Can this implementation of Damm algorithm be optimized?

[Nestor via Digitalmars-d-learn]

Hi,

I was trying to port C code from the article in Wikiversity [1] 
to D, but I'm not sure this implementation is the most efficient 
way to do it in D, so suggestions to optimize it are welcome:

import std.stdio;

static immutable char[] QG10Matrix =
  "03175986427092154863420687135917509834266123045978" ~
  "36742095815869720134894536201794386172052581436790";

char checkDigit(string str) {
  char tmpdigit = '0';
  foreach(chr; str) tmpdigit = QG10Matrix[(chr - '0') + (tmpdigit 
- '0') * 10];
  return tmpdigit;
}

enum {
  EXIT_SUCCESS = 0,
  EXIT_FAILURE = 1,
}

int main(string[] args) {
  scope(failure) {
    writeln("Invalid arguments. You must pass a number.");
    return EXIT_FAILURE;
  }
  assert(args.length == 2);
  char digit = checkDigit(args[1]);
  if(digit == '0') writefln("%s is a valid number.", args[1]);
  else {
    writefln("%s is not a valid number (but it would be, 
appending digit %s).",
      args[1], digit);
  }

  return EXIT_SUCCESS;
}

[1] https://en.wikiversity.org/wiki/Damm_algorithm