Uniform.hs

{-# LANGUAGE DeriveDataTypeable, DeriveGeneric #-}
-- |
-- Module    : Statistics.Distribution.Uniform
-- Copyright : (c) 2011 Aleksey Khudyakov
-- License   : BSD3
--
-- Maintainer  : bos@serpentine.com
-- Stability   : experimental
-- Portability : portable
--
-- Variate distributed uniformly in the interval.
module Statistics.Distribution.Uniform
    (
      UniformDistribution
    -- * Constructors
    , uniformDistr
    -- ** Accessors
    , uniformA
    , uniformB
    ) where

import Data.Binary (Binary)
import Data.Data (Data, Typeable)
import GHC.Generics (Generic)
import qualified Statistics.Distribution as D
import qualified System.Random.MWC       as MWC
import Data.Binary (put, get)
import Control.Applicative ((<$>), (<*>))


-- | Uniform distribution from A to B
data UniformDistribution = UniformDistribution {
      uniformA :: {-# UNPACK #-} !Double -- ^ Low boundary of distribution
    , uniformB :: {-# UNPACK #-} !Double -- ^ Upper boundary of distribution
    } deriving (Eq, Read, Show, Typeable, Data, Generic)

instance Binary UniformDistribution where
    put (UniformDistribution x y) = put x >> put y
    get = UniformDistribution <$> get <*> get

-- | Create uniform distribution.
uniformDistr :: Double -> Double -> UniformDistribution
uniformDistr a b
  | b < a     = uniformDistr b a
  | a < b     = UniformDistribution a b
  | otherwise = error "Statistics.Distribution.Uniform.uniform: wrong parameters"
-- NOTE: failure is in default branch to guard againist NaNs.

instance D.Distribution UniformDistribution where
  cumulative (UniformDistribution a b) x
    | x < a     = 0
    | x > b     = 1
    | otherwise = (x - a) / (b - a)

instance D.ContDistr UniformDistribution where
  density (UniformDistribution a b) x
    | x < a     = 0
    | x > b     = 0
    | otherwise = 1 / (b - a)
  quantile (UniformDistribution a b) p
    | p >= 0 && p <= 1 = a + (b - a) * p
    | otherwise        =
      error $ "Statistics.Distribution.Uniform.quantile: p must be in [0,1] range. Got: "++show p

instance D.Mean UniformDistribution where
  mean (UniformDistribution a b) = 0.5 * (a + b)

instance D.Variance UniformDistribution where
  -- NOTE: 1/sqrt 12 is not constant folded (#4101) so it's written as
  --       numerical constant. (Also FIXME!)
  stdDev   (UniformDistribution a b) = 0.2886751345948129 * (b - a)
  variance (UniformDistribution a b) = d * d / 12 where d = b - a

instance D.MaybeMean UniformDistribution where
    maybeMean = Just . D.mean

instance D.MaybeVariance UniformDistribution where
    maybeStdDev   = Just . D.stdDev

instance D.Entropy UniformDistribution where
  entropy (UniformDistribution a b) = log (b - a)

instance D.MaybeEntropy UniformDistribution where
  maybeEntropy = Just . D.entropy

instance D.ContGen UniformDistribution where
    genContVar (UniformDistribution a b) gen = MWC.uniformR (a,b) gen