face_north

function getHeading()
  local startPos = vector.new(gps.locate(3))
  turtle.refuel()

  for i=0,2,1 do
    local endPos = tryToMoveForward()
    if endPos then
      return getDirectionMoved(endPos - startPos)
    end
    turtle.up()
  end
  error('I have failed to get my bearings.')
end

local function tryToMoveForward()
  for i=0,3,1 do
    if turtle.forward() then
      local endPos = vector.new(gps.locate(3))
      turtle.back()
      return endPos
    end

    turtle.turnRight()
    sleep(1)
  end

  return false
end

local function getDirectionMoved(diff)
  if diff.z < 0 then
    return 0
  elseif diff.x > 0 then
    return 1
  elseif diff.z > 0 then
    return 2
  elseif diff.x < 0
    then return 3
  end
  error('diff of '.. diff.x..', '..diff.z..' indeterminate.  I tried.')
end