-- Do not lose state when reset (chunkloading etc)
local state = (function ()
    local path = ".state"
    local state_table = {}
    
    if fs.exists(path) then
        local f = fs.open(path, "r")
        state_table = textutils.unserialize(f.readAll())
        f.close()
    end
    
    return setmetatable({}, {
        __index = state_table,

        __newindex = function(self, k, v)
            rawset(state_table, k, v)
            local f = fs.open(path, "w")
            f.write(textutils.serialize(state_table))
            f.close()
        end,
    })
end)()

-- Not sure whether this is a good idea
--[[
if state.x == nil or state.y == nil or state.z == nil or state.o == nil then
    printError("[lib] position is not set")
    printError("[lib] defaulting to 0, 0, 0, +z")
    state.x = 0
    state.y = 0
    state.z = 0
    state.o = 0
end
]]

local function setPosition(x, y, z, direction)
    local o = ({
        ["+z"] = 0,
        ["-x"] = 1,
        ["-z"] = 2,
        ["+x"] = 3,
    })[direction]

    state.x = x
    state.y = y
    state.z = z
    state.o = o
end

--[[
    orientation:
    0 = South (+z)
    1 = West  (-x)
    2 = North (-z)
    3 = East  (+x)
]]

local function move(raw_move, position_update)
    return function()
        success, err = raw_move()
        if not success then
            return false, err
        end
        position_update()
        return true
    end
end

local fwd = move(turtle.forward, function()
    if state.o == 1 or state.o == 3 then
        state.x = state.x + ((state.o == 1) and -1 or 1)
    elseif state.o == 0 or state.o == 2 then
        state.z = state.z + ((state.o == 2) and -1 or 1)
    end
end)

local back = move(turtle.back, function()
    if state.o == 1 or state.o == 3 then
        state.x = state.x + ((state.o == 1 and 1 or -1))
    elseif state.o == 0 or state.o == 2 then
        state.z = state.z + ((state.o == 2) and 1 or -1)
    end
end)

local up = move(turtle.up, function()
    state.y = state.y + 1
end)

local down = move(turtle.down, function()
    state.y = state.y - 1
end)

local left = move(turtle.turnLeft, function()
    state.o = (state.o - 1) % 4
end)

local right = move(turtle.turnRight, function()
    state.o = (state.o + 1) % 4
end)

local function getOrientation()
    return ({"+z", "-x", "-z", "+x"})[1 + state.o]
end

local function rotateTowards(direction)
    local target_o = ({
        ["+z"] = 0,
        ["-x"] = 1,
        ["-z"] = 2,
        ["+x"] = 3,
    })[direction]

    if not target_o then
        error(tostring(direction) .. " is not a valid direction.")
    end

    local delta_o = (target_o - state.o) % 4

    if delta_o == 0 then
        -- orientation already correct
    elseif delta_o == 1 then
        right()
    elseif delta_o == 2 then
        right()
        right()
    elseif delta_o == 3 then
        left()
    end
end


return {
    state = state,

    fwd = fwd,
    back = back,
    up = up,
    down = down,
    left = left,
    right = right,

    setPosition = setPosition,
    getOrientation = getOrientation,
    rotateTowards = rotateTowards,
}