-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Ordering
-- Copyright   :  (c) 2020-2021 EMQ Technologies Co., Ltd.
-- License     :  BSD-style (see the LICENSE file)
--
-- Maintainer  :  Feng Lee, feng@emqx.io
--                Yang M, yangm@emqx.io
-- Stability   :  experimental
-- Portability :  portable
--
-- The Ordering datatype
--
-----------------------------------------------------------------------------
module Data.Ordering
  ( Ordering(..)
  , invert
  ) where

import Data.Eq (class Eq)
import Data.Semigroup (class Semigroup)
import Data.Monoid (class Monoid)

-- | The `Ordering` data type represents the three possible outcomes of
-- | comparing two values:
-- |
-- | `LT` - The first value is _less than_ the second.
-- | `GT` - The first value is _greater than_ the second.
-- | `EQ` - The first value is _equal to_ the second.
data Ordering = LT | GT | EQ

instance Eq Ordering where
  eq LT LT = true
  eq GT GT = true
  eq EQ EQ = true
  eq _  _  = false

instance Semigroup Ordering where
  append LT _ = LT
  append GT _ = GT
  append EQ y = y

-- | Reverses an `Ordering` value, flipping greater than for less than while
-- | preserving equality.
invert :: Ordering -> Ordering
invert GT = LT
invert EQ = EQ
invert LT = GT

instance Monoid Ordering where
  mempty = EQ