lingo / nlp-scores / NLP / Scores.hs

{-# LANGUAGE 
    BangPatterns 
  , NoMonomorphismRestriction
 #-}
-- | Scoring functions commonly used for evaluation of NLP
-- systems. Most functions in this module work on sequences which are
-- instances of 'Data.Foldable', but some take a precomputed table of
-- 'Counts'. This will give a speedup if you want to compute multiple
-- scores on the same data. For example to compute the Mutual
-- Information, Variation of Information and the Adjusted Rand Index
-- on the same pair of clusterings:
--
-- >>> let cs = counts "abcabc" "abaaba"
-- >>> mapM_ (print . ($ cs)) [mi, ari, vi]
-- >>> 0.9182958340544894
-- >>> 0.4444444444444445
-- >>> 0.6666666666666663

module NLP.Scores 
    ( 
    -- * Scores for classification and ranking
      accuracy
    , recipRank
    , avgPrecision
    -- * Scores for clustering
    , ari
    , mi
    , vi
    -- * Auxiliary types and functions
    , Count
    , Counts
    , counts
    , sum
    , mean
    , jaccard
    , entropy
    , histogram
    -- * Extracting joint and marginal counts from 'Counts'
    , countJoint
    , countFst
    , countSnd
    -- * Extracting lists of values from Counts
    , fstElems
    , sndElems
    )
where
import qualified Data.Foldable as F
import Data.Monoid
import Data.List hiding (sum)
import qualified Data.Set as Set
import qualified Data.Map as Map
import Prelude hiding (sum)

import NLP.Scores.Internals

-- | Accuracy: the proportion of elements in the first sequence equal
-- to elements at corresponding positions in second
-- sequence. Sequences should be of equal lengths.
accuracy :: (Eq a, Fractional c, F.Foldable t) =>  t a -> t a -> c
accuracy xs = mean . map fromEnum . zipWith (==) (F.toList xs) . F.toList
{-# SPECIALIZE accuracy :: [Double] -> [Double] -> Double #-}

-- | Reciprocal rank: the reciprocal of the rank at which the first arguments
-- occurs in the sequence given as the second argument.
recipRank :: (Eq a, Fractional b, F.Foldable t) => a -> t a -> b
recipRank y ys = 
    case [ r | (r,y') <- zip [1::Int ..] . F.toList $ ys , y' == y ] of
      []  -> 0
      r:_ -> 1/fromIntegral r
{-# SPECIALIZE recipRank :: Double -> [Double] -> Double #-}

-- | Average precision. 
-- <http://en.wikipedia.org/wiki/Information_retrieval#Average_precision>
avgPrecision :: (Fractional n, Ord a, F.Foldable t) => Set.Set a -> t a -> n
avgPrecision gold _ | Set.size gold == 0 = 0
avgPrecision gold xs =
      (/fromIntegral (Set.size gold))
    . sum 
    . map (\(r,rel,cum) -> if rel == 0 
                          then 0 
                          else fromIntegral cum / fromIntegral r)
    . takeWhile (\(_,_,cum) -> cum <= Set.size gold) 
    . snd 
    . mapAccumL (\z (r,rel) -> (z+rel,(r,rel,z+rel))) 0
    $ [ (r,fromEnum $ x `Set.member` gold) 
      | (x,r) <- zip (F.toList xs) [1::Int ..]]
{-# SPECIALIZE avgPrecision :: (Ord a) => Set.Set a -> [a] -> Double #-}

-- | Mutual information: MI(X,Y) = H(X) - H(X|Y) = H(Y) - H(Y|X). Also
-- known as information gain.
mi :: (Ord a, Ord b) => Counts a b -> Double
mi (Counts cxy cx cy) =
  let n = Map.foldl' (+) 0 cxy
      cell (P x y) nxy = 
        let nx = cx Map.! x
            ny = cy Map.! y
        in  nxy / n * logBase 2 (nxy * n / nx / ny)
  in sum [ cell (P x y) nxy | (P x y, nxy) <- Map.toList cxy ]

-- | Variation of information: VI(X,Y) = H(X) + H(Y) - 2 MI(X,Y)
vi :: (Ord a, Ord b) => Counts a b -> Double
vi cs@(Counts _ cx cy) = entropy (elems cx) + entropy (elems cy) - 2 * mi cs
  where elems = Map.elems

