
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 sCommand = tArgs[1]
if sCommand == "locate" then
    -- "gps locate"
    -- Just locate this computer (this will print the results)
    gps.locate( 2, true )
    
elseif sCommand == "host" then
    -- "gps host"
    -- Act as a GPS host
    if pocket then
        print( "GPS Hosts must be stationary" )
        return
    end

    -- Find a modem
    local sModemSide = nil
    for n,sSide in ipairs( rs.getSides() ) do
        if peripheral.getType( sSide ) == "modem" and peripheral.call( sSide, "isWireless" ) then    
            sModemSide = sSide
            break
        end
    end

    if sModemSide == nil then
        print( "No wireless modems found. 1 required." )
        return
    end

    -- Determine position
    local x,y,z
    if #tArgs >= 4 then
        -- Position is manually specified
        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
        -- Position is to be determined using locate        
        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
    
    -- Open a channel
    local modem = peripheral.wrap( sModemSide )
    print( "Opening channel on modem "..sModemSide )
    modem.open( gps.CHANNEL_GPS )

    -- Serve requests indefinately
    local nServed = 0
    while true do
        local e, p1, p2, p3, p4, p5 = os.pullEvent( "modem_message" )
        if e == "modem_message" then
            -- We received a message from a modem
            local sSide, sChannel, sReplyChannel, sMessage, nDistance = p1, p2, p3, p4, p5
            if sSide == sModemSide and sChannel == gps.CHANNEL_GPS and sMessage == "PING" and nDistance then
                -- We received a ping message on the GPS channel, send a response
                modem.transmit( sReplyChannel, gps.CHANNEL_GPS, { x, y, z } )
            
                -- Print the number of requests handled
                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
    -- "gps somethingelse"
    -- Error
    printUsage()
    
end
