From 07b5a3c3ba774ec93007827cf1233b4edb699bad Mon Sep 17 00:00:00 2001
From: marshmellow42 <marshmellowrf@gmail.com>
Date: Fri, 5 Jun 2015 22:39:56 -0400
Subject: [PATCH] Modified kermit crc to use existing crc calc code

---
 client/cmddata.c |  7 +++----
 common/crc16.c   | 28 +++++++++++++++-------------
 common/crc16.h   |  1 +
 3 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/client/cmddata.c b/client/cmddata.c
index e85b3a6c..cf105f88 100644
--- a/client/cmddata.c
+++ b/client/cmddata.c
@@ -1515,14 +1515,13 @@ int CmdFDXBdemodBI(const char *Cmd){
 	uint32_t crc16 = bytebits_to_byteLSBF(BitStream+64,16);
 	uint32_t extended = bytebits_to_byteLSBF(BitStream+80,24);
 
-	uint64_t rawid = ((uint64_t)bytebits_to_byteLSBF(BitStream,32)<<32) | bytebits_to_byteLSBF(BitStream+32,32);
+	uint64_t rawid = ((uint64_t)bytebits_to_byte(BitStream,32)<<32) | bytebits_to_byte(BitStream+32,32);
 	uint8_t raw[8];
 	num_to_bytes(rawid, 8, raw);
-	uint8_t *raw_ptr = SwapEndian64(raw, 8, 4);
 
-	if (g_debugMode) PrintAndLog("Raw ID Hex: %s", sprint_hex(raw_ptr,8));
+	if (g_debugMode) PrintAndLog("Raw ID Hex: %s", sprint_hex(raw,8));
 
-	uint16_t calcCrc = crc16_ccitt_kermit(raw_ptr, 8);
+	uint16_t calcCrc = crc16_ccitt_kermit(raw, 8);
 	PrintAndLog("Animal ID:     %04u-%012llu", countryCode, NationalCode);
 	PrintAndLog("National Code: %012llu", NationalCode);
 	PrintAndLog("CountryCode:   %04u", countryCode);
diff --git a/common/crc16.c b/common/crc16.c
index d48df3b2..a37f1d7e 100644
--- a/common/crc16.c
+++ b/common/crc16.c
@@ -8,7 +8,6 @@
 
 #include "crc16.h"
 
-
 unsigned short update_crc16( unsigned short crc, unsigned char c )
 {
 	unsigned short i, v, tcrc = 0;
@@ -44,16 +43,19 @@ uint16_t crc16_ccitt(uint8_t const *message, int length) {
 }
 
 uint16_t crc16_ccitt_kermit(uint8_t const *message, int length) {
-	if (length == 0) return 0;
-	uint32_t crc = 0, q = 0;
-	uint8_t c;
-
-	for (int i = 0; i < length; i++){
-		c = message[i];
-		q = (crc ^ c ) & 0xF;
-		crc = (crc >> 4) ^ (q * 0x1081);
-		q = (crc ^ (c >> 4)) & 0xF;
-		crc = (crc >> 4) ^ (q * 0x1081);
-	}
-	return crc;
+	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;
 }
diff --git a/common/crc16.h b/common/crc16.h
index 3656ce27..8eb4befb 100644
--- a/common/crc16.h
+++ b/common/crc16.h
@@ -13,4 +13,5 @@ unsigned short update_crc16(unsigned short crc, unsigned char c);
 uint16_t crc16(uint8_t const *message, int length, uint16_t remainder, uint16_t polynomial);
 uint16_t crc16_ccitt(uint8_t const *message, int length);
 uint16_t crc16_ccitt_kermit(uint8_t const *message, int length);
+uint16_t bit_reverse_uint16 (uint16_t value);
 #endif
-- 
2.39.2