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Haskell Quiz/Secret Santas/Solution Anton

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[[Category:Haskell Quiz solutions|Secret Santas]]
 
<haskell>
 
<haskell>
 
-- Ruby quiz 2 - Haskell solution
 
-- Ruby quiz 2 - Haskell solution
Line 18: Line 19:
 
mix = foldl1 (\a b -> ziprest a b ++ (concat $ zipWith (\x y -> x:[y]) a b))
 
mix = foldl1 (\a b -> ziprest a b ++ (concat $ zipWith (\x y -> x:[y]) a b))
 
where ziprest a b = let diff = length b - length a in
 
where ziprest a b = let diff = length b - length a in
if diff /= 0
+
if diff > 0
 
then drop (length b - diff) b
 
then drop (length b - diff) b
 
else drop (length a + diff) a
 
else drop (length a + diff) a

Latest revision as of 00:07, 12 August 2011

-- Ruby quiz 2 - Haskell solution
-- Copyright (C) 2011 Anton Pirogov
import Data.List
import Data.Function
 
main = do
  xs <- fmap (mix . prepare . parse) getContents
  putStr $ format xs
 
parse = map (\(a:b:c) -> (a,b)) . map words . lines
 
prepare = sortBy size . groupBy family . sortBy lastname
  where lastname = compare `on` snd
        family = (==) `on` snd
        size = compare `on` length
 
mix = foldl1 (\a b -> ziprest a b ++ (concat $ zipWith (\x y -> x:[y]) a b))
  where ziprest a b = let diff = length b - length a in
                        if diff > 0
                        then drop (length b - diff) b
                        else drop (length a + diff) a
 
format = unlines . map (\(a,b) -> a++" "++b)


It reads from standard input, splits, sorts and groups by family, then it merges the families by zipping them into each other, starting with the small ones, always prepending the "overhanging" rest members to the result (as zip drops the last elements of the longer list). By prepending the rests its ensured that in the next zipping these members get mixed up too (if possible at all).


Because no additional shuffling of the names is done, the result for a given list is always the same and depends on the name order (as the pre-sorting does not look at the first names), so to get different results, you have to pre-shuffle the list... example usage with shuffling (assuming a unix shell):

shuf namefile | runhaskell rubyquiz2.hs