zyxel checksum

zyxel_checksum.c

#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>

uint16_t zyxel_checksum1(const uint8_t *start, const uint8_t *end)
{
	const uint8_t *pos;
	uint32_t crc;
	size_t len;
	size_t i;

	crc = 0;
	pos = start;
	len = end - start + 1;

	if (len > 0) {
		for (i = 0; i < len; i++) {
			crc += pos[i];
			if (crc > UINT16_MAX) {
				crc++;
				crc &= UINT16_MAX;
			}
		}

		/* don't ask me, I have no idea - this is actually a out of
		 * bounds read in some situations
		 */
		if (len % 2) {
			crc += end[1] << 8;
			if (crc > UINT16_MAX) {
				crc++;
				crc &= UINT16_MAX;
			}
		}
	}

	return (uint16_t)crc;
}


uint16_t zyxel_checksum2(FILE *f, size_t start, size_t len)
{
	const uint8_t *pos;
	uint8_t buf[2048];
	size_t readsize;
	uint32_t crc;
	size_t ret;
	size_t i;

	crc = 0;
	fseek(f, start, SEEK_SET);

	while (len > 0 && !feof(f)) {
		readsize = len;
		if (readsize > sizeof(buf))
			readsize = sizeof(buf);

		len -= readsize;
		ret = fread(&buf, 1, readsize, f);
		assert(ret == readsize);

		pos = buf;

		for (i = 0; i < readsize; i++) {
			crc += pos[i];
			if (crc > UINT16_MAX) {
				crc++;
				crc &= UINT16_MAX;
			}
		}

		/* don't ask me, I have no idea */
		if (readsize % 2) {
			crc += buf[readsize] << 8;
			if (crc > UINT16_MAX) {
				crc++;
				crc &= UINT16_MAX;
			}
		}
	}

	return (uint16_t)crc;
}

int main(void)
{
	uint16_t crc;
	FILE *in;

	/* dd if=/dev/urandom of=test.img count=2000 */
	printf("make sure to create test file with:\ndd if=/dev/urandom of=test.img count=2000\n\n");
	in = fopen("test.img", "rb");
	assert(in);

	static const char *test = "abcdefg";
	crc = zyxel_checksum1(test, &test[strlen(test) - 1]);
	printf("Memory test: 0x%04x\n", crc);
	crc = zyxel_checksum2(in, 0, 1024000);
	printf("Checksum of test.img: 0x%04x\n", crc);

	return 0;
}