Algorithm for creating cells by spiral on the hexagonal field

Question

Help to find an algorithm for creating cells by spiral on the hexagonal field.

Look at the image:

Let's imagine an dimensionless 2d array. The X axis is the blue line, Y is horizontal, spiral is red.

I need to add cells from the central point x0y0 to point N by spiral

Tell me the way to solve the problem, please. Thank you!

Answer 1

I'd suggest changing the cells numbering sligtly, so that X remains the same when you go down and right (or up and left). Then simple algorithm like the following should work:

  int x=0, y=0;   
  add(x, y); // add the first cell
  int N=1 
  for( int N=1; <some condition>; ++N ) {
    for(int i=0; i<N; ++i) add(++x, y);  // move right
    for(int i=0; i<N-1; ++i) add(x, ++y); // move down right. Note N-1
    for(int i=0; i<N; ++i) add(--x, ++y); // move down left
    for(int i=0; i<N; ++i) add(--x, y); // move left
    for(int i=0; i<N; ++i) add(x, --y); // move up left
    for(int i=0; i<N; ++i) add(++x, --y); // move up right
  }

This generates the points as follows:

After a transformation we get:

Answer 2

(the circles have a diameter of 1)

Here's a function to get position i:

  void getHexPosition( int i, ref double x, ref double y )
  {
     if ( i == 0 ) { x = y = 0; return; }

     int layer = (int) Math.Round( Math.Sqrt( i/3.0 ) );

     int firstIdxInLayer = 3*layer*(layer-1) + 1;
     int side = (i - firstIdxInLayer) / layer; // note: this is integer division
     int idx  = (i - firstIdxInLayer) % layer;                  
     x =  layer * Math.Cos( (side - 1) * Math.PI/3 ) + (idx + 1) * Math.Cos( (side + 1) * Math.PI/3 );
     y = -layer * Math.Sin( (side - 1) * Math.PI/3 ) - (idx + 1) * Math.Sin( (side + 1) * Math.PI/3 );
  }

Scaling the result by Math.Sqrt(.75) gives

If you're interested in the skewed coordinates like in shura's answer:

  int[] h = { 1, 1, 0, -1, -1, 0, 1, 1, 0 };
  void getHexSkewedPosition( int i, ref int hx, ref int hy )
  {
     if ( i == 0 ) { hx = hy = 0; return; }

     int layer = (int) Math.Round( Math.Sqrt( i/3.0 ) );

     int firstIdxInLayer = 3*layer*(layer-1) + 1;
     int side = (i - firstIdxInLayer) / layer;
     int idx  = (i - firstIdxInLayer) % layer;

     hx = layer*h[side+0] + (idx+1) * h[side+2];
     hy = layer*h[side+1] + (idx+1) * h[side+3];
  }

  void getHexPosition( int i, ref double hx, ref double hy )
  {
     int x = 0, y = 0;
     getHexSkewedPosition( i, ref x, ref y );
     hx = x - y * .5;
     hy = y * Math.Sqrt( .75 );
  }