Associative Array with double[][]

[seany]

I have a perimeter of a shape, given by a `double [][]` . I want 
to keep the lengths of various diagonals in another associative 
array.

So,

     /// define some associative array to keep track of diagonals 
here..

     auto perimeter = new double[][] (0,0);

     /// --- fill up perimeter here ---

     for(int i = 0; i < perimeter.length; i++) {
       for ( int j = 0; j < perimeter.length; j++) {

       //// fill up the associative array here

       }
     }


So, I would like to do this:

     double[ double[][]] diagonalLengths;
     auto perimeter = new double[][] (0,0);

     /// --- fill up perimeter here ---

     for(int i = 0; i < perimeter.length; i++) {
       for ( int j = 0; j < perimeter.length; j++) {

       auto diag_point_i = perimeter[i];
       auto diag_point_j = perimeter[j];

       diagonalLengths [ [diag_point_i, diag_point_j]] = 
calculate_length (i,j);

       }
     }


This is necessary, as further processing will change the indices 
of the points in the perimeter. I can't therefore use ` 
diagonalLengths [ [i,j]] = calculate_length (i,j);`

However, trying to do this is resulting to :

`test.d(29): Error: associative arrays can only be assigned 
values with immutable keys, not `double[][]`

What are my options now?  Do I have to convert the array which i 
plan to use as a key to a struct and define opEquals and toHash?

Are there automatic hashing mechanisms for this?

If there are multiple possibilities, what is the fastest in terms 
of memory? Thank you.