do some arithmetic instead of if

mesecons_fpga/init.lua

@@ -143,11 +143,7 @@ plg.register_nodes({
 				if o and o.type == "io" then
 					local num = abcd2[o.port]
 					num = num + dir
-					if num > 4 then
+					num = (2 ^ num % 2) * 4 + num + math.floor(num / 5)
-						num = 1
-					elseif num < 1 then
-						num = 4
-					end
 					instr[i][op].port = abcd1[num]
 				end
 			end