Changed CRC to be generic CRC-CCITT and take a polynomial
Changed CRC to be generic CRC-CCITT and take a polynomial
Made packet use APRS CRC function which class CRC-CCITT with appropriate
polynomial and Check Sum subtracted from length.

--- a/src/crc/crcccitt.c
+++ b/src/crc/crcccitt.c
@@ -2,76 +2,26 @@
 
 #include "crcccitt.h"
 
-static unsigned short crc_table [256] = {
+unsigned short CRCCCITT(uint8_t *data, uint16_t len, uint16_t polynomial){
 
-0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5,
-0x60c6, 0x70e7, 0x8108, 0x9129, 0xa14a, 0xb16b,
-0xc18c, 0xd1ad, 0xe1ce, 0xf1ef, 0x1231, 0x0210,
-0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
-0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c,
-0xf3ff, 0xe3de, 0x2462, 0x3443, 0x0420, 0x1401,
-0x64e6, 0x74c7, 0x44a4, 0x5485, 0xa56a, 0xb54b,
-0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
-0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6,
-0x5695, 0x46b4, 0xb75b, 0xa77a, 0x9719, 0x8738,
-0xf7df, 0xe7fe, 0xd79d, 0xc7bc, 0x48c4, 0x58e5,
-0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
-0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969,
-0xa90a, 0xb92b, 0x5af5, 0x4ad4, 0x7ab7, 0x6a96,
-0x1a71, 0x0a50, 0x3a33, 0x2a12, 0xdbfd, 0xcbdc,
-0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
-0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03,
-0x0c60, 0x1c41, 0xedae, 0xfd8f, 0xcdec, 0xddcd,
-0xad2a, 0xbd0b, 0x8d68, 0x9d49, 0x7e97, 0x6eb6,
-0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
-0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a,
-0x9f59, 0x8f78, 0x9188, 0x81a9, 0xb1ca, 0xa1eb,
-0xd10c, 0xc12d, 0xf14e, 0xe16f, 0x1080, 0x00a1,
-0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
-0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c,
-0xe37f, 0xf35e, 0x02b1, 0x1290, 0x22f3, 0x32d2,
-0x4235, 0x5214, 0x6277, 0x7256, 0xb5ea, 0xa5cb,
-0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
-0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447,
-0x5424, 0x4405, 0xa7db, 0xb7fa, 0x8799, 0x97b8,
-0xe75f, 0xf77e, 0xc71d, 0xd73c, 0x26d3, 0x36f2,
-0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
-0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9,
-0xb98a, 0xa9ab, 0x5844, 0x4865, 0x7806, 0x6827,
-0x18c0, 0x08e1, 0x3882, 0x28a3, 0xcb7d, 0xdb5c,
-0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
-0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0,
-0x2ab3, 0x3a92, 0xfd2e, 0xed0f, 0xdd6c, 0xcd4d,
-0xbdaa, 0xad8b, 0x9de8, 0x8dc9, 0x7c26, 0x6c07,
-0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
-0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba,
-0x8fd9, 0x9ff8, 0x6e17, 0x7e36, 0x4e55, 0x5e74,
-0x2e93, 0x3eb2, 0x0ed1, 0x1ef0
-};
+	uint16_t crc = 0xFFFF;
+	uint8_t i, j;
 
-unsigned short CRCCCITT(char_array *data){
+	while(len--){
 
-	unsigned short crc = 0xFFFF;
-	unsigned short temp;
+		crc ^= *data++;
 
-	int i;
-	for(i = 0; i < data->len-2; ++i) {
-		unsigned char dat = data->data[i];
-		unsigned char rv = 0x00;
 		int j;
-		for(j = 0; j < 8; j++, dat >>= 1)
-			rv = (rv << 1) | (dat & 0x01);
-		dat = rv;
-		temp = (dat^(crc >> 8)) & 0xFF;
-		crc = crc_table[temp] ^ (crc << 8);
+		for(j = 0; j < 8; j++){
+			if(crc & 0x0001)
+				crc = (crc >> 1) ^ polynomial;
+			else
+				crc >>= 1;
+		}
+
 	}
 
 	crc ^= 0xFFFF;
-	unsigned short rv = 0x0000;
-	for(i = 0; i < 16; i++, crc >>= 1)
-		rv = (rv << 1) | (crc & 0x01);
-
-	crc = (rv>>8)|(rv<<8);
 
 	return crc;
 

--- a/src/crc/crcccitt.h
+++ b/src/crc/crcccitt.h
@@ -3,9 +3,9 @@
 #ifndef CRCCCITT_H
 #define CRCCCITT_H
 
-#include "../container/char_array.h"
+#include <stdint.h>
 
-unsigned short CRCCCITT(char_array *data);
+unsigned short CRCCCITT(uint8_t *data, uint16_t len, uint16_t polynomial);
 
 #endif
 

--- a/src/packet/APRSPacket.c
+++ b/src/packet/APRSPacket.c
@@ -42,7 +42,7 @@
 
 unsigned short APRSPacket_crc(APRSPacket *packet){
 
-	return CRCCCITT(&packet->data);
+	return CRCCCITT(packet->data.data, packet->data.len - 2, APRS_CRC_POLY);
 
 }
 

--- a/src/packet/APRSPacket.h
+++ b/src/packet/APRSPacket.h
@@ -7,6 +7,8 @@
 #include "../crc/crcccitt.h"
 
 #include "../container/char_array.h"
+
+#define APRS_CRC_POLY 0x8408
 
 typedef struct {
 

--- a/src/packet/packet_main.c
+++ b/src/packet/packet_main.c
@@ -145,11 +145,13 @@
 
 void print_packet(APRSPacket *packet, bool show_errors_bool, bool raw){
 
-	unsigned short fsc = CRCCCITT(&packet->data);
-
-	unsigned short fsc2 = packet->data.data[packet->data.len-2];
+	unsigned short fsc = APRSPacket_crc(packet);
+
+	/* CRC is added to packet with bytes reversed
+	 */
+	unsigned short fsc2 = packet->data.data[packet->data.len-1];
 	fsc2 <<= 8;
-	fsc2 |= packet->data.data[packet->data.len-1]&0xFF;
+	fsc2 |= packet->data.data[packet->data.len-2] & 0xFF;
 
 	if(fsc != fsc2 && !show_errors_bool) return;