「正八角形の分割」問題 ref: http://qiita.com/nobsun/items/2c227bab1156eddeb3df

F04.hs

module F04 where

import Data.Bool (bool)
import Data.List (sort,unfoldr)
import Text.Printf (printf)

readDiag :: String -> Word
readDiag = read

toBin :: Word -> String
toBin = printf "%08b"

angles :: String -> [Int]
angles = unfoldr phi
  where
    phi "" = Nothing
    phi xs = case break ('1'==) xs of
      (xs,_:ys) -> Just (length xs,ys)

type Problem = String
type Answer  = String

f04 :: Problem -> Answer
f04 = sort
    . concatMap (show . count)
    . angles
    . take 8
    . tail
    . dropWhile ('0'==)
    . cycle
    . toBin
    . readDiag

count :: Int -> Int
count n = bool (n+3) 5 (n == 3)

type Test = (Problem, Answer)

{- |
>>> test ( "165", "3445" )
True    
>>> test ( "80", "48" )
True    
>>> test ( "255", "33333333" )
True    
>>> test ( "68", "55" )
True    
>>> test ( "200", "355" )
True    
>>> test ( "82", "455" )
True    
>>> test ( "164", "455" )
True    
>>> test ( "73", "455" )
True    
>>> test ( "146", "455" )
True    
>>> test ( "37", "455" )
True    
>>> test ( "74", "455" )
True    
>>> test ( "148", "455" )
True    
>>> test ( "41", "455" )
True    
>>> test ( "38", "355" )
True    
>>> test ( "76", "355" )
True    
>>> test ( "152", "355" )
True    
>>> test ( "49", "355" )
True    
>>> test ( "98", "355" )
True    
>>> test ( "196", "355" )
True    
>>> test ( "137", "355" )
True    
>>> test ( "19", "355" )
True    
>>> test ( "20", "48" )
True    
>>> test ( "9", "57" )
True    
>>> test ( "209", "3345" )
True    
>>> test ( "121", "33345" )
True    
>>> test ( "239", "3333334" )
True    
>>> test ( "26", "347" )
True    
>>> test ( "111", "333344" )
True    
>>> test ( "95", "333344" )
True    
>>> test ( "85", "4444" )
True    
>>> test ( "24", "39" )
True    
>>> test ( "97", "347" )
True    
>>> test ( "234", "33444" )
True    
>>> test ( "59", "33345" )
True    
>>> test ( "187", "333344" )
True    
>>> test ( "34", "55" )
True    
>>> test ( "249", "333335" )
True    
>>> test ( "43", "3445" )
True    
>>> test ( "143", "33335" )
True    
>>> test ( "28", "338" )
True    
>>> test ( "79", "33345" )
True    
>>> test ( "173", "33444" )
True    
>>> test ( "55", "33345" )
True    
>>> test ( "77", "3445" )
True    
>>> test ( "35", "355" )
True    
>>> test ( "153", "3355" )
True    
>>> test ( "30", "3337" )
True    
>>> test ( "228", "3355" )
True    
>>> test ( "177", "3345" )
True    
>>> test ( "162", "445" )
True    
>>> test ( "184", "3345" )
True
-}
test :: Test -> Bool
test (p,a) = f04 p == a