どう書くE20の解答例 問題:http://nabetani.sakura.ne.jp/hena/orde20maze/

doukakuE20.hs

module Lib where

import Data.List.Split
import Data.List

solve :: String -> String
solve = show . solve'

solve' :: String -> Int
solve' [start,goal] = walk [start] [] 0
  where
    walk s r n | goal `elem` s = n
    walk s r n = walk (next s r) (r++s) (n+1)
    next s r = [b| (a,b) <- maze, a `elem` s, b `notElem` r]
solve' _ = error "unexpected input"

maze, field, walls :: [(Char,Char)]
maze = t ++ revt
  where
    revt = map (\(a,b) -> (b,a)) t
    t = field \\ walls

field = hol ++ ver
  where
    list = chunksOf 6 $ ['0'..'9'] ++ ['A'..'Z']
    hol = concatMap (\l -> zip l (tail l)) list
    ver = concatMap (\(x,y) -> zip x y) $ zip list (tail list)
 
walls = map (\[a,b] -> (a,b)) $ chunksOf 2 "12284A67AB7D8EAGBHDEEFGHDJFLIJMNKQLROPRSOUQWTZWXXY"

-- 以下説明用

walk :: [Char] -> Int
walk [start,goal] = length $ walk' start goal
walk _ = error "walk"

walk' :: Char -> Char -> [([Char], Int, [Char])]
walk' start goal = takeWhile (\(starts,_,_) -> goal `notElem` starts) $ iterate step ([start],0,[])

step :: ([Char], Int, [Char]) -> ([Char], Int, [Char])
step (starts,n,walked) = (next,(n+1),(starts++walked))
  where next = [to| (from,to) <- maze, from `elem` starts, to `notElem` walked]

printList :: (Show a2, Show a1, Show a, Foldable t) => t (a, a1, a2) -> IO ()
printList xs = mapM_ (\(starts,steps,walked) -> putStrLn $ "starts: " ++ show starts ++ "\tsteps: " ++ show steps ++ "\twalked: " ++ show walked) xs