I am trying to verify if a string (string_view) is one of the “allowed” combinations.

String is always 3 characters.

Currently thinking about something like this:

    constexpr uint32_t s2u(std::string_view s){
        using T = uint32_t;

        return
            (T(s[0]) << 8 * 0) |
            (T(s[1]) << 8 * 1) |
            (T(s[2]) << 8 * 2) ;
    }

    bool validate(std::string_view s){
        if (s.size() != 3)
            return false;

        switch( s2u(s) ){
        case s2u("abc") : case s2u("ABC") :
        case s2u("acb") : case s2u("ACB") :
        case s2u("bac") : case s2u("BAC") :
        case s2u("bca") : case s2u("BCA") :
        case s2u("cab") : case s2u("CAB") :
        case s2u("cba") : case s2u("CBA") : return true;
        default:
            return false;
        }
    }

Is there faster way to do it?

C solutions are welcome too.