Removing mutations for D metaprogramming/compiletime array generation

My plan is to write a mutation-less code in D-language so that my values are available by runtime. Someone spoke to me about loop-unrolling and compile time code generation but I have no clear idea how to do that. I have made the D-template below but it has no guarantee to be evaluated at compile-time because on the two assignment statements(mutations) . Advice would be greatly appreciated. Suggestions could be preferably in D or C++ without macros.

import std.stdio;
import std.string;
import std.conv;

const char[] ALPHABET="ABFCDFRGHDSTHFG";
const string pattern="ABA";


I[C]  computeAtCompileTime(S ,C,I)( const S  pattern ){
  I[C] table1;

  const int size = to!int(pattern.length) ;//Length of the pattern to be matched

  foreach( c; ALPHABET){   //Initialise array
          table1[c] = size;
    }

  foreach(i; 0..size-1){
             table1[pattern[i]] = size -i-1;
  }
  return table1;
}
enum TableFromCompiler  = computeAtCompileTime!(const string ,char, int)(pattern);
void main(){

    // enum TableFromCompiler  = computeAtCompileTime!(const string ,char, int)(pattern);

     writeln(TableFromCompiler);

 }