void
coap_decode_segment(const unsigned char *seg, size_t length, unsigned char *buf) {
while (length--) {
if (*seg == '%') {
*buf = (hexchar_to_dec(seg[1]) << 4) + hexchar_to_dec(seg[2]);
seg += 2; length -= 2;
} else {
*buf = *seg;
}
++buf; ++seg;
}
}
/* Validates the checksum of the specified NMEA sentence
* Returns 1 if calculated checksum matched the received checksum;
* Returns 0 otherwise
*/
static uint8_t validate_checksum(char * s) {
uint8_t checksum = 0;
uint8_t s_cursor;
// Calculate the checksum of the received characters in the sentence.
// Note: only the characters between '$' and '*' are used
for (s_cursor = 1; s_cursor < GPS_SENTENCE_BUFF_SZ; s_cursor++) {
if (*(s + s_cursor) != '*') {
checksum ^= *(s + s_cursor);
} else {
break;
}
}
// Advance the cursor to the position of the checksum's first char
s_cursor++;
// Verify that there was no overflow by checking whether the cursor
// went beyond the buffer limit, possibly due to not finding a '*'
if (s_cursor + GPS_CHECKSUM_LENGTH >= GPS_SENTENCE_BUFF_SZ) {
return 0;
}
// Convert the most significant and least significant nibbles of
// the checksum byte
uint8_t chk_upper = hexchar_to_dec(s[s_cursor]);
uint8_t chk_lower = hexchar_to_dec(s[s_cursor + 1]);
// Verify no error occurred during the conversion
if (chk_upper == GPS_INVALID_HEX_CHAR || chk_lower == GPS_INVALID_HEX_CHAR) {
return 0;
}
// Assemble the expected checksum provided by the GPS receiver
uint8_t expected_checksum = (chk_upper << 4) | chk_lower;
if (checksum == expected_checksum) {
return 1;
}
return 0;
}