1337GameDev

I am making a 2d grid out of a single array and need to access locations around a given index. The array represents a 2d grid, where the array spirals out from an origin location (index 0). The reason for the spiral is to allow my 2d grid to grow large in all directions. How do I calculate the indices of the locations to the right, left, above and below the given index location? Here is an image to help: