Farmer ComputerCraft

_G["xPos"] = 0
_G["yPos"] = 0
_G["dir"] = 0
_G["xSize"] = 0
_G["ySize"] = 0
_G["requiredFuel"] = 0
_G["farmTime"] = 0

args = {...}

if args[0] == nil then
    xSize = 9
    ySize = 9
    farmTime = 60
elseif args[0] ~= nil then
    if #args > 1 then
        xSize = tonumber(args[1])
        ySize = tonumber(args[2])
        farmTime = tonumber(args[3])
        if farmTime == nil then
            farmTime = 60
        end
        if xSize == nil or ySize == nil then
            print("Usage: "..shell.getRunningProgram().." [<xSize> <ySize>] [secondsToWait]")
            return
        end
    else
        farmTime = tonumber(args[1])
        if farmTime == nil then
            print("Usage: "..shell.getRunningProgram().." [secondsToWait]")
        end
    end
end

requiredFuel = xSize * ySize + xSize + ySize

function moveForward()
    while not turtle.forward() do
        sleep(1)
    end
    if dir == 0 then
        yPos = yPos + 1
    elseif dir == 1 then
        xPos = xPos + 1
    elseif dir == 2 then
        yPos = yPos - 1
    elseif dir == 3 then
        xPos = xPos - 1
    end
end

function moveBack()
    while not turtle.back() do
        sleep(1)
    end
    if dir == 0 then
        yPos = yPos - 1
    elseif dir == 1 then
        xPos = xPos - 1
    elseif dir == 2 then
        yPos = yPos + 1
    elseif dir == 3 then
        xPos = xPos + 1
    end
end

function turnLeft()
    turtle.turnLeft()
    dir = (dir + 1) % 4
end
function turnRight()
    turtle.turnRight()
    dir = (dir - 1) % 4
end
function faceDirection(n)
    while dir ~= n do
        if dir < n then
            if math.abs(dir - n) < 2 then
                turnLeft()
            else
                turnRight()
            end
        else
            if math.abs(dir - n) < 2 then
                turnRight()
            else
                turnLeft()
            end
        end
    end
end
function returnHome()
    faceDirection(2)
    for i = 1,yPos do
        turtle.forward()
    end
    faceDirection(3)
    for i = 1,xPos do
        turtle.forward()
    end
    faceDirection(0)
    xPos = 0
    yPos = 0
end

function dumpItems()
    for i = 1,16 do
        if turtle.getItemCount(i) > 0 then
            if string.find(turtle.getItemDetail(i).name, "charcoal") == nil then
                turtle.select(i)
                turtle.dropDown()
            end
        end
    end
end
function placeCrop()
    for i = 1,16 do
        if turtle.getItemCount(i) > 0 then
            if string.find(turtle.getItemDetail(i).name,"seed") then
                turtle.select(i)
                turtle.placeDown()
                break
            end
        end
    end
end
function harvestCrop()
    found, block = turtle.inspectDown()
    if found then
        if block["state"]["age"] then
            if block["state"]["age"] == 7 then
                turtle.digDown()
                placeCrop()
            end
        end
    end
end
function awaitFuel()
    rs.setOutput("top", true)
    for i = 1,16 do
        turtle.select(i)
        turtle.refuel()
        if turtle.getFuelLevel() >= requiredFuel then
            break
        end
    end
    rs.setOutput("top", false)
end

while true do
    if turtle.getFuelLevel() >= requiredFuel then
        moveForward()
        harvestCrop()
        for x = 1,9 do
            for y = 1,8 do
                moveForward()
                harvestCrop()
            end
            if x ~= 9 then
                faceDirection(1)
                moveForward()
                harvestCrop()
                if x % 2 == 0 then
                    faceDirection(0)
                else
                    faceDirection(2)
                end
            end
        end
        returnHome()

        parallel.waitForAll(
            function()
                sleep(farmTime)
            end,
            function()
                dumpItems()
            end)
        else
        awaitFuel()
    end
end