Change CRC algorithms to use slicing.

crc16_ccitt.c

@@ -15,7 +15,6 @@
  * You should have received a copy of the GNU Lesser General Public
  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  */
-
 #include <stdint.h>
 #include <stdlib.h>
 
@@ -35,11 +34,47 @@ AARU_EXPORT crc16_ccitt_ctx* AARU_CALL crc16_ccitt_init(void)
 
 AARU_EXPORT int AARU_CALL crc16_ccitt_update(crc16_ccitt_ctx* ctx, const uint8_t* data, uint32_t len)
 {
-    uint32_t i;
+    // Unroll according to Intel slicing by uint8_t
+    // http://www.intel.com/technology/comms/perfnet/download/CRC_generators.pdf
+    // http://sourceforge.net/projects/slicing-by-8/
+
     if(!ctx || !data) return -1;
 
-    for(i = 0; i < len; i++) ctx->crc = crc16_ccitt_table[(ctx->crc >> 8) ^ data[i]] ^ (ctx->crc << 8);
+    uint16_t       crc;
+    const uint8_t* current_char     = (const uint8_t*)data;
+    const size_t   unroll           = 4;
+    const size_t   bytes_at_once    = 8 * unroll;
+    uintptr_t      unaligned_length = (4 - (((uintptr_t)current_char) & 3)) & 3;
 
+    crc = ctx->crc;
+
+    while((len != 0) && (unaligned_length != 0))
+    {
+        crc = (crc << 8) ^ crc16_ccitt_table[0][(crc >> 8) ^ *current_char++];
+        len--;
+        unaligned_length--;
+    }
+
+    while(len >= bytes_at_once)
+    {
+        size_t unrolling;
+        for(unrolling = 0; unrolling < unroll; unrolling++)
+        {
+            crc = crc16_ccitt_table[7][current_char[0] ^ (crc >> 8)] ^
+                  crc16_ccitt_table[6][current_char[1] ^ (crc & 0xFF)] ^ crc16_ccitt_table[5][current_char[2]] ^
+                  crc16_ccitt_table[4][current_char[3]] ^ crc16_ccitt_table[3][current_char[4]] ^
+                  crc16_ccitt_table[2][current_char[5]] ^ crc16_ccitt_table[1][current_char[6]] ^
+                  crc16_ccitt_table[0][current_char[7]];
+
+            current_char += 8;
+        }
+
+        len -= bytes_at_once;
+    }
+
+    while(len-- != 0) crc = (crc << 8) ^ crc16_ccitt_table[0][(crc >> 8) ^ *current_char++];
+
+    ctx->crc = crc;
     return 0;
 }