(implementing suggestion #94)

[proxmark3-svn] / common / crc16.c

diff --git a/common/crc16.c b/common/crc16.c
index a5713333a66f2c6bc86b0967e64cec7174225d43..a37f1d7e35f69e63e6fba2259f0a011b8f1bf4ec 100644 (file)
--- a/common/crc16.c
+++ b/common/crc16.c
@@ -1,14 +1,61 @@
-#include "crc16.h"\r
-\r
-unsigned short update_crc16( unsigned short crc, unsigned char c )\r
-{\r
-  unsigned short i, v, tcrc = 0;\r
-\r
-  v = (crc ^ c) & 0xff;\r
-  for (i = 0; i < 8; i++) {\r
-      tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;\r
-      v >>= 1;\r
-  }\r
-\r
-  return ((crc >> 8) ^ tcrc)&0xffff;\r
-}\r
+//-----------------------------------------------------------------------------
+// This code is licensed to you under the terms of the GNU GPL, version 2 or,
+// at your option, any later version. See the LICENSE.txt file for the text of
+// the license.
+//-----------------------------------------------------------------------------
+// CRC16
+//-----------------------------------------------------------------------------
+
+#include "crc16.h"
+
+unsigned short update_crc16( unsigned short crc, unsigned char c )
+{
+       unsigned short i, v, tcrc = 0;
+
+       v = (crc ^ c) & 0xff;
+       for (i = 0; i < 8; i++) {
+               tcrc = ( (tcrc ^ v) & 1 ) ? ( tcrc >> 1 ) ^ 0x8408 : tcrc >> 1;
+               v >>= 1;
+       }
+
+       return ((crc >> 8) ^ tcrc)&0xffff;
+}
+
+uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial) {
+
+       if (length == 0) return (~remainder);
+
+       for (int byte = 0; byte < length; ++byte) {
+               remainder ^= (message[byte] << 8);
+               for (uint8_t bit = 8; bit > 0; --bit) {
+                       if (remainder & 0x8000) {
+                               remainder = (remainder << 1) ^ polynomial;
+                       } else {
+                               remainder = (remainder << 1);
+                       }
+               }
+       }
+       return remainder;
+}
+
+uint16_t crc16_ccitt(uint8_t const *message, int length) {
+       return crc16(message, length, 0xffff, 0x1021);
+}
+
+uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
+       return bit_reverse_uint16(crc16(message, length, 0x0000, 0x1021));
+}
+
+uint16_t bit_reverse_uint16 (uint16_t value) {
+       const uint16_t mask0 = 0x5555;
+       const uint16_t mask1 = 0x3333;
+       const uint16_t mask2 = 0x0F0F;
+       const uint16_t mask3 = 0x00FF;
+
+       value = (((~mask0) & value) >> 1) | ((mask0 & value) << 1);
+       value = (((~mask1) & value) >> 2) | ((mask1 & value) << 2);
+       value = (((~mask2) & value) >> 4) | ((mask2 & value) << 4);
+       value = (((~mask3) & value) >> 8) | ((mask3 & value) << 8);
+
+       return value;
+}