-- Do not lose state when reset (chunkloading etc)
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)()

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

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

return {
    state = state,

    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)
        else
            state.z = state.z + ((state.o == 2) and -1 or 1)
        end
    end),

    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))
        else
            state.z = state.z + ((state.o == 2) and 1 or -1)
        end
    end),

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

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

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

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