// Scanner; a simple string parser.
struct scan {
    int !x;                     // Cursor
    bool !bad;                  // True after a malformed token
    u8[~] buf;                  // Buffer to parse

    new scan = { u8[~] b -> buf = b; };

    // Skip whitespace
    val skip = { ->
        while( x < buf# && buf[x] <= ' ' )
            x++;
        return self;
    };

    // True if find required character, and skip it; else False.
    val match = { u8 c ->
        skip();
        if( x >= buf# ) return false;
        if( buf[x] != c ) return false;
        x++;
        return true;
    };

    // Parse a simple number
    val number = { ->
        skip();
        bool neg = match('-');
        if( x >= buf# || buf[x] < '0' || '9' < buf[x] ) {
            bad = true;
            return 0;
        }
        i64 n = 0;
        while( x < buf# && '0' <= buf[x] & buf[x] <= '9' )
            n = n*10 + (buf[x++] - '0');
        return neg ? -n : n;
    };
};