utf8_encode.h

#include <stdint.h>

/**
 * Encode a code point using UTF-8
 *
 * @author Ondřej Hruška <ondra@ondrovo.com>
 * @license MIT
 *
 * @param out - output buffer (min 5 characters), will be 0-terminated
 * @param utf - code point 0-0x10FFFF
 * @return number of bytes on success, 0 on failure (also produces U+FFFD, which uses 3 bytes)
 */
static int utf8_encode(char *out, uint32_t utf)
{
	if (utf <= 0x7F) {
		// Plain ASCII
		out[0] = (char) utf;
		out[1] = 0;
		return 1;
	}
	else if (utf <= 0x07FF) {
		// 2-byte unicode
		out[0] = (char) (((utf >> 6) & 0x1F) | 0xC0);
		out[1] = (char) (((utf >> 0) & 0x3F) | 0x80);
		out[2] = 0;
		return 2;
	}
	else if (utf <= 0xFFFF) {
		// 3-byte unicode
		out[0] = (char) (((utf >> 12) & 0x0F) | 0xE0);
		out[1] = (char) (((utf >>  6) & 0x3F) | 0x80);
		out[2] = (char) (((utf >>  0) & 0x3F) | 0x80);
		out[3] = 0;
		return 3;
	}
	else if (utf <= 0x10FFFF) {
		// 4-byte unicode
		out[0] = (char) (((utf >> 18) & 0x07) | 0xF0);
		out[1] = (char) (((utf >> 12) & 0x3F) | 0x80);
		out[2] = (char) (((utf >>  6) & 0x3F) | 0x80);
		out[3] = (char) (((utf >>  0) & 0x3F) | 0x80);
		out[4] = 0;
		return 4;
	}
	else {
		// error - use replacement character
		out[0] = (char) 0xEF;
		out[1] = (char) 0xBF;
		out[2] = (char) 0xBD;
		out[3] = 0;
		return 0;
	}
}