Best shuffle

Best shuffle
You are encouraged to solve this task according to the task description, using any language you may know.

Shuffle the characters of a string in such a way that as many of the characters are in a different position as possible. Print the result as follows: original string, shuffled string, (num characters ignored)

For example: tree, eetr, (0)

The words to test with are: abracadabra, seesaw, elk, grrrrrr, up, a

D

D v.2 <lang d>int bestShuffle(string s) {

   int countSamePositions(T, U)(T s1, U s2, uint len) {
       int count;
       for (int i; i < len; i++) {
           if (s2[i] != '-' && s2[i] == s1[i]) {
               count++;
           }
       }
       return count;        
   }

   const len = s.length;
   if (len == 0) {
       throw new Exception("input string cannot have zero length");
   }

   char[] ch = s.dup.sort;

   auto problemChar = sort!("a[1] > b[1]")(array(group(ch)))[0];
   if ((problemChar[1] - len / 2) > 0) { 
       int numToRemove = problemChar[1] - (len - problemChar[1]);
       for (int i, j; i < len && j < numToRemove; i++) {
           if (ch[i] == problemChar[0]) {
               ch[i] = '-';
               j++;
           }
       }
   }

   do {
       foreach (ref e; ch) {
           swap(e, ch[uniform(0, len)]);
       }
   } while(countSamePositions(s, ch, len) > 0);

   string result = replace(to!string(ch), "-", to!string(problemChar[0]));
   int samePos = countSamePositions(s, result, len);

   writefln("%s %s (%s)", s, result, samePos);

   return samePos;

}</lang>

output:

abracadabra baadacbraar (0)
seesaw easwes (0)
elk lke (0)
grrrrrr rrrrrgr (5)
up pu (0)
a a (1)

<lang d>unittest {

   assert(bestShuffle("abracadabra") == 0);
   assert(bestShuffle("seesaw") == 0);
   assert(bestShuffle("elk") == 0);
   assert(bestShuffle("grrrrrr") == 5);
   assert(bestShuffle("up") == 0);
   assert(bestShuffle("a") == 1);

}</lang>