function _init()
	frame = 0

	-- Set up userdata so we can draw the screen to itself
	-- by using memcpy() to a memmapped region of memory.
	-- The userdata is now the contents of the previous frame,
	-- and it is now possible to modify the current frame.
	-- We can also call spr() with userdata
	-- to draw the previous frame to the current one

	screen = userdata("u8", 480, 270)
	memmap(0x30000, screen)
end

function set_col_table(new, current, col)
	poke(0x8000 + 64*new + current, col)
end

function _draw()
	frame += 1

	if frame == 1 then
		-- Randomise the screen for the first frame.
		for y=0,269 do
			for x=0,479 do
				if (rnd() < 0.2) pset(x, y, 7)
			end
		end

		-- Copy the current screen to the buffer, just for the first frame
		memcpy(0x30000, 0x10000, 0x20000)

	end

	--if (frame % 32 > 0) return

	cls()

	-- White cells drawn onto colour 0 will set the colour to 1, white onto 1 will be 2
	-- and so on. This counts the number of neighbouring cells very quickly
	for i=0, 7 do
		set_col_table(7, i, i+1)
	end

	-- Draw the screen 8 times in a ring, for each neighbour. The colour tables
	-- do the counting
	for y=-1,1 do
		for x=-1,1 do
			if (x!=0 or y!=0) spr(screen, x, y)
		end
	end

	-- Set up colour tables to turn the "counted" screen into the next frame
	-- Set every colour to black except for drawing black onto 3 (brought back alive)
	-- and 2 or 3 neighbours for alive (stay alive)
	for i=0, 9 do
		set_col_table(0, i, 0)
		set_col_table(7, i, 0)
	end
	set_col_table(0, 3, 7)
	set_col_table(7, 2, 7)
	set_col_table(7, 3, 7)

	-- Draw the screen to the "counted" version, with the rules set above
	spr(screen, 0, 0)

	-- Reset the draw state to make sure we have a predictable next frame
	reset()

	-- Copy the screen to the buffer for the next frame,
	-- before we pollute it with the FPS counter
	memcpy(0x30000, 0x10000, 0x20000)

	-- FPS counter
	if key("x") then
		rectfill(0, 0, 44, 8, 0)
		print("FPS: "..stat(7), 1, 1, 8)
	end

end