-- | Adjusted Rand Index: <http://en.wikipedia.org/wiki/Rand_index>
ari :: (Ord a, Ord b) => Counts a b -> Double
ari (Counts cxy cx cy) =  (sum1 - sum2*sum3/choicen2) 
                        / (1/2 * (sum2+sum3) - (sum2*sum3) / choicen2)
  where choicen2 = choice (sum . Map.elems $ cx) 2
        sum1 = sum [ choice nij 2 | nij <- Map.elems cxy ]
        sum2 = sum [ choice ni 2 | ni <- Map.elems cx ]
        sum3 = sum [ choice nj 2 | nj <- Map.elems cy ]

-- | The sum of a sequence of numbers
sum :: (F.Foldable t, Num a) => t a -> a
sum = F.foldl' (+) 0
{-# SPECIALIZE sum :: [Double] -> Double #-}
{-# SPECIALIZE sum :: [Int] -> Int #-}
{-# INLINE sum #-}

-- | The mean of a sequence of numbers.
mean :: (F.Foldable t, Fractional n, Real a) => t a -> n
mean xs = 
    let (P tot len) = F.foldl' (\(P s l) x -> (P (s+x) (l+1))) (P 0 0) xs
    in realToFrac tot/len
{-# SPECIALIZE mean :: [Double] -> Double #-}

-- | The binomial coefficient: C^n_k = PROD^k_i=1 (n-k-i)/i
choice :: (Enum b, Fractional b) => b -> b -> b
choice n k = foldl' (*) 1 [n-k+1 .. n] / foldl' (*) 1 [1 .. k]
{-# SPECIALIZE choice :: Double -> Double -> Double #-}

-- | Jaccard coefficient
-- J(A,B) = |AB| / |A union B|
jaccard :: (Fractional n, Ord a) => Set.Set a -> Set.Set a -> n
jaccard a b = 
  fromIntegral (Set.size (Set.intersection a b))
  / 
  fromIntegral (Set.size (Set.union a b))
{-# SPECIALIZE jaccard :: (Ord a) => Set.Set a -> Set.Set a -> Double #-}  

-- | Entropy: H(X) = -SUM_i P(X=i) log_2(P(X=i)). @entropy xs@ is the
-- entropy of the random variable represented by the sequence @xs@,
-- where each element of @xs@ is the count of the one particular 
-- value the random variable can take. If you need to compute the 
-- entropy from a sequence of outcomes, the following will work:
--
-- > entropy . elems . histogram
--
entropy :: (Floating c, F.Foldable t) => t c -> c
entropy cx = negate . getSum . F.foldMap  (Sum . f)  $ cx
    where n    = sum cx
          logn = logBase 2 n
          f nx = nx / n * (logBase 2 nx - logn)

-- | @histogram xs@ is returns the map of the frequency counts of the
-- elements in sequence @xs@
histogram :: (Num a, Ord k, F.Foldable t) => t k -> Map.Map k a
histogram = F.foldl' (\ z k -> Map.insertWith' (+) k 1 z) Map.empty

-- | Creates count table 'Counts'
counts :: (Ord a, Ord b, F.Foldable t) => t a -> t b -> Counts a b
counts xs = foldl' f empty . zipWith P (F.toList xs) . F.toList
    where f cs@(Counts cxy cx cy) p@(P x y) = 
            cs { joint       = Map.insertWith' (+) p 1 cxy
               , marginalFst = Map.insertWith' (+) x 1 cx
               , marginalSnd = Map.insertWith' (+) y 1 cy }

-- | Joint count
countJoint :: (Ord a, Ord b) => a -> b -> Counts a b -> Count          
countJoint x y = Map.findWithDefault 0 (P x y) . joint
-- | Count of first element
countFst :: Ord k => k -> Counts k b -> Count
countFst x = Map.findWithDefault 0 x . marginalFst
-- | Count of second element
countSnd :: Ord k => k -> Counts a k -> Count
countSnd y = Map.findWithDefault 0 y . marginalSnd

-- | List of values of first element
fstElems :: Counts k b -> [k]
fstElems = Map.keys . marginalFst
-- | List of values of second element
sndElems :: Counts a k -> [k]
sndElems = Map.keys . marginalSnd
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