#include <stdio.h>

/*
 * http://www.unicode.org/versions/Unicode6.0.0/ch03.pdf - page 94
 *
 * Table 3-7. Well-Formed UTF-8 Byte Sequences
 *
 * +--------------------+------------+-------------+------------+-------------+
 * | Code Points        | First Byte | Second Byte | Third Byte | Fourth Byte |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+0000..U+007F     | 00..7F     |             |            |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+0080..U+07FF     | C2..DF     | 80..BF      |            |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+0800..U+0FFF     | E0         | A0..BF      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+1000..U+CFFF     | E1..EC     | 80..BF      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+D000..U+D7FF     | ED         | 80..9F      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+E000..U+FFFF     | EE..EF     | 80..BF      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+10000..U+3FFFF   | F0         | 90..BF      | 80..BF     | 80..BF      |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+40000..U+FFFFF   | F1..F3     | 80..BF      | 80..BF     | 80..BF      |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+100000..U+10FFFF | F4         | 80..8F      | 80..BF     | 80..BF      |
 * +--------------------+------------+-------------+------------+-------------+
 */

/* Return 0 - success,  >0 - index(1 based) of first error char */
int utf8_naive(const unsigned char *data, int len)
{
    int err_pos = 1;

    while (len) {
        int bytes;
        const unsigned char byte1 = data[0];

        /* 00..7F */
        if (byte1 <= 0x7F) {
            bytes = 1;
        /* C2..DF, 80..BF */
        } else if (len >= 2 && byte1 >= 0xC2 && byte1 <= 0xDF &&
                (signed char)data[1] <= (signed char)0xBF) {
            bytes = 2;
        } else if (len >= 3) {
            const unsigned char byte2 = data[1];

            /* Is byte2, byte3 between 0x80 ~ 0xBF */
            const int byte2_ok = (signed char)byte2 <= (signed char)0xBF;
            const int byte3_ok = (signed char)data[2] <= (signed char)0xBF;

            if (byte2_ok && byte3_ok &&
                     /* E0, A0..BF, 80..BF */
                    ((byte1 == 0xE0 && byte2 >= 0xA0) ||
                     /* E1..EC, 80..BF, 80..BF */
                     (byte1 >= 0xE1 && byte1 <= 0xEC) ||
                     /* ED, 80..9F, 80..BF */
                     (byte1 == 0xED && byte2 <= 0x9F) ||
                     /* EE..EF, 80..BF, 80..BF */
                     (byte1 >= 0xEE && byte1 <= 0xEF))) {
                bytes = 3;
            } else if (len >= 4) {
                /* Is byte4 between 0x80 ~ 0xBF */
                const int byte4_ok = (signed char)data[3] <= (signed char)0xBF;

                if (byte2_ok && byte3_ok && byte4_ok &&
                         /* F0, 90..BF, 80..BF, 80..BF */
                        ((byte1 == 0xF0 && byte2 >= 0x90) ||
                         /* F1..F3, 80..BF, 80..BF, 80..BF */
                         (byte1 >= 0xF1 && byte1 <= 0xF3) ||
                         /* F4, 80..8F, 80..BF, 80..BF */
                         (byte1 == 0xF4 && byte2 <= 0x8F))) {
                    bytes = 4;
                } else {
                    return err_pos;
                }
            } else {
                return err_pos;
            }
        } else {
            return err_pos;
        }

        len -= bytes;
        err_pos += bytes;
        data += bytes;
    }

    return 0;
}