$GPVTG,126.59,T,,M,0.22,N,0.42,K,A*32
$GPZDA,135558.000,09,11,2021,,*51
$GPGGA,135559.000,3540.5648,N,13944.6910,E,1,7,1.42,59.3,M,38.8,M,,*61
$GPGLL,3540.5648,N,13944.6910,E,135559.000,A,A*58
$GPGSA,A,3,08,26,27,10,21,194,195,,,,,,1.70,1.42,0.93*03
$GPGSV,3,1,12,194,88,165,29,195,76,186,23,27,65,017,37,08,63,289,32*74
$GPGSV,3,2,12,21,34,195,24,26,26,109,17,10,12,093,22,23,09,062,18*77
$GPGSV,3,3,12,01,08,201,,04,07,228,20,40,02,261,,193,,,*71
$GPRMC,135559.000,A,3540.5648,N,13944.6910,E,0.23,126.59,091121,,,A*6D


$GPVTG,126.59,T,,M,0.23,N,0.43,K,A*32
$GPZDA,135559.000,09,11,2021,,*50
$GPGGA,135600.000,3540.5648,N,13944.6910,E,1,7,1.42,59.3,M,38.8,M,,*6E
$GPGLL,3540.5648,N,13944.6910,E,135600.000,A,A*57
$GPGSA,A,3,08,26,27,10,21,194,195,,,,,,1.70,1.42,0.93*03
$GPGSV,3,1,12,194,88,165,29,195,76,186,24,27,65,017,38,08,63,290,32*74
$GPGSV,3,2,12,21,34,195,25,26,26,109,17,10,12,093,22,23,09,062,19*77
$GPGSV,3,3,12,01,08,201,,04,07,228,21,40,02,261,,193,,,*70
$GPRMC,135600.000,A,3540.5648,N,13944.6910,E,0.22,126.59,091121,,,A*63

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <wiringPi.h>
#include <wiringSerial.h>

#define false   0
#define true    1
#define bool unsigned int
#define BUF_SIZE 256
#define TOKEN_MAX 32
#define null '\0'
#define CR 0x0d
#define LF 0x0a

typedef struct {
	char  date[64];
	char  latitude[12];
	char  longitude[12];
	char  altitude[6];
} GPS_INFO;

GPS_INFO gps;

float nmea2dd(char *fstr) {
	float fval = atof(fstr);
	int   d = fval / 100;
	return  ((fval - (d * 100)) / 60) + d;
}

bool check(int checkval, char *checksum) {
	char high, low;
	int  ival;
	ival = checkval>>4;
	if (ival<=9) {
		high = (char)(ival + 0x30);
	} else {
		high = (char)(ival + 0x37);
	}
	ival = checkval&0x0f;
	if (ival<=9) {
		low = (char)(ival + 0x30);
	} else {
		low = (char)(ival + 0x37);
	}
	if ((high==checksum[0])&&(low==checksum[1])) {
		return true;
	} else {
		return false;
	}
}

bool edit(char *sentence) {
	char *pt, *cpt;
	int  cnt = 0;
	unsigned char checksum = 0;
	char timestr[15];
	time_t t;
	struct tm tm;
	char *token[TOKEN_MAX];
	static int gpgga=0;
	static int gpzda=0;

	pt = sentence + 1;
	token[cnt] = pt;
	for (;*pt!=LF;++pt) {
		switch(*pt) {
			case ',':
				token[++cnt] = pt + 1;
				break;
			case '*':
				for (cpt=sentence+1;*cpt!='*';++cpt) checksum ^= *cpt;
				*pt = null;
				token[++cnt] = pt + 1;
				break;
			case 0x0d:
				*pt = null;
				break;
			default:
				break;
		}
	}
	for (pt=sentence;*pt!=LF;++pt) if (*pt==',') *pt = null;

	if (strcmp(token[0],"GPGGA")==0) {
		if (check(checksum, token[15])) {
			sprintf(gps.latitude, "%9.5f%s", nmea2dd(token[2]), token[3]);
			sprintf(gps.longitude,"%9.5f%s", nmea2dd(token[4]), token[5]);
			sprintf(gps.altitude, "%s",   token[9]);
			gpgga = true;
		}
	}
	if (strcmp(token[0],"GPZDA")==0) {
		if (check(checksum, token[7])) {
			memset(&tm, 0, sizeof(struct tm));
			*(token[4]+6) = null;
			sprintf(timestr,"%6s%s%s%s", token[4],token[3],token[2],token[1]);
			strptime(timestr,"%Y%m%d%H%M%S", &tm);
			t = mktime(&tm) + (9 * 60 * 60);
			strftime(gps.date, sizeof(gps.date), "%Y/%m/%d %a %H:%M:%S", localtime(&t));
			gpzda = true;
		}
	}
	if (gpgga && gpzda) {
		gpgga=false;
		gpzda=false;
		return true;
	}
	return false;
}

bool encode(char c) {
	static char sentence[BUF_SIZE];
	static bool flag;
	static int  pos;
	if (c=='$') {
		flag = true;
		pos = 0;
	}
	if (flag) {
		sentence[pos] = c;
		pos++;
	}
	if (c==LF)  {
		flag = false;
		sentence[pos] = null;
		pos = 0;
		return edit(sentence);
	}
	return false;
}

int main(int argc, char** argv) {

	int fd = serialOpen("/dev/ttyAMA0",9600);
	if(fd!=-1){
		while(1){
			while(serialDataAvail(fd)){
				if (encode(serialGetchar(fd))) {
					printf("LOCALTIME     %s\n",gps.date);
					printf("LATITUDE(DD)  %s\n",gps.latitude);
					printf("LONGITUDE(DD) %s\n",gps.longitude);
					printf("ALTITUDE      %sM\n",gps.altitude);
				}
			}
		}
	}
}