Cotton Turtle Farmer

local xPos, zPos = 0,0
local xDir, zDir = 0,1
local depth = 0

local function turnLeft()
    turtle.turnLeft()
    xDir, zDir = -zDir, xDir
end

local function turnRight()
    turtle.turnRight()
    xDir, zDir = zDir, -xDir
end

local function tryForwards()

-- TODO:  check fuel levels

   while not turtle.forward() do
      if turtle.detect() then
         --blocked and we're a pacifist so wait
         sleep(0.5)
      end
   end

   xPos = xPos + xDir
   zPos = zPos + zDir
   return true
end

function goTo( x, y, z, xd, zd )
    while depth > y do
        if turtle.up() then
            depth = depth - 1
        elseif turtle.digUp() or turtle.attackUp() then
            collect()
        else
            sleep( 0.5 )
        end
    end

    if xPos > x then
        while xDir ~= -1 do
            turnLeft()
        end
        while xPos > x do
            if turtle.forward() then
                xPos = xPos - 1
            elseif turtle.dig() or turtle.attack() then
                collect()
            else
                sleep( 0.5 )
            end
        end
    elseif xPos < x then
        while xDir ~= 1 do
            turnLeft()
        end
        while xPos < x do
            if turtle.forward() then
                xPos = xPos + 1
            elseif turtle.dig() or turtle.attack() then
                collect()
            else
                sleep( 0.5 )
            end
        end
    end

    if zPos > z then
        while zDir ~= -1 do
            turnLeft()
        end
        while zPos > z do
            if turtle.forward() then
                zPos = zPos - 1
            elseif turtle.dig() or turtle.attack() then
                collect()
            else
                sleep( 0.5 )
            end
        end
    elseif zPos < z then
        while zDir ~= 1 do
            turnLeft()
        end
        while zPos < z do
            if turtle.forward() then
                zPos = zPos + 1
            elseif turtle.dig() or turtle.attack() then
                collect()
            else
                sleep( 0.5 )
            end
        end
    end

    while depth < y do
        if turtle.down() then
            depth = depth + 1
        elseif turtle.digDown() or turtle.attackDown() then
            collect()
        else
            sleep( 0.5 )
        end
    end

    while zDir ~= zd or xDir ~= xd do
        turnLeft()
    end
end

function findSeeds()
  for n=1,16 do
    local slotData = turtle.getItemDetail(n)
    if slotData then
       if (slotData.name == "Natura:barley.seed") then
          turtle.select(n)
       end
    end
  end
end

function inspectCrop()
   if turtle.detectDown() then
     local success, data = turtle.inspectDown()
     if success then
       if ((data.name == "Natura:N Crops") and (data.metadata == 8)) then
          turtle.digDown()
          findSeeds()
          turtle.placeDown()
       end
     end
   end
end

function advanceRow()
  if xPos % 2 == 0 then
    turnRight()
    tryForwards()
    turnRight()
  else
    turnLeft()
    tryForwards()
    turnLeft()
  end
end

function walkRow()
  for n=1,6 do
    tryForwards()
    inspectCrop()
  end
end

function turnAround()
  turnLeft()
  turnLeft()
end

if turtle.up() then
  depth = depth - 1
end


-- currently this only works on my test farm ;-)
for i=1,8 do
  walkRow()
  advanceRow()
  print("My position is: X"..xPos.." Z:"..zPos)
end
goTo(0,0,0,0,-1)
turnAround()