code clean up, added some comments to hitag

[proxmark3-svn] / common / crc16.c

diff --git a/common/crc16.c b/common/crc16.c
index 4225ceb1cbaf6ae3e47756a715886877effa23d6..07090fc6fbbeec4f56300dbb6246c4a5adf74041 100644 (file)
--- a/common/crc16.c
+++ b/common/crc16.c
@@ -1,4 +1,13 @@
+//-----------------------------------------------------------------------------
+// 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"
+#define CRC16_MASK_CCITT 0x1021
 
 unsigned short update_crc16( unsigned short crc, unsigned char c )
 {
@@ -12,3 +21,42 @@ unsigned short update_crc16( unsigned short crc, unsigned char c )
 
   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, CRC16_MASK_CCITT);
+}
+
+uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
+    return bit_reverse_uint16(crc16(message, length, 0x0000, CRC16_MASK_CCITT));
+}
+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;
+}