Lift-Call

--Calls lift

--CONST
    POS = 0 --Number of blocks between position 0 and the level we're at (if negative, means we're below level 0)
    M_SIDE = "back" --Modem side
    R_SIDE = "bottom"   --Side we get lift signal from
    B_SIDE = "left" --Side we get button signal from
    CHANNEL = 1 --Channel to send calls to
--

--FUNC
    function waitForAnswer() --Waits for the lift to answer
        local event
        local timer = os.startTimer(1)  --Time to wait for an answer
        while true do
            event = {os.pullEvent()}
            if event[1] == "modem_message" and event[5] == "pong" then
                return true --The lift got our message
            elseif event[1] == "timer" and event[2] == timer then
                return false --The lift did not have time to answer (we're assuming it didn't get our message)
            end
        end
    end
--

--MAIN
--Wrapping and opening modem
local modem = peripheral.wrap(M_SIDE)
modem.open(CHANNEL)

--Main loop
while true do
    os.pullEvent("redstone") --Waiting for a redstone state change (there is no parameter)
    if rs.getInput(R_SIDE) then rs.setOutput(B_SIDE, false) end --Got signal from lift, we turn the lamp off

    if rs.getInput(B_SIDE) and not rs.getInput(R_SIDE) then --Got signal from button, and if the lift is not already there
        --We send position to lift
        print("Calling elevator to position "..POS)
        modem.transmit(CHANNEL, CHANNEL, POS)
        if waitForAnswer() then --If the lift got the message
            print("Elevator incomming")
            rs.setOutput(B_SIDE, true) --We keep light on to say it's comming
        else    --Otherwise we do nothing, the lamp will turn off
            print("Failed to call elevator")
        end
    end
end