unicode.c

#include <stdint.h>
#include <stddef.h>
#include <string.h>
#include "unicode.h"

int utf8_last_size(const char *str) {
	int len = 0;
	char *pos = strchr(str, '\0');
	while (pos > str) {
		--pos; ++len;
		if ((*pos & 0xc0) != 0x80) {
			return len;
		}
	}
	return 0;
}

size_t utf8_chsize(uint32_t ch) {
	if (ch < 0x80) {
		return 1;
	} else if (ch < 0x800) {
		return 2;
	} else if (ch < 0x10000) {
		return 3;
	}
	return 4;
}

size_t utf8_encode(char *str, uint32_t ch) {
	size_t len = 0;
	uint8_t first;

	if (ch < 0x80) {
		first = 0;
		len = 1;
	} else if (ch < 0x800) {
		first = 0xc0;
		len = 2;
	} else if (ch < 0x10000) {
		first = 0xe0;
		len = 3;
	} else {
		first = 0xf0;
		len = 4;
	}

	for (size_t i = len - 1; i > 0; --i) {
		str[i] = (ch & 0x3f) | 0x80;
		ch >>= 6;
	}

	str[0] = ch | first;
	return len;
}


static const struct {
	uint8_t mask;
	uint8_t result;
	int octets;
} sizes[] = {
	{ 0x80, 0x00, 1 },
	{ 0xE0, 0xC0, 2 },
	{ 0xF0, 0xE0, 3 },
	{ 0xF8, 0xF0, 4 },
	{ 0xFC, 0xF8, 5 },
	{ 0xFE, 0xF8, 6 },
	{ 0x80, 0x80, -1 },
};

int utf8_size(const char *s) {
	uint8_t c = (uint8_t)*s;
	for (size_t i = 0; i < sizeof(sizes) / sizeof(*sizes); ++i) {
		if ((c & sizes[i].mask) == sizes[i].result) {
			return sizes[i].octets;
		}
	}
	return -1;
}