utf8.h

#ifndef VIOLET_UTF8_H
#define VIOLET_UTF8_H

b32 utf8_can_decode_codepoint(const char *nptr, size_t max_size);
s32 utf8_next_codepoint(const char *nptr, char **endptr);
s32 utf8_prev_codepoint(const char *nptr, char **beginptr);

#endif // VIOLET_UTF8_H

/* Implementation */

#ifdef UTF8_IMPLEMENTATION

static
b32 utf8__can_decode_remainder(const char *p, size_t n)
{
	for (size_t i = 0; i < n; ++i)
		if ((p[1] & 0xc0) != 0x80)
			return false;
	return true;
}

/* doesn't validate the codepoint or exclude overlong encodings */
b32 utf8_can_decode_codepoint(const char *nptr, size_t max_size)
{
	const char *p = nptr;
	if (max_size == 0)
		return false;
	else if (max_size >= 1 && (p[0] & 0x80) == 0x00)
		return true;
	else if (max_size >= 2 && (p[0] & 0xe0) == 0xc0 && utf8__can_decode_remainder(&p[1], 1))
		return true;
	else if (max_size >= 3 && (p[0] & 0xf0) == 0xe0 && utf8__can_decode_remainder(&p[1], 2))
		return true;
	else if (max_size >= 4 && (p[0] & 0xf8) == 0xf0 && utf8__can_decode_remainder(&p[1], 3))
		return true;
	else
		return false;
}

s32 utf8_next_codepoint(const char *nptr, char **endptr)
{
	const char *p = nptr;
	if (*p == 0) {
		if (endptr) *endptr = (char*)nptr;
		return 0;
	} else if (!(p[0] & 0x80)) {
		if (endptr) *endptr = (char*)nptr + 1;
		return p[0];
	} else if (!(p[0] & 0x20)) {
		if (endptr) *endptr = (char*)nptr + 2;
		return ((p[0] & 0x1f) << 6)  | (p[1] & 0x3f);
	} else if (!(p[0] & 0x10)) {
		if (endptr) *endptr = (char*)nptr + 3;
		return ((p[0] & 0x0f) << 12) | ((p[1] & 0x3f) << 6) | (p[2] & 0x3f);
	} else {
		if (endptr) *endptr = (char*)nptr + 4;
		return ((p[0] & 0x07) << 18) | ((p[1] & 0x3f) << 12)
		     | ((p[2] & 0x3f) << 6)  | (p[3] & 0x3f);
	}
}

s32 utf8_prev_codepoint(const char *nptr, char **beginptr)
{
	const char *p = nptr-1;
	if (!(*p & 0x80)) {
		if (beginptr) *beginptr = (char*)p;
		return *p;
	} else {
		while (!(*p & 0x40))
			--p;
		if (beginptr) *beginptr = (char*)p;
		return utf8_next_codepoint(p, NULL);
	}
}

#undef UTF8_IMPLEMENTATION
#endif // UTF8_IMPLEMENTATION