local function ensure_equipped(ptype, item)
	local cur = peripheral.getType"left"
	print(cur)
	if cur == ptype then return true end
	for i = 1,16 do
		local det = turtle.getItemDetail(i)
		if det and det.name == item then
			turtle.select(i)
			return turtle.equipLeft()
		end
	end
	return false, "couldn't find "..item
end

local function ensure_modem() return ensure_equipped("modem","computercraft:wireless_modem_normal") end
local function ensure_scanner() return error("aaa") end

assert(ensure_modem())

local POS = vector.new(gps.locate())
local function get_facing()
	while turtle.detect() do turtle.turnRight() end
	turtle.forward()
	local newpos = vector.new(gps.locate())
	turtle.back()
	return newpos - POS
end
local FACING = get_facing()

local function moveby_y(delta)
	local fn = turtle.up
	if delta < 0 then fn = turtle.down  delta = -delta  end
	for i=1,delta do fn() end
end

local function rotatev(v,nq)
	-- rotates v by n quarters
	nq = nq%4
	if nq == 0     then return vector.new( v.x,  v.y,  v.z)
	elseif nq == 1 then return vector.new(-v.z,  v.y,  v.x)
	elseif nq == 2 then return vector.new(-v.x,  v.y, -v.z)
	elseif nq == 3 then return vector.new( v.z,  v.y, -v.x)
	end
end

local v = vector.new(1,0,2)
for i=0,4 do print(i,rotatev(v,i)) end

local function moveby_xz(dv)
	-- y component of dv is ignored

end




local datum = vector.new(-18, 128, -1837)