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{-# LANGUAGE BangPatterns, ScopedTypeVariables #-}
-- |
-- Module : Statistics.Distribution
-- Copyright : (c) 2009 Bryan O'Sullivan
-- License : BSD3
--
-- Maintainer : bos@serpentine.com
-- Stability : experimental
-- Portability : portable
--
-- Types classes for probability distrubutions
module Statistics.Distribution
(
-- * Type classes
Distribution(..)
, DiscreteDistr(..)
, ContDistr(..)
-- ** Distribution statistics
, MaybeMean(..)
, Mean(..)
, MaybeVariance(..)
, Variance(..)
-- ** Random number generation
, ContGen(..)
, DiscreteGen(..)
, genContinous
-- * Helper functions
, findRoot
, sumProbabilities
) where
import Control.Applicative ((<$>), Applicative(..))
import Control.Monad.Primitive (PrimMonad,PrimState)
import qualified Data.Vector.Unboxed as U
import System.Random.MWC
-- | Type class common to all distributions. Only c.d.f. could be
-- defined for both discrete and continous distributions.
class Distribution d where
-- | Cumulative distribution function. The probability that a
-- random variable /X/ is less or equal than /x/,
-- i.e. P(/X/&#8804;/x/). Cumulative should be defined for
-- infinities as well:
--
-- > cumulative d +∞ = 1
-- > cumulative d -∞ = 0
cumulative :: d -> Double -> Double
-- | One's complement of cumulative distibution:
--
-- > complCumulative d x = 1 - cumulative d x
--
-- It's useful when one is interested in P(/X/&#8805;/x/) and
-- expression on the right side begin to lose precision. This
-- function have default implementation but implementors are
-- encouraged to provide more precise implementation.
complCumulative :: d -> Double -> Double
complCumulative d x = 1 - cumulative d x
-- | Discrete probability distribution.
class Distribution d => DiscreteDistr d where
-- | Probability of n-th outcome.
probability :: d -> Int -> Double
-- | Continuous probability distributuion
class Distribution d => ContDistr d where
-- | Probability density function. Probability that random
-- variable /X/ lies in the infinitesimal interval
-- [/x/,/x+/&#948;/x/) equal to /density(x)/&#8901;&#948;/x/
density :: d -> Double -> Double
-- | Inverse of the cumulative distribution function. The value
-- /x/ for which P(/X/&#8804;/x/) = /p/. If probability is outside
-- of [0,1] range function should call 'error'
quantile :: d -> Double -> Double
-- | Type class for distributions with mean. 'maybeMean' should return
-- 'Nothing' if it's undefined for current value of data
class Distribution d => MaybeMean d where
maybeMean :: d -> Maybe Double
-- | Type class for distributions with mean. If distribution have
-- finite mean for all valid values of parameters it should be
-- instance of this type class.
class MaybeMean d => Mean d where
mean :: d -> Double
-- | Type class for distributions with variance. If variance is
-- undefined for some parameter values both 'maybeVariance' and
-- 'maybeStdDev' should return Nothing.
--
-- Minimal complete definition is 'maybeVariance' or 'maybeStdDev'
class MaybeMean d => MaybeVariance d where
maybeVariance :: d -> Maybe Double
maybeVariance d = (*) <$> x <*> x where x = maybeStdDev d
maybeStdDev :: d -> Maybe Double
maybeStdDev = fmap sqrt . maybeVariance
-- | Type class for distributions with variance. If distibution have
-- finite variance for all valid parameter values it should be
-- instance of this type class.
--
-- Minimal complete definition is 'variance' or 'stdDev'
class (Mean d, MaybeVariance d) => Variance d where
variance :: d -> Double
variance d = x * x where x = stdDev d
stdDev :: d -> Double
stdDev = sqrt . variance
-- | Generate discrete random variates which have given
-- distribution.
class Distribution d => ContGen d where
genContVar :: PrimMonad m => d -> Gen (PrimState m) -> m Double
-- | Generate discrete random variates which have given
-- distribution. 'ContGen' is superclass because it's always possible
-- to generate real-valued variates from integer values
class (DiscreteDistr d, ContGen d) => DiscreteGen d where
genDiscreteVar :: PrimMonad m => d -> Gen (PrimState m) -> m Int
-- | Generate variates from continous distribution using inverse
-- transform rule.
genContinous :: (ContDistr d, PrimMonad m) => d -> Gen (PrimState m) -> m Double
genContinous d gen = do
x <- uniform gen
return $! quantile d x
{-# INLINE genContinous #-}
data P = P {-# UNPACK #-} !Double {-# UNPACK #-} !Double
-- | Approximate the value of /X/ for which P(/x/>/X/)=/p/.
--
-- This method uses a combination of Newton-Raphson iteration and
-- bisection with the given guess as a starting point. The upper and
-- lower bounds specify the interval in which the probability
-- distribution reaches the value /p/.
findRoot :: ContDistr d =>
d -- ^ Distribution
-> Double -- ^ Probability /p/
-> Double -- ^ Initial guess
-> Double -- ^ Lower bound on interval
-> Double -- ^ Upper bound on interval
-> Double
findRoot d prob = loop 0 1
where
loop !(i::Int) !dx !x !lo !hi
| abs dx <= accuracy || i >= maxIters = x
| otherwise = loop (i+1) dx'' x'' lo' hi'
where
err = cumulative d x - prob
P lo' hi' | err < 0 = P x hi
| otherwise = P lo x
pdf = density d x
P dx' x' | pdf /= 0 = P (err / pdf) (x - dx)
| otherwise = P dx x
P dx'' x''
| x' < lo' || x' > hi' || pdf == 0 = let y = (lo' + hi') / 2
in P (y-x) y
| otherwise = P dx' x'
accuracy = 1e-15
maxIters = 150
-- | Sum probabilities in inclusive interval.
sumProbabilities :: DiscreteDistr d => d -> Int -> Int -> Double
sumProbabilities d low hi =
-- Return value is forced to be less than 1 to guard againist roundoff errors.
-- ATTENTION! this check should be removed for testing or it could mask bugs.
min 1 . U.sum . U.map (probability d) $ U.enumFromTo low hi
{-# INLINE sumProbabilities #-}
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