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39 lines
1.3 KiB
Haskell
39 lines
1.3 KiB
Haskell
-- |
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-- Module : GenAlph
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-- Copyright : (c) 2013-2016 Galois, Inc.
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-- License : BSD3
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-- Maintainer : cryptol@galois.com
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-- Stability : provisional
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-- Portability : portable
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-- alph generates a random permutation of the alphabet
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-- symAlph gives you a permutation that is symmetric (i.e., if A maps to B, then
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-- B maps to A); and also one that never maps anything back to itself.
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module GenAlph(alph, symAlph) where
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import System.Random
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import qualified Data.Map as M
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import Control.Monad
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letters = ['A' .. 'Z']
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genAlph = go []
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where go xs | length xs == 26 = return xs
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| True = do s <- randomRIO (0, 25)
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if s `elem` xs then go xs else go (s:xs)
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alph = do s <- genAlph
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return $ [letters !! i | i <- s ]
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symAlph = do s <- foldM add M.empty letters
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return $ map snd $ M.toAscList s
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where add m l
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| l `elem` M.keys m = return m
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| True = do i <- randomRIO (0, 25)
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let e = letters !! i
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if e == l
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then add m l
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else case e `M.lookup` m of
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Nothing -> return $ M.insert l e $ M.insert e l $ m
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Just _ -> add m l
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