I want to pack a letters from a char[] into an array of 5-bit encoded English lower letters in a size_t[2].

How do I accomplish that with as few instructions as possible?

I currently have

        size_t[2] words;
        enum charBits = 5;
        foreach (const ch_index, const char ch; expr)
        {
            ubyte ub = 0;
            if (ch >= 'a' && ch <= 'z') // English lower letter
            {
                ub = cast(ubyte)(ch - 'a');
            }
            else if (ch == ' ') // space
            {
                ub = cast(ubyte)(ch - 'z' + 1);
            }
            else
            {
                dln("Invalid char ", ch);
                assert(0);
            }
            assert(ub < (1 << charBits), "Too large ubyte code");
            const bit_index = 8 + charBits*ch_index; // first 8 bits reserved for length
            // TODO store first `charBits` bits of `ub` at `bit_index` in `words`
        }