbe, 0x46, 0xdb, 0x65, 0xf8,
+    0x8c, 0x11, 0xaf, 0x32, 0xca, 0x57, 0xe9, 0x74
+};

src/utils/Crc.cxxtest

@@ -1,6 +1,8 @@
-#include "utils/test_main.hxx"
 #include "utils/Crc.hxx"
 
+#include <stdlib.h>
+#include "utils/test_main.hxx"
+
 extern uint8_t reverse(uint8_t data);
 
 
@@ -30,11 +32,76 @@ TEST(CrcIbmTest, Example3) {
     EXPECT_EQ("0459", actual);
 }
 
+TEST(Crc3Test, Example)
+{
+    uint16_t data[3];
+    crc3_crc16_ibm("12345678", 8, data);
+    EXPECT_EQ(0x3c9d, data[0]);
+    EXPECT_EQ(0x75a8, data[1]);
+    EXPECT_EQ(0x0459, data[2]);
+}
+
+TEST(Crc8Test, Example)
+{
+    // This test vector comes from the RCN-218 document by RailCommunity. It is
+    // the same as an example message from the BiDiB examples page:
+    // http://www.bidib.org/support/example1_e.html
+    static const uint8_t sample_message[] = {
+        0x0B, 0x0A, 0x00, 0x00, 0x8E, 0x40, 0x00, 0x0D, 0x67, 0x00, 0x01, 0x00};
+
+    Crc8DallasMaxim c0;
+    Crc8DallasMaxim c16;
+    Crc8DallasMaxim c256;
+
+    for (unsigned i = 0; i < ARRAYSIZE(sample_message); i++) {
+        c0.update0(sample_message[i]);
+        c16.update16(sample_message[i]);
+        c256.update256(sample_message[i]);
+
+        EXPECT_EQ(c0.get(), c16.get());
+        EXPECT_EQ(c0.get(), c256.get());
+    }
+    EXPECT_EQ(0x4Cu, c0.get());
+
+    EXPECT_TRUE(c0.check_ok(0x4C));
+    EXPECT_TRUE(c16.check_ok(0x4C));
+    EXPECT_TRUE(c256.check_ok(0x4C));
+
+    // Consumes the CRC byte
+    c0.update0(0x4C);
+    c16.update16(0x4C);
+    c256.update256(0x4C);
+    // The message should check to zero now.
+    EXPECT_TRUE(c0.check_ok());
+    EXPECT_TRUE(c16.check_ok());
+    EXPECT_TRUE(c256.check_ok());
+}
+
+TEST(Crc8Test, Fuzz)
+{
+    // In the fuzz test we test the three different implementations of CRC8
+    // against each other on random data.
+    unsigned int seed = 42;
+
+    Crc8DallasMaxim c0;
+    Crc8DallasMaxim c16;
+    Crc8DallasMaxim c256;
+
+    for (unsigned i = 0; i < 100000; i++)
+    {
+        int r1 = rand_r(&seed);
+        if (r1 % 100 == 0)
+        {
+            c0.init();
+            c16.init();
+            c256.init();
+        }
+        uint8_t m = rand_r(&seed) & 0xFF;
+        c0.update0(m);
+        c16.update16(m);
+        c256.update256(m);
 
-TEST(Crc3Test, Example) {
-  uint16_t data[3];
-  crc3_crc16_ibm("12345678", 8, data);
-  EXPECT_EQ(0x3c9d, data[0]);
-  EXPECT_EQ(0x75a8, data[1]);
-  EXPECT_EQ(0x0459, data[2]);
+        EXPECT_EQ(c0.get(), c16.get());
+        EXPECT_EQ(c0.get(), c256.get());
+    }
 }

src/utils/Crc.hxx

@@ -59,3 +59,130 @@ uint16_t crc_16_ibm(const void* data, size_t length_bytes);
  * @param checksum is the output buffer where to store the 48-bit checksum.
  */
 void crc3_crc16_ibm(const void* data, size_t length_bytes, uint16_t* checksum);
+
+
+/// Helper class for computing CRC-8 according to Dallas/Maxim specification
+/// for 1-wire protocol. This specification is used for BiDiB, RCN-218 and
+/// S-9.2.1.1.
+///
+/// This class can incrementally compute CRC byte by byte. There are three
+/// implementations available, with different code space requirements.
+class Crc8DallasMaxim {
+public:
+    Crc8DallasMaxim()
+        : state_(0)
+    {
+    }
+
+    /// Re-sets the state machine for checksumming a new message.
+    void init()
+    {
+        state_ = 0;
+    }
+
+    /// @return the checksum of the currently consumed message.
+    uint8_t get()
+    {
+        return state_;
+    }
+
+    /// Checks that the message has a correct CRC. This function assumes that
+    /// the CRC byte has already been consumed.
+    /// @return true if the message checksum is correct.
+    bool check_ok()
+    {
+        return (state_ == 0);
+    }
+
+    /// Checks that the message has a correct CRC. This function assumes that
+    /// the CRC byte was not part of the message.
+    /// @param checksum_byte the CRC8 byte of the received message.
+    /// @return true if the message checksum is correct.
+    bool check_ok(uint8_t checksum_byte)
+    {
+        return (state_ == checksum_byte);
+    }
+
+    /// Processes one byte of the incoming message. No lookup table will be
+    /// used.
+    /// @param message_byte next byte in the message.
+    void update0(uint8_t message_byte)
+    {
+        uint8_t data = state_ ^ message_byte;
+        uint8_t crc = 0;
+        if (data & 1)
+        {
+            crc ^= 0x5e;
+        }
+        if (data & 2)
+        {
+            crc ^= 0xbc;
+        }
+        if (data & 4)
+        {
+            crc ^= 0x61;
+        }
+        if (data & 8)
+        {
+            crc ^= 0xc2;
+        }
+        if (data & 0x10)
+        {
+            crc ^= 0x9d;
+        }
+        if (data & 0x20)
+        {
+            crc ^= 0x23;
+        }
+        if (data & 0x40)
+        {
+            crc ^= 0x46;
+        }
+        if (data & 0x80)
+        {
+            crc ^= 0x8c;
+        }
+        state_ = crc;
+    }
+
+    /// Processes one byte of the incoming message. A small lookup table will be
+    /// used.
+    /// @param message_byte next byte in the message.
+    void update16(uint8_t message_byte)
+    {
+        uint8_t data = state_ ^ message_byte;
+        state_ = tableLo16[data & 0xf] ^ tableHi16[data >> 4];
+    }
+
+    /// Processes one byte of the incoming message. A 256-byte lookup table
+    /// will be used.
+    /// @param message_byte next byte in the message.
+    void update256(uint8_t message_byte)
+    {
+        state_ = table256[state_ ^ message_byte];
+    }
+
+#ifdef CRC8_TABLE_SIZE
+    /// Processes one byte of the incoming message.
+    /// @param message_byte next byte in the message.
+    void update(uint8_t message_byte)
+    {
+        update##CRC8_TABLE_SIZE(message_byte);
+    }
+#endif
+
+private:
+    // Of the static tables here only those will be linked into a binary which
+    // have been used there.
+
+    /// 256-entry lookup table for the update256 function.
+    static uint8_t table256[256];
+
+    /// 16-entry lookup table for the update16 function.
+    static uint8_t tableHi16[16];
+    /// 16-entry lookup table for the update16 function.
+    static uint8_t tableLo16[16];
+
+    /// Current value of the state register for the CRC computation.
+    uint8_t state_;
+};