Oh. I couldn't read this all now. But the orange peeling idea demonstrates why it's really important to specify what you mean by a "spiral" and a "list of coordinates". My instinct would have been totally different from peeling the edges of the array back. It would have been to draw a circle or ellipse and keep shrinking the radius(radii). I wouldn't have assumed from the definition of "spiral" that every outer shell coordinate would be included. I would have assumed the opposite, actually.
Oh. I couldn't read this all now. But the orange peeling idea demonstrates why it's really important to specify what you mean by a "spiral" and a "list of coordinates". My instinct would have been totally different from peeling the edges of the array back. It would have been to draw a circle or ellipse and keep shrinking the radius(radii). I wouldn't have assumed from the definition of "spiral" that every outer shell coordinate would be included. I would have assumed the opposite, actually.