-- api

t = turtle

pos = {
	x = 1;
	y = 1;
	z = 29;
	f = 1;
}

-- internal functions

-- returns 1 if positive, -1 if negative, or 0 if 0
local function sign(n)
	return n > 0 and 1 or (n < 0 and -1 or 0)
end

-- recieves case/command pairs in {}s, optional default value
-- ie switch(dir, {"up", "placeUp"}, {"down", "placeDown"}, "place")
local function switch(exp, ...)
	local args = {...}
	local def = table.remove(args)
	for pair, case in ipairs(args) do
		if exp == case[1] then
			return case[2]
		end
	end
	return def
end

local function isint(n)
	return type(n) == "number" and (math.modf(n) == n)
end

-- errors msg if any expression in ... fails
local function check(msg, ...)
	for _, exp in pairs({...}) do
		if not exp then
			error(msg, 2)
		end
	end
end

-- turns left or right f times
-- -f = left, +f = right
-- @return new_f
local function fturn(f)
	for rep = 1, math.abs(f) do
		t[f > 0 and "turnRight" or "turnLeft"]()
		pos.f = switch(pos.f + sign(f), {5, 1}, {0, 4}, pos.f + sign(f))
	end
	return pos.f
end

-- moves x, y, z on each axis
-- @return [true + new_coords or false + error]
local function fmove(x, y, z)
	local axes = {
		x = {x, "forward", "back"};
		y = {y, "forward", "back"};
		z = {z, "up", "down"};
	}
	for axis, data in ipairs(axes) do
		for rep = 1, math.abs(data[1]) do
			if t[sign(data[1]) > 0 and data[2] or data[3]]() then
				pos[axis] = sign(data[1])
			else
				return false, "Failed move "..rep.." of "..math.abs(data[1])
			end
		end
	end
	return true, pos
end

-- external functions

-- turns left or right
-- @return new_f
function turn(d, r)
	check("Expected 'left' or 'right', got "..d, d == "left" or d == "right")
	check("Expected a positive integer, got "..r, type(r) == "number", r > 0, isint(r))
	d = d == "left" and -1 or 1
	r = r % 4 == 3 and -1 or r % 4
	fturn(r * d)
	return pos.f
end

-- sets direction to f
-- @return new_f
function setf(f)
	check("Expected an integer between 1 and 4, got "..f, f > 1, f < 4, isint(f))
	turn(sign(f - pos.f) == 1 and "right" or "left", math.abs(f - pos.f))
	return pos.f
end

-- moves x, y, z on each axis and turns to f
-- @return success + [new_coords or error]
function move(x, y, z, f)
	check("Expected an integer, got "..(isint(x) and x or (isint(y) and y or z), isint(x), isint(y), isint(z)))
	check("Expected an integer between 1 and 4, got "..f, f > 1, f < 4, isint(f))
	local ok, err = fmove(x, y, z)
	setf(f)
	return ok, ok and pos or err
end

-- moves to the specified coords and turns to f
-- @return success + [new_coords or error]
function goto(x, y, z, f)
	check("Expected an integer, got "..(isint(x) and x or (isint(y) and y or z), isint(x), isint(y), isint(z)))
	check("Expected an integer between 1 and 4, got "..f, f > 1, f < 4, isint(f))
	return move(x - pos.x, y - pos.y, z - pos.z, f)
end

-- places the item in slot s in direction d (defaults to front)
-- @return success + remaining_items
function place(s, d)
	check("Expected an integer between 1 and 16, got "..s, type(s) == "number", s >= 1, s <= 16)
	check("Expected 'front', 'up', or 'down', got "..d, d == "front" or d == "up" or d == "down")
	d = switch(d, {"up", "placeUp"}, {"down", "placeDown"}, "place")
	t.select(s)
	return t[d](), t.getItemCount(s)
end

-- resets the coordinates and direction
-- @return old_pos
function reset(x, y, z, f)
	check("Expected an integer, got "..(isint(x) and x or (isint(y) and y or z), isint(x), isint(y), isint(z)))
	check("Expected an integer between 1 and 4, got "..f, f > 1, f < 4, isint(f))
	local old = pos
	pos.x = x
	pos.y = y
	pos.z = z
	pos.f = f
	return old
end