vla_receiver.lua

local modem = peripheral.find("modem", function(n,o) return o.isWireless() end)
local origin = nil
local toid = 31415
modem.open(toid)

local function printHeader(str, ny)
    local w, h = term.getSize()
    local x, y = term.getCursorPos()
    term.setCursorPos(math.ceil(w/2) - math.floor(#str/2), ny or 1)
    term.clearLine()
    term.write(str)
    term.setCursorPos(x, y)
end

local function select_caster()
    print("Scanning for casters")

    local pingmsg = "Cast_Ping"
    local origins = {}
    local timeout = os.startTimer(1)

    modem.transmit(0, 0, pingmsg)
    while true do
        local event, side, chan, reply, msg = os.pullEvent()
        if event == "modem_message" and chan == toid and type(msg) == "table" and msg.origin and msg.senderChannel == 0 and msg.replyChannel == 0 and msg.dimension and type(msg.message) == "string" and msg.message == pingmsg then
            origins[#origins+1] = msg.origin
        elseif event == "timer" and side == timeout then break
        end
    end

    if #origins == 0 then return false
    elseif #origins == 1 then return origins[1]
    end

    local sel = 1
    while true do
        term.clear()
        term.setCursorPos(1,1)
        term.write("Select a caster")
        for i = 1, #origins do
            term.setCursorPos(1, i+2)
            term.write(("%s%s%s"):format(i==sel and "[" or " ", origins[i], i==sel and "]" or " "))
        end
        local event, char = os.pullEvent("key")
        if char == keys.up then sel = sel-1
        elseif char == keys.down then sel = sel+1
        elseif char == keys.enter then break
        end
        sel = math.max(1, math.min(#origins, sel))
    end
    return origins[sel]
end

term.setTextColor(colors.yellow)
if not origin then
    origin = select_caster()
    if not origin then
        error("No casters found", 0)
    end
end

term.clear()
term.setCursorPos(1, 3)
while true do
    printHeader("Connected to", 1)
    printHeader(origin, 2)
    local event, side, chan, reply, msg = os.pullEvent()
    if event == "modem_message" then
        if chan == toid and type(msg) == "table" and msg.origin == origin and msg.senderChannel and msg.replyChannel and msg.message and msg.dimension then
            os.queueEvent("global_message", msg.senderChannel, msg.replyChannel, msg.message, msg.senderDistance)
        end
    elseif event == "key" then
        if side == keys.q then sleep(0) return
        elseif side == keys.c then
            origin = select_caster() or origin
            term.clear()
            term.setCursorPos(1, 3)
        end
    end
end