Can this implementation of Damm algorithm be optimized?

[Nestor via Digitalmars-d-learn]

On Thursday, 9 February 2017 at 18:34:30 UTC, Era Scarecrow wrote:
> On Thursday, 9 February 2017 at 17:36:11 UTC, Nestor wrote:
>> 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;
>> }
>
> Well one thing is you can probably reduce them from chars to 
> just bytes, instead of having to subtract you can instead add 
> at the end. Although unless you're working with a VERY large 
> input you won't see a difference.
>
> Actually since you're also multiplying by 10, you can 
> incorporate that in the table too... (although a mixin might be 
> better for the conversion than by hand)
>
>
>  static immutable char[] QG10Matrix = [
>     00,30,10,70,50,90,80,60,40,20,
>     70,00,90,20,10,50,40,80,60,30,
>     40,20,00,60,80,70,10,30,50,90,
>     10,70,50,00,90,80,30,40,20,60,
>     60,10,20,30,00,40,50,90,70,80,
>     30,60,70,40,20,00,90,50,80,10,
>     50,80,60,90,70,20,00,10,30,40,
>     80,90,40,50,30,60,20,00,10,70,
>     90,40,30,80,60,10,70,20,00,50,
>     20,50,80,10,40,30,60,70,90,00];
>
>  char checkDigit(string str) {
>    char tmpdigit = 0;
>    foreach(chr; str) tmpdigit = QG10Matrix[(chr - '0') +
>  tmpdigit];
>    return (tmpdigit/10) + '0';
>  }



OK I changed the approach using a multidimensional array for the 
matrix so I could ditch arithmetic operations altogether, but 
curiously after measuring a few thousand runs of both 
implementations through avgtime, I see no noticeable difference. 
Why?

import std.stdio;

static immutable ubyte[][] QG10Matrix = [
   [0,3,1,7,5,9,8,6,4,2],[7,0,9,2,1,5,4,8,6,3],
   [4,2,0,6,8,7,1,3,5,9],[1,7,5,0,9,8,3,4,2,6],
   [6,1,2,3,0,4,5,9,7,8],[3,6,7,4,2,0,9,5,8,1],
   [5,8,6,9,7,2,0,1,3,4],[8,9,4,5,3,6,2,0,1,7],
   [9,4,3,8,6,1,7,2,0,5],[2,5,8,1,4,3,6,7,9,0],
];

static int charToInt(char chr) {
   scope(failure) return -1;
   return cast(int)(chr - '0');
}

ubyte checkDigit(string str) {
   ubyte tmpdigit;
   foreach(chr; str) tmpdigit = 
QG10Matrix[tmpdigit][charToInt(chr)];
   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);
   ubyte 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;
}