Some documentation

[proxmark3-svn] / common / crc16.c

diff --git a/common/crc16.c b/common/crc16.c
index d181bb2a7f8a649d6fa2795be1251606e80589bd..973cd103c009b16697b7523b1cbf60b107500f7a 100644 (file)
--- a/common/crc16.c
+++ b/common/crc16.c
@@ -8,6 +8,7 @@
 
 #include "crc16.h"
 
+
 unsigned short update_crc16( unsigned short crc, unsigned char c )
 {
   unsigned short i, v, tcrc = 0;
@@ -20,3 +21,25 @@ 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, 0x1021);
+}