[Haskell-cafe] Tournament knock out

Xinyu LIU liuxinyu95 at gmail.com
Mon Feb 25 06:13:08 CET 2013


Tournament knock out is explained in [1] by Knuth. The limitation is that
it can only handle the sequence of the length 2^m for some integer m. When
the champion element is removed, it was replaced with negative infinity.
Typically negative infinity is represented by a predefined big negative

Although heap sort solves all these limitation, I found tournament knock
out itself can work out.
Here is the Haskell code:

data Infinite a = NegInf | Only a | Inf deriving (Eq, Show, Ord)

only (Only x) = x

data Tr a = Empty | Br (Tr a) a (Tr a) deriving Show

key (Br _ k _ ) = k

wrap x = Br Empty (Only x) Empty

branch t1 t2 = Br t1 (min (key t1) (key t2)) t2

fromList :: (Ord a) => [a] -> Tr (Infinite a)
fromList = build . (map wrap) where
  build [] = Empty
  build [t] = t
  build ts = build $ pair ts
  pair (t1:t2:ts) = (branch t1 t2):pair ts
  pair ts = ts

pop (Br Empty _ Empty) = Br Empty Inf Empty
pop (Br l k r) | k == key l = let l' = pop l in Br l' (min (key l') (key r)) r
               | k == key r = let r' = pop r in Br l (min (key l) (key r')) r'

top = only . key

tsort :: (Ord a) => [a] -> [a]
tsort = sort' . fromList where
    sort' Empty = []
    sort' (Br _ Inf _) = []
    sort' t = (top t) : (sort' $ pop t)

The detailed explanation can be found at:

[1]. Donald E. Knuth. The art of computer programming, Volume 3, sorting
and searching.

Larry, LIU Xinyu
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