-- | Representation of an axis-aligned rectangle on a 2D plane, with one of its
-- corners being a designated origin point.
module Data.Text.ParagraphLayout.Internal.Rect
    ( Bias (LL, LH, HL, HH)
    , Rect (Rect, x_origin, y_origin, x_size, y_size)
    , height
    , union
    , width
    , x_max
    , x_min
    , x_terminus
    , y_max
    , y_min
    , y_terminus
    )
where

-- | An axis-aligned rectangle on a 2D plane.
data Rect a = Rect
    { x_origin :: a
    -- ^ X coordinate of the corner designated as the rectangle's origin.
    , y_origin :: a
    -- ^ Y coordinate of the corner designated as the rectangle's origin.
    , x_size :: a
    -- ^ Signed difference between the X coordinates of the rectangle's sides.
    , y_size :: a
    -- ^ Signed difference between the Y coordinates of the rectangle's sides.
    }
    deriving (Eq, Read, Show)

-- | Absolute difference between the X coordinates of the rectangle's sides.
width :: Num a => Rect a -> a
width r = abs $ x_size r

-- | Absolute difference between the Y coordinates of the rectangle's sides.
height :: Num a => Rect a -> a
height r = abs $ y_size r

-- | X coordinate of the corner opposite of the origin.
x_terminus :: Num a => Rect a -> a
x_terminus r = x_origin r + x_size r

-- | Y coordinate of the corner opposite of the origin.
y_terminus :: Num a => Rect a -> a
y_terminus r = y_origin r + y_size r

-- | The smaller of the two X coordinates of the rectangle's edges.
x_min :: (Num a, Ord a) => Rect a -> a
x_min r = x_origin r `min` x_terminus r

-- | The smaller of the two Y coordinates of the rectangle's edges.
y_min :: (Num a, Ord a) => Rect a -> a
y_min r = y_origin r `min` y_terminus r

-- | The larger of the two X coordinates of the rectangle's edges.
x_max :: (Num a, Ord a) => Rect a -> a
x_max r = x_origin r `max` x_terminus r

-- | The larger of the two Y coordinates of the rectangle's edges.
y_max :: (Num a, Ord a) => Rect a -> a
y_max r = y_origin r `max` y_terminus r

-- | Determines which corner of a calculated rectangle should be its origin.
data Bias
    = LL
    -- ^ Set the origin as the corner with low X and low Y coordinates.
    | LH
    -- ^ Set the origin as the corner with low X and high Y coordinates.
    | HL
    -- ^ Set the origin as the corner with high X and low Y coordinates.
    | HH
    -- ^ Set the origin as the corner with high X and high Y coordinates.

-- | The smallest rectangle completely containing the given two rectangles.
--
-- The origin of the output rectangle will be set according to `Bias`,
-- regardless of which corners of the input rectangles are designated
-- as their origins.
--
-- Note that this operation has no identity element. A rectangle whose
-- `x_size` and/or `y_size` are zero is not considered null or neutral,
-- but effectively acts as a point, which will be contained in the union.
--
-- You can use `Nothing` as an identity element if you lift this operation
-- over the `Maybe` applicative functor:
--
-- @
-- `Control.Applicative.liftA2` (`union` bias)
-- @
union :: (Num a, Ord a) => Bias -> Rect a -> Rect a -> Rect a
union bias a b =
    case bias of
        LL -> Rect lx ly dx dy
        LH -> Rect lx hy dx (-dy)
        HL -> Rect hx ly (-dx) dy
        HH -> Rect hx hy (-dx) (-dy)
    where
        lx = x_min a `min` x_min b
        ly = y_min a `min` y_min b
        hx = x_max a `max` x_max b
        hy = y_max a `max` y_max b
        dx = hx - lx
        dy = hy - ly