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With our easy coordinate transformation to- and from-Canonical coordinates, this becomes dead simple:

From Coords.cs:
    private int Range(ICoordsCanon coords) {
      var deltaX = coords.X - VectorCanon.X;
      var deltaY = coords.Y - VectorCanon.Y;
      return (Math.Abs(deltaX) + Math.Abs(deltaY) + Math.Abs(deltaX-deltaY)) / 2;

Invoking this method is as simple as this code fragement from hexgridPanel_MouseMove:
void hexgridPanel_MouseMove(object sender, MouseEventArgs e) {
  HotSpotHex = hexgridPanel.GetHexCoords(e.Location).User;
  Range      = CurrentHex.Range(HotSpotHex);
  // etc.

This internal method is aliased, with the same name and signature, in all three of the Coordinate interfaces: ICoordsCanon, ICoordsUser, and ICoordsCustom.

The proof of this formula (in Canonical coordinates) is readiy available all over the web; I will not repeat it here. Note that this variant of the formual is for X- and Y-axes at 120 degrees; the formula varies slightly if they are chosen at 60 degrees instead.

Last edited Feb 25, 2013 at 1:10 AM by pgeerkens, version 4