chunker

local function copyVec(vec)
    return vector.new(vec.x, vec.y, vec.z)
end
local function updateX(pos, update)
    return vector.new(update(pos.x), pos.y, pos.z)
end
local function updateZ(pos, update)
    return vector.new(pos.x, pos.y, update(pos.z))
end
local function updateY(pos, update)
    return vector.new(pos.x, update(pos.y), pos.z)
end
local function incr(x)
    return x + 1
end
local function decr(x)
    return x - 1
end
local function eqVec(f, s)
    return f.x == s.x and f.y == s.y and f.z == s.z
end

local N = 0
local S = 1
local W = 2
local E = 3

local function oppositeFace(f)
    if f == N then return S end
    if f == S then return N end
    if f == W then return E end
    if f == E then return W end
end

local function faceTo(c, t)
    if (c == N and t == E) or (c == E and t == S) or (c == S and t == W) or (c == W and t == N) then
        turtle.turnRight()
    elseif (c == N and t == W) or (c == W and t == S) or (c == S and t == E) or (c == E and t == N) then
        turtle.turnLeft()
    elseif (c == N and t == S) or (c == S and t == N) or (c == W and t == E) or (c == E and t == W) then
        turtle.turnRight()
        turtle.turnRight()
    end
end

local function newMove(m, l, f)
    return {move = m, loc = l, face = f}
end

local RTurn = 0
local LTurn = 1
local FwdMv = 2
local DwnMv = 3

local function tern(x, y, z)
    if x then return y else return z end
end

local function next(pos, face)
    local x = pos.x
    local y = pos.y
    local z = pos.z
    local r2l = y % 2 == 0
    local evenRow = z % 2 == 0
    local d2t = tern(r2l, evenRow, not evenRow)
    local rowEnd = tern(d2t, x == 15, x == 0)
    local lastRow = tern(r2l, z == 15, z == 0)
    local chunkEnd = rowEnd and lastRow
    if chunkEnd then
        local loc = updateY(pos, decr)
        return newMove(DwnMv, loc, oppositeFace(face))
    elseif rowEnd then
        local turn = tern(d2t ~= r2l, RTurn, LTurn)
        local update = tern(r2l, incr, decr)
        local loc = updateZ(pos, update)
        return newMove(turn, loc, oppositeFace(face))
    else
        local update = tern(d2t, incr, decr)
        local loc = updateX(pos, update)
        return newMove(FwdMv, loc, face)
    end
end

local function newStep(d, f, l)
    return {dir = d, face = f, loc = l}
end

local StepUp = 0
local StepDwn = 1
local StepFwd = 2
local Arrived = 3

local function pathTo(curr, face, trgt)

    local function flat()
        if curr.x ~= trgt.x then
            if curr.x < trgt.x then
                return newStep(StepFwd, N, updateX(curr, incr))
            else
                return newStep(StepFwd, S, updateX(curr, decr))
            end
        elseif curr.z ~= trgt.z then
            if curr.z < trgt.z then
                return newStep(StepFwd, W, updateZ(curr, incr))
            else
                return newStep(StepFwd, E, updateZ(curr, decr))
            end
        else return Arrived end
    end

    if curr.y < trgt.y then
        return newStep(StepUp, face, updateY(curr, incr))
    elseif curr.y > trgt.y then
        local res = flat()
        if res == Arrived then
            return newStep(StepDwn, face, updateY(curr, decr))
        else return res end
    else return flat() end
end

local function tryRefuel(furthest)
    local x = math.abs(furthest.x)
    local y = math.abs(furthest.y)
    local z = math.abs(furthest.z)
    local path = x + y + z
    local cost = path * 2
    local enough = false
    local level = turtle.getFuelLevel()
    if cost > level then
        local consume = turtle.getItemCount(1) - 1
        if consume > 0 then
            turtle.select(1)
            turtle.refuel(consume)
            enough = cost < turtle.getFuelLevel()
        end
    else enough = true end
    return enough
end

local function destination()
    local absolute = vector.new(gps.locate())
    if absolute.y < -57 then
        error("position is not vaild")
    else
        local y = -(absolute.y + 57)
        return vector.new(16, y, 16)
    end
end

local finish = destination()
local start = vector.new(0, 0, 0)
local location = vector.new(0, 0, 0)
local face = N

local function moveTo(target)
    local loop = true
    while loop do
        local step = pathTo(location, face, target)
        if step == Arrived then
            loop = false
        else
            location = step.loc
            faceTo(face, step.face)
            face = step.face
            local direction = step.dir
            if direction == StepUp then
                turtle.up()
            elseif direction == StepDwn then
                turtle.down()
            elseif direction == StepFwd then
                turtle.forward()
            end
        end
    end
end

local function unload()
    moveTo(start)
    faceTo(face, S)
    face = S
    for i=2,16 do
        turtle.select(i)
        turtle.drop()
    end
end

if not tryRefuel(finish) then
    error("not enough fuel")
end
while not eqVec(location, finish) do
    local nextMove = next(location, face)
    location = nextMove.loc
    face = nextMove.face
    local move = nextMove.move
    if move == RTurn then
        turtle.turnRight()
        turtle.dig()
        turtle.forward()
        turtle.turnRight()
    elseif move == LTurn then
        turtle.turnLeft()
        turtle.dig()
        turtle.forward()
        turtle.turnLeft()
    elseif move == FwdMv then
        turtle.dig()
        turtle.forward()
    elseif move == DwnMv then
        turtle.digDown()
        turtle.down()
        turtle.turnLeft()
        turtle.turnLeft()
    end
    tryRefuel(location)
    if turtle.getItemCount(16) > 0 then
        local contPos = copyVec(location)
        local contFace = face
        unload()
        turtle.select(1)
        moveTo(contPos)
        faceTo(face, contFace)
        face = contFace
    end
end
unload()