A-M baby

Task/Fast-Fourier-transform/Haskell/fast-fourier-transform.hs

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+import Data.Complex
+
+-- Cooley-Tukey
+fft [] = []
+fft [x] = [x]
+fft xs = zipWith (+) ys ts ++ zipWith (-) ys ts
+    where n = length xs
+          ys = fft evens
+          zs = fft odds
+          (evens, odds) = split xs
+          split [] = ([], [])
+          split [x] = ([x], [])
+          split (x:y:xs) = (x:xt, y:yt) where (xt, yt) = split xs
+          ts = zipWith (\z k -> exp' k n * z) zs [0..]
+          exp' k n = cis $ -2 * pi * (fromIntegral k) / (fromIntegral n)
+
+main = mapM_ print $ fft [1,1,1,1,0,0,0,0]