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module Algo.Spectral ( spectral ) where
import Data.Vector.Unboxed as V
import Data.Bits
spectral :: Vector Double -> Vector Double
{-# NOINLINE spectral #-}
spectral us = us `seq` V.map row (V.enumFromTo 0 (n-1))
where
n = V.length us
row i = i `seq` V.sum (V.imap (\j u -> eval_A i j * u) us)
eval_A i j = 1 / fromIntegral r
where
r = u + (i+1)
u = t `shiftR` 1
t = n * (n+1)
n = i+j
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