local direction = 0

local function GetXFromDirection()
        if              (direction == 1)                 then    return -1
        elseif   (direction == 3)               then     return 1
        else                                                                     return 0
        end
end

local function GetZFromDirection()
        if              (direction == 0)                 then    return 1
        elseif   (direction == 2)               then     return -1
        else                                                                     return 0
        end
end

local x = 0
local y = 0
local z = 0

-- Save direction and coordinates to file.
local function Store()
        local h = fs.open("location", "w")
        h.writeLine(x)
        h.writeLine(y)
        h.writeLine(z)
        h.writeLine(direction)
        h.close()
end

-- Reads direction and coordinates from file.
local function Read()
        if fs.exists("location") then
                local h = fs.open("location", "r")
                x                       = tonumber(h.readLine())
                y                       = tonumber(h.readLine())
                z                       = tonumber(h.readLine())
                direction       = tonumber(h.readLine())
                h.close()

                return true
        end

        return false
end

function moveBackOnce()
        -- Try and move.
        moved = turtle.back()
        -- If move success increment coordinates.
        if(moved) then
                x = x - GetXFromDirection()
                z = z - GetZFromDirection()

                Store()
        end

        return moved
end

function moveForwardOnce()
        -- Try and move.
        moved = turtle.forward()
        -- If move success increment coordinates.
        if(moved) then
                x = x + GetXFromDirection()
                z = z + GetZFromDirection()

                Store()
        end
                return moved
end

-- Turn Right.
local function TurnLeftOnce()
        if(direction > 0) then
                direction = direction - 1
        else
                direction = 3
        end

        turtle.turnLeft()

        Store()
end

-- Turn Left.
local function TurnRightOnce()
        if(direction < 3) then
                direction = direction + 1
        else
                direction = 0
        end

        turtle.turnRight()

        Store()
end

-- Move up.
local function moveUpOnce()
        moved = turtle.up()

        if(moved) then
                y = y + 1

                Store()
        end

        return moved
end

-- Move down.
local function moveDownOnce()
        moved = turtle.down()

        if(moved) then
                y = y - 1

                Store()
        end

        return moved
end

-- =======================================================================================================================================================
-- Public API
-- =======================================================================================================================================================
-- Initalization:
-- =======================================================================================================================================================
-- Sets the position and heading of the turtle. Use for initialization.
-- FACING:
--[[
        0 = South
        1 = West
        2 = North
        3 = East
--]]
function SetPosition(X, Y, Z, FACING)
        x                       = X
        y                       = Y
        z                       = Z
        direction       = FACING
        Store()
end

-- Initializes the navigator from file backed storage.
function InitializeNavigator()
        return Read()
end

-- Prints the direction enumeration in a user readable format.
function PrintDirections()
        print("0 = South")
        print("1 = West")
        print("2 = North")
        print("3 = East")
end

-- Takes N, S, E, W and converts it to numbers.
function GetDirectionINT(dir)
        dir = string.lower(dir)

        if(dir == "s") or (dir == "south")       then return 0 end
        if(dir == "n") or (dir == "north")      then return 2 end
        if(dir == "e") or (dir == "east")         then return 3 end
        if(dir == "w") or (dir == "west")        then return 1 end
end

-- Returns the current position of the turtle.
function GetPosition()
        Read()
        return x, y, z, direction
end
-- =======================================================================================================================================================
-- Rotation:
-- =======================================================================================================================================================

-- Turns left.
-- Goal (optinal): The target heading to turn toward.
-- Returns if movement succesfull.
function TurnLeft( goal )
        goal = goal or "T"

        if(goal == "T") then
                TurnLeftOnce()
        else
                while (direction ~= goal) do
                        TurnLeftOnce()
                end
        end
end

-- Turns right.
-- Goal (optinal): The target heading to turn toward.
-- Returns if movement succesfull.
function TurnRight( goal )
        goal = goal or "T"
        --print(goal .. " : " .. direction)

        if(goal == "T") then
                TurnRightOnce()
        else
                while (direction ~= goal) do
                        --print(goal .. " : " .. direction)
                        TurnRightOnce()
                end
        end
end
-- =======================================================================================================================================================
-- Moving:
-- =======================================================================================================================================================
local function Navigate(mainMovementFunction, resetMovementFunction, goal)
        goal = goal or "T"
        steps = 0

        -- If no steps required return true.
        if(goal == 0) then return true end

        -- If only a single step is required than do it.
        if(goal == "T") then return mainMovementFunction() end

        -- Try and move to the target position
        moved = false
        while goal > steps do
                moved = mainMovementFunction()

                if(moved == true) then
                        steps = steps + 1
                else
                        -- If cant move reset
                        while steps > 0 do
                                resetMovementFunction()
                                steps = steps - 1
                        end
                        -- And break out of the movement loop
                        return false
                end
        end

        return true
end

-- Moves Up and returns if succesfull.
-- Goal (optinal): Number of steps taken. If movement is obstructed turtle will reset to its starting position.
-- Returns if movement succesfull.
function Up(goal)
        return Navigate(moveUpOnce, moveDownOnce, goal)
end

-- Moves Down and returns if succesfull.
-- Goal (optinal): Number of steps taken. If movement is obstructed turtle will reset to its starting position.
-- Returns if movement succesfull.
function Down(goal)
        return Navigate(moveDownOnce, moveUpOnce, goal)
end

-- Moves back and returns if succesfull.
-- Goal (optinal): Number of steps taken. If movement is obstructed turtle will reset to its starting position.
-- Returns if movement succesfull.
function Back(goal)
        return Navigate(moveBackOnce, moveForwardOnce, goal)
end

-- Moves forward and returns if succesfull.
-- Goal (optinal): Number of steps taken. If movement is obstructed turtle will reset to its starting position.
-- Returns if movement succesfull.
function Forward(goal)
        return Navigate(moveForwardOnce, moveBackOnce, goal)
end

-- Initialize Navigator
if (NAVIGATOR.InitializeNavigator()) then
   print("Navigator file found, reading position.")
else
   print("Program running first time. Setting initial position.")
   NAVIGATOR.SetPosition(x, y, z, direction)
end