Optimizes the computation speed for crc16-ibm. (#612)

Switches to a 4-bit translation table.

src/utils/Crc.cxx

@@ -85,17 +85,41 @@ inline uint16_t crc_16_ibm_finish(uint16_t state) {
     //return state;
     }*/
 
+/// Translation table for crc16-ibm, high nibble.
+static const uint16_t CRC16_IBM_HI[16] =
+{
+    0x0000, 0xcc01, 0xd801, 0x1400, 0xf001, 0x3c00, 0x2800, 0xe401, 0xa001, 0x6c00, 0x7800, 0xb401, 0x5000, 0x9c01, 0x8801, 0x4400, 
+};
+/// Translation table for crc16-ibm, low nibble.
+static const uint16_t CRC16_IBM_LO[16] =
+{
+    0x0000, 0xc0c1, 0xc181, 0x0140, 0xc301, 0x03c0, 0x0280, 0xc241, 0xc601, 0x06c0, 0x0780, 0xc741, 0x0500, 0xc5c1, 0xc481, 0x0440, 
+};
+
 /// Appends a byte to a CRC16 state machine.
 ///
 /// @param state the state machine of the CRC computer.
 /// @param data next byte to add.
 ///
-inline void crc_16_ibm_add(uint16_t& state, uint8_t data) {
+inline void __attribute__((always_inline)) crc_16_ibm_add(uint16_t &state, uint8_t data)
+{
     state ^= data;
-    for (int i = 0; i < 8; i++) {
-        if (state & 1) {
+    state = (state >> 8) ^ CRC16_IBM_LO[state & 0xf] ^
+        CRC16_IBM_HI[(state >> 4) & 0xf];
+}
+
+/// Bitwise implementation of the crc16 ibm add.
+void crc_16_ibm_add_basic(uint16_t &state, uint8_t data)
+{
+    state ^= data;
+    for (int i = 0; i < 8; i++)
+    {
+        if (state & 1)
+        {
             state = (state >> 1) ^ crc_16_ibm_poly;
-        } else {
+        }
+        else
+        {
             state = (state >> 1);
         }
     }

src/utils/Crc.cxxtest

@@ -42,6 +42,30 @@ TEST(Crc3Test, Example)
     EXPECT_EQ(0x0459, data[2]);
 }
 
+
+extern void crc_16_ibm_add_basic(uint16_t& state, uint8_t data);
+
+// This is not a test. It generates the translation table for the CRC16-IBM.
+TEST(Crc3Generate, generate16)
+{
+    printf("static const uint16_t CRC16_IBM_HI[16] =\n{\n    ");
+    for (unsigned nib = 0; nib < 16; nib++)
+    {
+        uint16_t state = 0;
+        crc_16_ibm_add_basic(state, nib << 4);
+        printf("0x%04x, ", state);
+    }
+    printf("\n};\n");
+    printf("static const uint16_t CRC16_IBM_LO[16] =\n{\n    ");
+    for (unsigned nib = 0; nib < 16; nib++)
+    {
+        uint16_t state = 0;
+        crc_16_ibm_add_basic(state, nib);
+        printf("0x%04x, ", state);
+    }
+    printf("\n};\n");
+}
+
 TEST(Crc8Test, Example)
 {
     // This test vector comes from the RCN-218 document by RailCommunity. It is