    {-# LANGUAGE PatternGuards #-}

    module Numeric.Histogram ( Range
                             , binBounds
                             , histValues
                             , histWeightedValues
                             , histWithBins
                             ) where

    import qualified Data.Vector as V
    import qualified Data.Vector.Mutable as MV
    import Control.Monad.ST

    type Range a = (a,a)

    -- | 'binBounds a b n' generates bounds for 'n' bins spaced linearly between
    -- 'a' and 'b'
    --
    -- Examples:
    --
    -- >>> binBounds 0 3 4
    -- [(0.0,0.75),(0.75,1.5),(1.5,2.25),(2.25,3.0)]
    binBounds :: RealFrac a => a -> a -> Int -> [Range a]
    binBounds a b n = map (\i->(lbound i, lbound (i+1))) [0..n-1]
            where lbound i = a + (b-a) * realToFrac i / realToFrac n

    -- | 'histValues a b n vs' returns the bins for the histogram of
    -- 'vs' on the range from 'a' to 'b' with 'n' bins
    histValues :: RealFrac a => a -> a -> Int -> [a] -> V.Vector (Range a, Int)
    histValues a b n = histWithBins (V.fromList $ binBounds a b n) . zip (repeat 1)

    -- | 'histValues a b n vs' returns the bins for the weighted histogram of
    -- 'vs' on the range from 'a' to 'b' with 'n' bins
    histWeightedValues :: RealFrac a => a -> a -> Int -> [(Double,a)] -> V.Vector (Range a, Double)
    histWeightedValues a b n = histWithBins (V.fromList $ binBounds a b n)

    -- | 'histWithBins bins xs' is the histogram of weighted values 'xs' with 'bins'
    --
    -- Examples:
    --
    -- >>> :{
    -- histWithBins
    --     (V.fromList [(0.0, 0.75), (0.75, 1.5), (1.5, 2.25), (2.25, 3.0)])
    --     [(1, 0), (1, 0), (1, 1), (1, 2), (1, 2), (1, 2), (1, 3)]
    -- :}
    -- [((0.0,0.75),2),((0.75,1.5),1),((1.5,2.25),3),((2.25,3.0),1)]
    histWithBins :: (Num w, RealFrac a) => V.Vector (Range a) -> [(w, a)] -> V.Vector (Range a, w)
    histWithBins bins xs =
        let n = V.length bins
            testBin :: RealFrac a => a -> (Int, Range a) -> Bool
            testBin x (i, (a,b)) =
                if i == n - 1
                    then x >= a && x <= b
                    else x >= a && x < b

            f :: (RealFrac a, Num w)
              => V.Vector (Range a) -> MV.STVector s w -> (w, a)
              -> ST s ()
            f bins1 bs (w,x) =
                case V.dropWhile (not . testBin x) $ V.indexed bins1 of
                    v | V.null v  -> return ()
                    v | (idx,_) <- V.head v  -> do
                        m <- MV.read bs idx
                        MV.write bs idx $! m+w

            counts = runST $ do b <- MV.replicate n 0
                                mapM_ (f bins b) xs
                                V.freeze b
        in V.zip bins counts