Make sure GPS host positions are not nil
function locate( _nTimeout, _bDebug )
-- Let command computers use their magic fourth-wall-breaking special abilities
if commands then
return commands.getBlockPosition()
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
if _bDebug then
print( "No wireless modem attached" )
end
return nil
end
if _bDebug then
print( "Finding position..." )
end
-- Open a channel
local modem = peripheral.wrap( sModemSide )
local bCloseChannel = false
if not modem.isOpen( os.getComputerID() ) then
modem.open( os.getComputerID() )
bCloseChannel = true
end
-- Send a ping to listening GPS hosts
modem.transmit( CHANNEL_GPS, os.getComputerID(), "PING" )
-- Wait for the responses
local tFixes = {}
local pos1, pos2 = nil, nil
sBadPositionError = "GPS replied with bad position, check your GPS hosts."
local timeout = os.startTimer( _nTimeout or 2 )
while true do
local e, p1, p2, p3, p4, p5 = os.pullEvent()
if e == "modem_message" then
-- We received a reply from a modem
local sSide, sChannel, sReplyChannel, tMessage, nDistance = p1, p2, p3, p4, p5
if sSide == sModemSide and sChannel == os.getComputerID() and sReplyChannel == CHANNEL_GPS and nDistance then
-- Received the correct message from the correct modem: use it to determine position
if type(tMessage) == "table" and #tMessage == 3 then
local tFix = { vPosition = vector.new( assert(tMessage[1], sBadPositionError), assert(tMessage[2], sBadPositionError),assert(tMessage[3], sBadPositionError) ), nDistance = nDistance }
if _bDebug then
print( tFix.nDistance.." metres from "..tostring( tFix.vPosition ) )
end
if tFix.nDistance == 0 then
pos1, pos2 = tFix.vPosition, nil
else
table.insert( tFixes, tFix )
if #tFixes >= 3 then
if not pos1 then
pos1, pos2 = trilaterate( tFixes[1], tFixes[2], tFixes[#tFixes] )
else
pos1, pos2 = narrow( pos1, pos2, tFixes[#tFixes] )
end
end
end
if pos1 and not pos2 then
break
end
end
end
elseif e == "timer" then
-- We received a timeout
local timer = p1
if timer == timeout then
break
end
end
end
-- Close the channel, if we opened one
if bCloseChannel then
modem.close( os.getComputerID() )
end
-- Return the response
if pos1 and pos2 then
if _bDebug then
print( "Ambiguous position" )
print( "Could be "..pos1.x..","..pos1.y..","..pos1.z.." or "..pos2.x..","..pos2.y..","..pos2.z )
end
return nil
elseif pos1 then
if _bDebug then
print( "Position is "..pos1.x..","..pos1.y..","..pos1.z )
end
return pos1.x, pos1.y, pos1.z
else
if _bDebug then
print( "Could not determine position" )
end
return nil
end
end