Calculate XY to D for hilbert curve (Convert C code)

Question

I am trying to convert this bit of code from this wikipedia article from C to Delphi.

//convert (x,y) to d
int xy2d (int n, int x, int y) {
    int rx, ry, s, d=0;
    for (s=n/2; s>0; s/=2) {
        rx = (x & s) > 0;
        ry = (y & s) > 0;
        d += s * s * ((3 * rx) ^ ry);
        rot(s, &x, &y, rx, ry);
    }
    return d;
}

I cannot find an equivalent to the rot function used there

I can't believe I missed it. Since the question was answered I could not delete it. I have edited the question to make it about code conversion instead. — Ali, Aug 13 '15 at 06:21
@Ali I rolled back your edit. It's too late to ask a different question. You've already asked and accepted an answer. — David Heffernan, Aug 13 '15 at 06:24

samgak · Accepted Answer · 2015-08-13T06:30:33.483

2

You can use the the Var keyword to pass functions by reference in Delphi, which achieves the same result as passing pointers to ints in the linked C code:

procedure Rot(N : Integer; RX, RY : Boolean; Var X, Y : Integer);
 var
   T : Integer;
 begin
   If Not RY then
   begin
     If RX then
     begin
       X := N - 1 - X;
       Y := N - 1 - Y;
     end;
       T := X;
       X := Y;
       Y := T;
   end;
 end;

Note that the parameter order has changed. I grouped the parameters passed by reference and those passed by value together. You can use Booleans instead of Integers for RX and RY (make sure you adapt the calling code appropriately though).

edited Aug 13 '15 at 06:30

answered Aug 13 '15 at 06:12

samgak

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You didn't explain where this function came from, nor why you changed the parameter order. – David Heffernan Aug 13 '15 at 06:17
@DavidHeffernan I assumed that the OP had seen the function since it was right there in the linked article but wasn't sure how to translate it – samgak Aug 13 '15 at 06:19
I don't think so. I read it that asker expected rot to be a standard function and simply not read the article carefully. Furthermore, if you feel inclined to translate this for the user please don't use `int` where `Boolean` should be used. – David Heffernan Aug 13 '15 at 06:22
@DavidHeffernan yes you are correct based on the comment posted to the question after the answers – samgak Aug 13 '15 at 06:23

David Heffernan · Answer 2 · 2015-08-13T06:30:50.340

This rot is not a standard function. Rather it is part of the code which you are translating. You simply need to keep reading the article to which you link. A few lines on you will find this:

//rotate/flip a quadrant appropriately
void rot(int n, int *x, int *y, int rx, int ry) {
    if (ry == 0) {
        if (rx == 1) {
            *x = n-1 - *x;
            *y = n-1 - *y;
        }

        //Swap x and y
        int t  = *x;
        *x = *y;
        *y = t;
    }
}

You need to translate this function alongside the code in your question. I'm sure you don't need me to do the translation of such a simple function.

I would say though that rx and ry are really booleans and you may be better coding them that way. The code in the article is written in a C flavour that pre-dates C boolean types. That said, the function in the question performs arithmetic on these "booleans" so it's a little tricky to write a clean literal translation.

The parameters passed as pointers are probably better as var parameters and so passed by reference. Again a feature that C lacks.

Calculate XY to D for hilbert curve (Convert C code)

2 Answers2