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I'm building simple match 3 game.
I have the parts where I move the gems with the mouse only in the straight X or straight Y, up and down and both sides, no 45 degrees movements on grid.
I'm looking for simple collision detection algorithm for the situation when for example:

a gem is moved up or down or left or right and there is another gem. How can it detect it touched a gem and which gem it is?

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    \$\begingroup\$ It sounds more like you are looking for an algorithm to detect a valid move rather than collision. \$\endgroup\$ – UnderscoreZero Jul 22 '13 at 20:08
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    \$\begingroup\$ You don't even need collision detection per se. Since your gems are arranged in a grid, just check the grid location where you want to move to and see if it's occupied. \$\endgroup\$ – MichaelHouse Jul 22 '13 at 20:22
  • \$\begingroup\$ Thanks , any idea on how to make it generic? \$\endgroup\$ – user3689 Jul 23 '13 at 7:01
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You could have all your gems contain their position (you probably do) and attach a method to the setter of that position so that it checks its surroundings for the gems around it.

In C# it would probably look something like this

private Vector2 mPos = something; // position of the gem
public Vector2 Position {
  get { return mPos; }
  set { mPos = value; GemMoved(mPos); }
}

public void GemMoved(Vector2 pPos) 
{
   var sameneighbours = map.FindNeighbours(this.Position, this.Color);
   ...
}

Another approach would simply to be check the board through for connected areas once a move is made. It's quite fast to check connected areas every time something happens unless your map is massive. By doing something like

bool[,] boardMarked = new bool[width, height];
boardMarked.Fill(false);
for(int x = 0; x < width; x++)
  for(int y = 0; y < height; y++) {
    if(!boardMarked[x,y]) {
      int count = FindAndMarkConnected(x,y);
      if(count >= 3) EliminateAreAndGivePoints(x,y);
    }
  }
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