Reverse GPS

local cChannel = 15150

local tArgs = {...}
if #tArgs < 3 then
    printError("Usage: " .. shell.getRunningProgram() .. " <x> <y> <z>")
    return
end

local coords = {tonumber(tArgs[1]) or 0, tonumber(tArgs[2]) or 0, tonumber(tArgs[3]) or 0}

local function openChannel(channel)
    for i,v in pairs(peripheral.getNames()) do
        if peripheral.getType(v) == "modem" then
            peripheral.call(v, "open", channel)
        end
    end
end

local function closeChannel(channel)
    for i,v in pairs(peripheral.getNames()) do
        if peripheral.getType(v) == "modem" then
            peripheral.call(v, "close", channel)
        end
    end
end

local function closeAll()
    for i,v in pairs(peripheral.getNames()) do
        if peripheral.getType(v) == "modem" then
            peripheral.call(v, "closeAll")
        end
    end
end

local function transmit(channel, reply, message)
    local modem = peripheral.find("modem")
    if modem then
        modem.transmit(channel, reply, message)
    end
end

local channels = {}

openChannel(cChannel)

local lastMessages = {}

print("Hosting reverse GPS at (" .. coords[1] .. ", " .. coords[2] .. ", " .. coords[3] .. ")")

local running = true
while running do
    local success, err = pcall(function()
        local event, side, channel, reply, message, distance = os.pullEventRaw()
        if event == "modem_message" then
            if channel == cChannel and type(message) == "table" then
                if reply == 0 then
                    if message.openChannel then
                        openChannel(message.channel)
                        table.insert(channels, message.channel)
                        print("Opened channel " .. message.channel)
                    elseif message.closeChannel then
                        closeChannel(message.channel)
                        local toRemove = {}
                        for _,channel in ipairs(channels) do
                            if channel == message.channel then
                                table.insert(toRemove, 1, _)
                            end
                        end
                        for _,i in ipairs(toRemove) do
                            table.remove(channels, i)
                        end
                        print("Closed channel " .. message.channel)
                    elseif message.closeAll then
                        closeAll()
                        channels = {}
                        openChannel(cChannel)
                        print("Closed all channels")
                    elseif message.update then
                        local file = fs.open(shell.getRunningProgram(), "w")
                        file.write(message.source)
                        file.close()
                        os.reboot()
                    elseif message.discard then
                        local last = lastMessages[message.channel]
                        if last then
                            last = last[message.reply]
                            if last and #last ~= 0 then
                                table.remove(last, 1)
                            end
                        end
                    elseif message.getDistance then
                        local last = lastMessages[message.channel]
                        if last then
                            last = last[message.reply]
                            if last and #last ~= 0 then
                                table.insert(last[1], message.id)
                                transmit(cChannel, 1, last[1])
                                table.remove(last, 1)
                                print("Sent gps data")
                            end
                        end
                    elseif message.clear then
                        lastMessages = {}
                        print("Cleared messages")
                    end
                end
            else
                if not lastMessages[channel] then
                    lastMessages[channel] = {}
                end
                if not lastMessages[channel][reply] then
                    lastMessages[channel][reply] = {}
                end
                table.insert(lastMessages[channel][reply], {coords, distance})
            end
        elseif event == "key" then
            if side == keys.q then
                os.pullEvent("char")
                running = false
            end
        elseif event == "peripheral" then
            if peripheral.getType(side) == "modem" then
                for _,channel in pairs(channels) do
                    peripheral.call(side, "open", channel)
                end
                peripheral.call(side, "open", cChannel)
            end
        end
    end)

    if not success then
        print("Error: " .. err)
    end
end