Grid neighbor search

grid_neighbors.p8

--[[

Given a 1D-array of cells in a 2D-grid,
and a target cell with x and y coordinates,
it returns a table of neigboring cells.

Given a truthy third parameter,
it also returns diagonally adjacent cells.

]]

function get_adjacent(board,cell,diagonals)
	local left_edge, right_edge,
		top_edge, bottom_edge=
		cell.x==0, cell.x==board_width-1,
		cell.y==0, cell.y==board_height-1
	local offsets={}
	if(not top_edge)    add(offsets,-board_width)
	if(not bottom_edge) add(offsets, board_width)
	if(not left_edge)   add(offsets,-1)
	if(not right_edge)  add(offsets, 1)
	if diagonals then
		if not top_edge then
			if not left_edge then
				add(offsets,-board_width-1)
			end
			if not right_edge then
				add(offsets,-board_width+1)
			end
		end
		if not bottom_edge then
			if not left_edge then
				add(offsets,board_width-1)
			end
			if not right_edge then
				add(offsets,board_width+1)
			end
		end
	end
	
	local adjacents={}
	local cell_index=find_index(board,cell)
	for offset in all(offsets) do
		if board[cell_index+offset] then
			add(adjacents, board[cell_index+offset])
		end
	end
	return adjacents
end