module Algo.ListRank
where
import Data.Vector.Unboxed as V
listRank :: Int -> Vector Int
{-# NOINLINE listRank #-}
listRank n = pointer_jump xs val
where
xs = 0 `V.cons` V.enumFromTo 0 (n-2)
val = V.zipWith (\i j -> if i == j then 0 else 1)
xs (V.enumFromTo 0 (n-1))
pointer_jump pt val
| npt == pt = val
| otherwise = pointer_jump npt nval
where
npt = V.backpermute pt pt
nval = V.zipWith (+) val (V.backpermute val pt)