naïve utf8 encode/decode

gistfile1.c

#include <stdio.h>
#include <string.h>
#include <stdint.h>

typedef uint32_t rune;

uint8_t *
encode(rune *runes, uint8_t *buf, size_t length)
{
    for (size_t i = 0; i < length; i++) {
        rune r = runes[i];
        unsigned n, k;

        if (r > 0x10FFFF) {
            return NULL; // Out of range per RFC 3629.
        } else if (r >= 0x10000) {
            n = 4;
        } else if (r >= 0x800) {
            n = 3;
        } else if (r >= 0x80) {
            n = 2;
        } else {
            // We have an ASCII character. Echo as-is.
            *buf++ = (uint8_t) r;
            continue;
        }

        /* This is hairy. The high-order bits must come first, so we'll
         * emit the bytes "backwards", ending with the first one.
         */
        for (k = n; k > 1; k--) {
            *(buf + k - 1) = 0x80 | (r & 0x3f);
            r >>= 6;
        }

        *buf = ~(0xff >> n) | ((uint8_t) r & (0x7f >> n));
        buf += n;
    }

    *buf = 0;
    return buf;
}

rune *
decode(uint8_t *u, rune *buf, size_t *length)
{
    rune r = 0;
    *length = 0;

    while (*u) {
        // Start of UTF-8 sequence?
        if (*u >= 0xC0) {
            unsigned n = 0;

            // Figure out the total length of the sequence.
            while (*u & (0x80 >> n))
                n++;

            // The first few bits of data come from the start sequence.
            r = *u++ & (0xff >> n);

            // Consume all continuation bytes.
            for (; n > 1; n--) {
                if ((*u & 0xC0) != 0x80)
                    return NULL;
                r = (r << 6) | (*u++ & 0x3f);
            }
        } else {
            r = (rune) *u++;
        }

        buf[(*length)++] = r;
    }

    return buf;
}

int main()
{
    char utf8[1024];
    rune runes[1024];
    size_t length;
    fgets(utf8,sizeof(utf8),stdin);
    decode(utf8, runes, &length);
    for (size_t i = 0; i < length; i++) {
        printf("%x ", runes[i]);
    }
    memset(utf8,0,sizeof(utf8));
    encode(runes,utf8,length);
    printf("\n%s\n",utf8);
}