Removing mutations for D metaprogramming/compiletime array generation

问题

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);

 }

回答1:

import std.stdio;

immutable char[] ALPHABET = "ABFCDFRGHDSTHFG";

int[char] compute(in string pattern)
{
    int[char] table;

    foreach (c; ALPHABET) {
        table[c] = cast(int)pattern.length;
    }

    foreach (i, c; pattern) {
        table[c] = cast(int)(pattern.length - i - 1);
    }

    return table;
}

void main()
{
    enum table = compute("ABA");
    writeln(table);
}

Output:

['A':0, 'B':1, 'C':3, 'D':3, 'F':3, 'G':3, 'H':3, 'R':3, 'S':3, 'T':3]

Code on dpaste.

来源：https://stackoverflow.com/questions/31781188/removing-mutations-for-d-metaprogramming-compiletime-array-generation