GPS for turtles

local function printUsage()
    print( "Usages:" )
    print( "gps host" )
    print( "gps host <x> <y> <z>" )
    print( "gps locate" )
end

local tArgs = { ... }
if #tArgs < 1 then
    printUsage()
    return
end

local function readNumber()
    local num = nil
    while num == nil do
        num = tonumber(read())
        if not num then
            write( "Not a number. Try again: " )
        end
    end
    return math.floor( num + 0.5 )
end

local function open()
    local bOpen, sFreeSide = false, nil
    for n,sSide in pairs(rs.getSides()) do
        if peripheral.getType( sSide ) == "modem" then
            sFreeSide = sSide
            if rednet.isOpen( sSide ) then
                bOpen = true
                break
            end
        end
    end

    if not bOpen then
        if sFreeSide then
            print( "No modem active. Opening "..sFreeSide.." modem" )
            rednet.open( sFreeSide )
            return true
        else
            print( "No modem attached" )
            return false
        end
    end
    return true
end

local sCommand = tArgs[1]
if sCommand == "locate" then
    if open() then
        gps.locate( 2, true )
    end

elseif sCommand == "host" then
    if open() then
        local x,y,z
        if #tArgs >= 4 then
            x = tonumber(tArgs[2])
            y = tonumber(tArgs[3])
            z = tonumber(tArgs[4])
            if x == nil or y == nil or z == nil then
                printUsage()
                return
            end
            print( "Position is "..x..","..y..","..z )
        else
            x,y,z = gps.locate( 2, true )
            if x == nil then
                print( "Run \"gps host <x> <y> <z>\" to set position manually" )
                return
            end
        end

        print( "Serving GPS requests" )

        local nServed = 0
        while true do
            sender,message,distance = rednet.receive()
            if message == "PING" then
                rednet.send(sender, textutils.serialize({x,y,z}))

                nServed = nServed + 1
                if nServed > 1 then
                    local x,y = term.getCursorPos()
                    term.setCursorPos(1,y-1)
                end
                print( nServed.." GPS Requests served" )
            end
        end
    end

else
    printUsage()
    return
end