c2hex.c

int8_t c2hex(int8_t c)
{
    return (unsigned) (c - '0') < 10 ? c - '0' : ((unsigned) (c | 0x20) - 'a' < 6 ? 9 + (c&7) : -1);
}

int8_t _c2hex(int8_t c)
{
    if (c - '0' >= 0 && c - '0' < 10) return c - '0';
    if (c - 'a' >= 0 && c - 'a' <  7) return c - 'a' + 10;
    if (c - 'A' >= 0 && c - 'A' <  7) return c - 'A' + 10;
    return -1;
}

#define c2hex(_c) \
    ((unsigned) ((_c) - '0') < 10 ? (_c) - '0' : ((unsigned)((_c) | 0x20) - 'a' < 6 ? 9 + ((_c)&7) : -1))