[SINGLE PLAYER] Half Network

--[[
    A complete train signal system has two on sensors (one per side), two off sensors (one per side),
    and two signal lights. Given a signal light A and a signal light B, this script handles
    the on sensor for B and the off sensor for A. Since these two sensors are close to A,
    it is efficient to link them and A to a single computer, and to link the rest of the components
    on the other side to another computer.

    Each train signal network should have two computers, and each computer should use this script
    and name it "startup".
]]


----| config |----------------------------


local MODEM_SIDE = "top"
local LEFT_SENSOR_SIDE = "left"
local RIGHT_SENSOR_SIDE = "right"
local SIGNAL_SIDE = "back"
local CENTRAL_CPU_ID = 4 -- the id of the computer at spawn


----| setup |----------------------------


local protocolEnum = {
    GET_SIGNAL_STATE = "GET_SIGNAL_STATE",
    SENSOR_CHANGE = "SENSOR_CHANGE",
}

local sensorStateEnum = { -- compatible with bitwise operations
    off = 0,
    leftOn = 1,
    rightOn = 2,
    bothOn = 3,
}

local lastSensorStates = sensorStateEnum.off


----| functions |----------------------------


function getSensorStates()
    local leftValue = redstone.getInput(LEFT_SENSOR_SIDE) and sensorStateEnum.leftOn or sensorStateEnum.off -- 1 if on, 0 if off
    local rightValue = redstone.getInput(RIGHT_SENSOR_SIDE) and sensorStateEnum.rightOn or sensorStateEnum.off -- 2 if on, 0 if off
    return leftValue + rightValue -- corresponds to one of the sensorStateEnum values
end


function sendSensorStates()
    local states = getSensorStates()
    if states ~= lastSensorStates and states ~= sensorStateEnum.off and states ~= sensorStateEnum.bothOn then -- if the state changed and just one of the sensors is on
        rednet.send(CENTRAL_CPU_ID, states, protocolEnum.SENSOR_CHANGE)
    end
    lastSensorStates = states -- record the state change
end


function setSignalState()
    rednet.send(CENTRAL_CPU_ID, nil, protocolEnum.GET_SIGNAL_STATE)
    local id, signalState = rednet.receive(protocolEnum.GET_SIGNAL_STATE)
    print("Received message from computer " .. id .. "\n\t> Message: " .. tostring(signalState))
    if id ~= CENTRAL_CPU_ID then return end

    if signalState == true then -- activate signal
        print("Activating signal...")
        redstone.setOutput(SIGNAL_SIDE, true)
    elseif signalState == false then -- deactivate signal
        redstone.setOutput(SIGNAL_SIDE, false)
        print("Deactivating signal...")
    end
end


----| main |----------------------------


rednet.open(MODEM_SIDE)
print("Rednet opened.")


while true do
    sendSensorStates()
    setSignalState()
    sleep(1)
end