Enderchest Melon Farm

--By xXxMoNkEyMaNxXx
--Slot 1 is for a fuel supply enderchest
--Slot 2 is for a melon unloading enderchest
local ceil=math.ceil
local max=math.max
local min=math.min
local farm_len=...
local s=3
turtle.select(3)
local function unload(slot)
        turtle.select(slot)
        if (not turtle.detect() or turtle.dig()) and turtle.place() then
                for i=1,16 do
                        if i~=slot and turtle.getItemCount(i)>0 then
                                turtle.select(i)
                                turtle.drop()
                        end
                end
                turtle.select(slot)
                turtle.dig()
        end
        turtle.select(s)
end

local function getRealFuel()
        local f=turtle.getFuelLevel()
        return type(f)=="number" and f or math.huge
end

local function fillTo(slot,req)
        turtle.select(slot)
        local ret
        if (not turtle.detect() or turtle.dig()) and turtle.place() then
                local i=0
                local f_0=getRealFuel()
                print("Fuel: "..f_0)
                print("Required: "..req)
                if f_0<req then
                        print'Refuelling...'
                        local t=1
                        while turtle.getItemCount(t)>0 and t<=16 do
                                t=t+1
                        end
                        if t<=16 and turtle.select(t) then
                                while getRealFuel()<req and turtle.suck() do
                                        local f0=getRealFuel()
                                        local i0=turtle.getItemSpace(t)
                                        assert(turtle.refuel(1),"That's not fuel!")
                                        local f1=getRealFuel()
                                        local i1=turtle.getItemSpace(t)

                                        local df,di=f1-f0,i1-i0
                                        local rate=df/di
                                        local f_req=ceil((req-f1)/rate)

                                        local c=turtle.getItemCount(t)
                                        if c>0 then
                                                assert(turtle.refuel(min(max(0,f_req),c)),"That's not fuel!")
                                        end
                                        i=i+turtle.getItemSpace(t)-i0
                                        local c=turtle.getItemCount(t)
                                        if c>0 then
                                                if turtle.drop(c) then
                                                        break
                                                else
                                                        turtle.turnRight()
                                                        turtle.turnRight()
                                                        turtle.drop(c)
                                                        turtle.turnLeft()
                                                        turtle.turnLeft()
                                                end
                                        end
                                end
                                turtle.select(s)
                                local f_1=getRealFuel()
                                print("Gained "..f_1-f_0.." fuel from "..i.." fuel items.")
                                ret=f_1>=req
                        else
                                print'Unable to refuel.\n(No room in the turtle)'
                                ret=false
                        end
                else
                        ret=true
                end
                turtle.select(slot)
                turtle.dig()
        end
        turtle.select(s)
        return ret
end
turtle.turnRight()
local runs=0
local fuel=fillTo(2*(farm_len or 2^runs)+2,1)
while fuel do
        turtle.turnLeft()
        while farm_len and not turtle.detect() do sleep(1) end
        local distance=0
        while farm_len and distance<farm_len or turtle.detectDown() and distance+3<getRealFuel() do
                if turtle.getItemSpace(s)>0 then
                        if turtle.detect() then
                                turtle.dig()
                        end
                        if turtle.forward() then
                                distance=distance+1
                        end
                elseif s==15 then
                        unload(2)
                        s=2
                        turtle.select(2)
                else
                        s=s+1
                        turtle.select(s)
                end
        end
        if not turtle.detectDown() then
                farm_len=distance-1
        end
        turtle.turnLeft()
        turtle.forward()
        turtle.turnLeft()
        local d=distance
        while d>0 do
                if turtle.getItemSpace(s)>0 then
                        if turtle.detect() then
                                turtle.dig()
                        end
                        if turtle.forward() then
                                d=d-1
                        end
                elseif s==15 then
                        unload(2)
                        s=2
                        turtle.select(2)
                else
                        s=s+1
                        turtle.select(s)
                end
        end
        turtle.turnLeft()
        turtle.forward()
        runs=runs+1
        if farm_len then
                fuel=fillTo(2*farm_len+2,1)
        else
                fuel=fillTo(2*2^runs+2,1)
        end
end
turtle.turnLeft()
print'Out of fuel.'