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aprsispass.c
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aprsispass.c
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/**
* Generates an APRS-IS passcode for the provided callsign, using the algorithm
* described in the APRS-IS source code.
*
* Daria Juniper
* Github: @juniberry
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* Maximum length of callsign */
#define MAX_CALLSIGN 12
/* Function Prototypes */
unsigned int generateAPRSISPasscode(const char* callsign);
int main(int argc, const char* argv[])
{
unsigned int passcode = 0;
if (argc < 2) {
printf("APRS-IS Passcode Generator\n");
printf("Usage: %s <callsign>\nNote -- only supply the base callsign, do not supply an SSID or prefix.\n", argv[0]);
return -1;
}
/* Validate callsign length */
if (strlen(argv[1]) > MAX_CALLSIGN) {
printf("Error: Callsign is too long. Maximum length is %i characters.\n", MAX_CALLSIGN);
return -1;
}
char callsign[MAX_CALLSIGN] = {0};
strncpy(callsign, argv[1], MAX_CALLSIGN - 1);
passcode = generateAPRSISPasscode(callsign);
printf("%s passcode: %u\n", callsign, passcode);
return 0;
}
/**
* Generates an APRS-IS passcode for the provided callsign.
*
* @param callsign The callsign for which to generate the passcode.
* @return The passcode for the provided callsign.
*/
unsigned int generateAPRSISPasscode(const char* callsign)
{
if (callsign == NULL)
return 0;
const short int INITIAL_HASH = 0x73e2; /* Hash value from APRS-IS source code */
short int hash = INITIAL_HASH;
const char *ptr = callsign;
size_t len = strlen(callsign);
/* XOR the callsign with the hash */
for (size_t i = 0; i < len; i += 2) {
if (ptr[i] != '\0')
hash ^= ptr[i] << 8;
if (ptr[i + 1] != '\0')
hash ^= ptr[i + 1];
}
/* drop sign and return the passcode */
return (unsigned int)abs(hash);
}