
CHANNEL_GPS = 65534

local function trilaterate( A, B, C )
	local a2b = B.vPosition - A.vPosition
	local a2c = C.vPosition - A.vPosition
		
	if math.abs( a2b:normalize():dot( a2c:normalize() ) ) > 0.999 then
		return nil
	end
	
	local d = a2b:length()
	local ex = a2b:normalize( )
	local i = ex:dot( a2c )
	local ey = (a2c - (ex * i)):normalize()
	local j = ey:dot( a2c )
	local ez = ex:cross( ey )

	local r1 = A.nDistance
	local r2 = B.nDistance
	local r3 = C.nDistance
		
	local x = (r1*r1 - r2*r2 + d*d) / (2*d)
	local y = (r1*r1 - r3*r3 - x*x + (x-i)*(x-i) + j*j) / (2*j)
		
	local result = A.vPosition + (ex * x) + (ey * y)

	local zSquared = r1*r1 - x*x - y*y
	if zSquared > 0 then
		local z = math.sqrt( zSquared )
		local result1 = result + (ez * z)
		local result2 = result - (ez * z)
		
		local rounded1, rounded2 = result1:round( 0.01 ), result2:round( 0.01 )
		if rounded1.x ~= rounded2.x or rounded1.y ~= rounded2.y or rounded1.z ~= rounded2.z then
			return rounded1, rounded2
		else
			return rounded1
		end
	end
	return result:round( 0.01 )
	
end

local function narrow( p1, p2, fix )
	local dist1 = math.abs( (p1 - fix.vPosition):length() - fix.nDistance )
	local dist2 = math.abs( (p2 - fix.vPosition):length() - fix.nDistance )
	
	if math.abs(dist1 - dist2) < 0.01 then
		return p1, p2
	elseif dist1 < dist2 then
		return p1:round( 0.01 )
	else
		return p2:round( 0.01 )
	end
end

function locate( _nTimeout, _bDebug )
    if _nTimeout ~= nil and type( _nTimeout ) ~= "number" then
        error( "bad argument #1 (expected number, got " .. type( _nTimeout ) .. ")", 2 ) 
    end
    if _bDebug ~= nil and type( _bDebug ) ~= "boolean" then
        error( "bad argument #2 (expected boolean, got " .. type( _bDebug) .. ")", 2 ) 
    end
    -- 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
	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 and tonumber(tMessage[1]) and tonumber(tMessage[2]) and tonumber(tMessage[3]) then
					local tFix = { vPosition = vector.new( tMessage[1], tMessage[2], tMessage[3] ), 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
