Can this implementation of Damm algorithm be optimized?

[Era Scarecrow via Digitalmars-d-learn]

On Saturday, 11 February 2017 at 21:02:40 UTC, Era Scarecrow 
wrote:
>  Yes i know, which is why i had 3 to calculate 2 inputs because 
> the third is the temp/previous calculation.

  Alright I've found the bug and fixed it, and it passes with 
flying colors (brute force tests up to 6 digits); However it 
doesn't use the original function to build the table. So I'm 
satisfied it will handle any length now.

  But it seriously is a lot of overhead for such a simple function.

int[] buildMatrix2() {
     string digits = "0123456789";
     int[] l = new int[16*16*10];
     l[] = -1; //printing the array it's obvious to see what is 
padding
     foreach(a; digits)
     foreach(b; digits)
     foreach(c; digits) {
         int t = (a-'0')*10,
             t2 = (QG10Matrix[(b - '0') + t]-'0') * 10,
             off = (a - '0') << 8 | (b - '0') << 4 | (c - '0');
         l[off] = (QG10Matrix[(c - '0') + t2]-'0')<<8;
     }

     return l;
}

char checkDigit2(string str) {
     int tmpdigit = 0;
     for(;str.length >= 2;str = str[2 .. $])
         tmpdigit = 
QG10Matrix2[tmpdigit|(str[0]-'0')<<4|(str[1]-'0')];

     tmpdigit>>=8;
     if (str.length==1)
         return QG10Matrix[(str[0]-'0')+tmpdigit*10];

     return (tmpdigit+'0') & 0xff;
}