/*
 * 2014 lovewilliam <ztong@vt.edu>
 * from http://www.lemoda.net/c/ucs2-to-utf8/ucs2-to-utf8.c
 */
/* Input: a Unicode code point, "ucs2". 

   Output: UTF-8 characters in buffer "utf8". 

   Return value: the number of bytes written into "utf8", or -1 if
   there was an error.

   This adds a zero byte to the end of the string. It assumes that the
   buffer "utf8" has at least four bytes of space to write to. */

#define UNICODE_SURROGATE_PAIR -2
#define UNICODE_BAD_INPUT -1

int ucs2_to_utf8 (int ucs2, unsigned char * utf8)
{
    if (ucs2 < 0x80) {
        utf8[0] = ucs2;
        utf8[1] = '\0';
        return 1;
    }
    if (ucs2 >= 0x80  && ucs2 < 0x800) {
        utf8[0] = (ucs2 >> 6)   | 0xC0;
        utf8[1] = (ucs2 & 0x3F) | 0x80;
        utf8[2] = '\0';
        return 2;
    }
    if (ucs2 >= 0x800 && ucs2 < 0xFFFF) {
	if (ucs2 >= 0xD800 && ucs2 <= 0xDFFF) {
	    /* Ill-formed. */
	    return UNICODE_SURROGATE_PAIR;
	}
        utf8[0] = ((ucs2 >> 12)       ) | 0xE0;
        utf8[1] = ((ucs2 >> 6 ) & 0x3F) | 0x80;
        utf8[2] = ((ucs2      ) & 0x3F) | 0x80;
        utf8[3] = '\0';
        return 3;
    }
    if (ucs2 >= 0x10000 && ucs2 < 0x10FFFF) {
	/* http://tidy.sourceforge.net/cgi-bin/lxr/source/src/utf8.c#L380 */
	utf8[0] = 0xF0 | (ucs2 >> 18);
	utf8[1] = 0x80 | ((ucs2 >> 12) & 0x3F);
	utf8[2] = 0x80 | ((ucs2 >> 6) & 0x3F);
	utf8[3] = 0x80 | ((ucs2 & 0x3F));
        utf8[4] = '\0';
        return 4;
    }
    return UNICODE_BAD_INPUT;
}