Associative arrays, symetrical algorithm, CTFE

I want to implement something like a 开发者_Python百科Vigenère cipher, and I chose to use D's associative arrays. I use the

struct Codes
{
    int[char] enc;
    char[int] dec;
}

and then i populate the AA's with

foreach(immutable char c; letters ~ whitespace ~ digits)
{
    codes.enc[c] = codes.enc.length;
    codes.dec[codes.enc.length] = c;
}

It works as it is suppose to work in my [en/de]crypting functions, but I have two questions:

1. How can I, if I can, use just one AA. Or is it some other data structure that is as easy to use, more efficient or jut fits better with point 2.
2. How am I suppose to use CTFE to generate some data structure at compile time and use the output at runtime.

(I'm using the dmd v2.053 compiler if that matter whatsoever this days).

---

the dec can be a normal array (it's equal to letters ~ whitespace ~ digits btw)

in enc you can also use a normal array if you only allow ASCII as characters (with length 255 and casting c to int for the index)

edit

you can create a mixin string from the letters ~ whitespace ~ digits string like so

string createSwitch(string seq,string inp){
    string out = "switch("~inp~"){";
    int i=0;
    foreach(ch;seq){
        out~="case "~ch~":return "~i";";
    }
    out~="default: return -1;";
    return out;
}

which you can then use as

int intFromChar(char ch){
    mixin(createSwitch(letters ~ whitespace ~ digits,"ch"));
}