#include <string.h>

int utf8_encode(int codepoint, char *buffer, int *size)
{
    if(codepoint < 0)
        return -1;
    else if(codepoint < 0x80)
    {
        buffer[0] = (char)codepoint;
        *size = 1;
    }
    else if(codepoint < 0x800)
    {
        buffer[0] = 0xC0 + ((codepoint & 0x7C0) >> 6);
        buffer[1] = 0x80 + ((codepoint & 0x03F));
        *size = 2;
    }
    else if(codepoint < 0x10000)
    {
        buffer[0] = 0xE0 + ((codepoint & 0xF000) >> 12);
        buffer[1] = 0x80 + ((codepoint & 0x0FC0) >> 6);
        buffer[2] = 0x80 + ((codepoint & 0x003F));
        *size = 3;
    }
    else if(codepoint <= 0x10FFFF)
    {
        buffer[0] = 0xF0 + ((codepoint & 0x1C0000) >> 18);
        buffer[1] = 0x80 + ((codepoint & 0x03F000) >> 12);
        buffer[2] = 0x80 + ((codepoint & 0x000FC0) >> 6);
        buffer[3] = 0x80 + ((codepoint & 0x00003F));
        *size = 4;
    }
    else
        return -1;

    return 0;
}

int utf8_check_first(char byte)
{
    unsigned char u = (unsigned char)byte;

    if(u < 0x80)
        return 1;

    if(0x80 <= u && u <= 0xBF) {
        /* second, third or fourth byte of a multi-byte
           sequence, i.e. a "continuation byte" */
        return 0;
    }
    else if(u == 0xC0 || u == 0xC1) {
        /* overlong encoding of an ASCII byte */
        return 0;
    }
    else if(0xC2 <= u && u <= 0xDF) {
        /* 2-byte sequence */
        return 2;
    }

    else if(0xE0 <= u && u <= 0xEF) {
        /* 3-byte sequence */
        return 3;
    }
    else if(0xF0 <= u && u <= 0xF4) {
        /* 4-byte sequence */
        return 4;
    }
    else { /* u >= 0xF5 */
        /* Restricted (start of 4-, 5- or 6-byte sequence) or invalid
           UTF-8 */
        return 0;
    }
}

int utf8_check_full(const char *buffer, int size)
{
    int i, value = 0;
    unsigned char u = (unsigned char)buffer[0];

    if(size == 2)
    {
        value = u & 0x1F;
    }
    else if(size == 3)
    {
        value = u & 0xF;
    }
    else if(size == 4)
    {
        value = u & 0x7;
    }
    else
        return 0;

    for(i = 1; i < size; i++)
    {
        u = (unsigned char)buffer[i];

        if(u < 0x80 || u > 0xBF) {
            /* not a continuation byte */
            return 0;
        }

        value = (value << 6) + (u & 0x3F);
    }

    if(value > 0x10FFFF) {
        /* not in Unicode range */
        return 0;
    }

    else if(0xD800 <= value && value <= 0xDFFF) {
        /* invalid code point (UTF-16 surrogate halves) */
        return 0;
    }

    else if((size == 2 && value < 0x80) ||
            (size == 3 && value < 0x800) ||
            (size == 4 && value < 0x10000)) {
        /* overlong encoding */
        return 0;
    }

    return 1;
}

int utf8_check_string(const char *string, int length)
{
    int i;

    if(length == -1)
        length = strlen(string);

    for(i = 0; i < length; i++)
    {
        int count = utf8_check_first(string[i]);
        if(count == 0)
            return 0;
        else if(count > 1)
        {
            if(i + count > length)
                return 0;

            if(!utf8_check_full(&string[i], count))
                return 0;

            i += count - 1;
        }
    }

    return 1;
}