import           Data.Vector.Generic (Vector, (!))
import qualified Data.Vector.Generic as V

conv_1, conv_2, conv_3 :: (Num a, Vector v a) => v a -> v a -> v a
{-# INLINE conv_1 #-}
conv_1 h x = V.generate (l+m) f
    where
        m = V.length h - 1
        l = V.length x
        {-# INLINE f #-}
        f n = g 0 n (max 0 (n-l+1)) (min n m)
        g y n m k = if m <= k
                    then let y' = y + (h ! m) * (x ! (n-m))
                         in y' `seq` g y' n (m+1) k
                    else y
{-# INLINE conv_2 #-}
conv_2 h x = V.generate (l+m) f
    where
        l = V.length x
        m = V.length h - 1
        {-# INLINE f #-}
        f n = let j = max 0 (n-l+1)
                  k = (min n m) - j + 1
              in V.sum (V.zipWith (*) (V.slice j k h) (V.reverse (V.slice (n - j - k + 1) k x)))
{-# INLINE conv_3 #-}
conv_3 h x = V.generate (l+m-1) f
    where
        m   = V.length h
        l   = V.length x
        p   = V.replicate (m-1) 0
        x'  = p ++ x ++ p
        {-# INLINE f #-}
        f i = V.sum (V.zipWith (*) (V.reverse h) (V.slice i m x'))