module Selector
( Selector (..)
, Coordinate
, Direction (..)

, initSelector
, move
, moveBy
, select
, deselect
, toggleSelection
, getSelected

, moveUp
, moveDown
, moveLeft
, moveRight
) where

----------------------------------------------------------------------
-- Base of a general purpose selector.                              --
-- Can be used to show a selector, move up, down, left and right,   --
-- to 'hold' the currently selected card and remember that held.    -- 
-- card.                                                            --
----------------------------------------------------------------------

----------------------------- Constants ------------------------------

-- A position on the playboard.
type Coordinate = (Int, Int)

-- The direction in which the selector can move.
data Direction = U | D | L | R deriving (Show)

diff = [(0,1), (0,-1), (-1,0), (1,0)]

-- A selector can highlight a coordinate.
data Selector = Selector {
    -- The current position of the selector.
    position :: Coordinate,
    -- The card(s) that the selector currently holds.
    selected :: Maybe Coordinate
} deriving (Show, Eq)

----------------------------------------------------------------------

-- Get the default selector.
initSelector :: Selector
initSelector = Selector {
    position = (0,0),
    selected = Nothing
}

-- Sum two coordinates.
sumCoords :: Coordinate -> Coordinate -> Coordinate
sumCoords (x, y) (a, b) = (x + a, y + b)

-- Move the selector by a given coordinate.
moveBy :: Coordinate -> Selector -> Selector
moveBy c1 s@Selector{ position = c2 } = s{ position = sumCoords c1 c2 }

-- Move the selector one position into the the given direction.
move :: Direction -> Selector -> Selector
move U = moveBy (head diff)
move D = moveBy (diff !! 1)
move L = moveBy (diff !! 2)
move R = moveBy (diff !! 3)

-- Select the current position.
select :: Selector -> Selector
select s@Selector{ position = pos } = s{ selected = Just pos }

-- Deselect the current selection.
deselect :: Selector -> Selector
deselect s = s{ selected = Nothing }

-- Toggle the selection of the selector. Deselect if any position is
-- selected, otherwise select the current position.
toggleSelection :: Selector -> Selector
toggleSelection s@Selector{ selected = Nothing } = select s
toggleSelection s                                = deselect s

-- Get the selected coordinate, otherwise get (0,0) by default.
getSelected :: Selector -> Coordinate
getSelected s@Selector{ selected = Just c  } = c
getSelected s@Selector{ selected = Nothing } = (0,0)

----------------------------------------------------------------------

-- Move the selector up one position.
moveUp :: Selector -> Selector
moveUp = move U
-- Move the selector down one position.
moveDown :: Selector -> Selector
moveDown = move D
-- Move the selector left one position.
moveLeft :: Selector -> Selector
moveLeft = move L
-- Move the selector right one position.
moveRight :: Selector -> Selector
moveRight = move R