順列の索引付け ref: http://qiita.com/nobsun/items/aba3bda2bd8a2f9b5f88

file0.txt

permNth :: [a] -> Int -> [a]

file1.txt

permNth xs = (perms xs !!)

file2.txt

permNth :: [a] -> Integer -> [a]

file3.txt

permNth xs idx = snd $ mapAccumL f (xs,idx) $ reverse $ take n facts

file4.txt

    f (ys,jdx) fact = case divMod jdx fact of
      (d,m) -> case splitAt (fromIntegral d) ys of
         (zs,w:ws) -> ((zs++ws,fromIntegral m),w)

file5.txt

permNth :: [a] -> Integer -> [a]
permNth xs idx = snd $ mapAccumL f (xs,idx) $ reverse $ take n facts
  where
    n = length xs
    f (ys,jdx) fact = case divMod jdx fact of
      (d,m) -> case splitAt (fromIntegral d) ys of
         (zs,w:ws) -> ((zs++ws,fromIntegral m),w)

facts :: [Integer]
facts = scanl (*) 1 [1..]

file6.txt

permIndex :: Eq a => [a] -> [a] -> Integer

file7.txt

permIndex []       []       = 0
permIndex xs@(_:_) ys@(_:_) = sum $ zipWith (*) zs $ snd $ mapAccumL f xs ys
  where
    zs = reverse $ take (length xs) facts
    f ps q = case elemIndex q ps of
      Just i -> (delete q ps,fromIntegral i)

permIndex.hs

import Data.List (mapAccumL,elemIndex,delete)

permNth :: [a] -> Integer -> [a]
permNth xs idx = snd $ mapAccumL f (xs,idx) $ reverse $ take n facts
  where
    n = length xs
    f (ys,jdx) fact = case divMod jdx fact of
      (d,m) -> case splitAt (fromIntegral d) ys of
         (zs,w:ws) -> ((zs++ws,fromIntegral m),w)

permIndex :: Eq a => [a] -> [a] -> Integer
permIndex []       []       = 0
permIndex xs@(_:_) ys@(_:_) = sum $ zipWith (*) zs $ snd $ mapAccumL f xs ys
  where
    zs = reverse $ take (length xs) facts
    f ps q = case elemIndex q ps of
      Just i -> (delete q ps,fromIntegral i)

facts :: [Integer]
facts = scanl (*) 1 [1..]