parity.c

#include <stdbool.h>
#include <stdio.h>
#include <stdint.h>

static const int bits_per_nibble[] = { 0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4 };
#define BITS_PER_U8(x) (bits_per_nibble[(x) & 0xf] + bits_per_nibble[((x) >> 4) & 0xf])
#define BITS_PER_U16(x) (BITS_PER_U8((x) & 0xff) + BITS_PER_U8(((x) >> 8) & 0xff))
#define __builtin_popcount(x) (BITS_PER_U16((x) & 0xffff) + BITS_PER_U16(((x) >> 16) & 0xffff))

#define SERIAL_FLAG_EXT_PAREN	(1 << 6)
#define SERIAL_FLAG_EXT_PARODD	(1 << 7)

uint8_t set_parity(uint8_t flags, uint8_t flags_ext) {
	flags_ext |= SERIAL_FLAG_EXT_PAREN;
	flags_ext &= ~SERIAL_FLAG_EXT_PARODD;

	if (1 & (BITS_PER_U8(flags) + BITS_PER_U8(flags_ext)))
		return flags_ext | SERIAL_FLAG_EXT_PARODD;

	return flags_ext;
}

bool check_parity(uint8_t flags, uint8_t flags_ext) {
	if ((flags_ext & SERIAL_FLAG_EXT_PAREN) == 0) {
		printf("no parity\n");
		return false;
	}

	return 1 & (BITS_PER_U8(flags) + BITS_PER_U8(flags_ext));
}


int main(void) {
	uint8_t flags     = 0b00000000;
	uint8_t flags_ext = 0b00000000;

	for (uint8_t i = 0; i < 64; ++i) {
		flags_ext = i;
		for (flags = 0; flags < 0xff; ++flags) {

			flags_ext = set_parity(flags, flags_ext);

			printf("%02x%02x\n", flags, flags_ext);

			if (check_parity(flags, flags_ext)) {
				printf("parity error!\n");
				return 1;
			}
		}
	}

	return 0;
}