(Added another, easily understandable, solution.)
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[[Category:Programming exercise spoilers]]
Latest revision as of 19:53, 18 January 2014
(***) Huffman codes.
We suppose a set of symbols with their frequencies, given as a list of fr(S,F) terms. Example: [fr(a,45),fr(b,13),fr(c,12),fr(d,16),fr(e,9),fr(f,5)]. Our objective is to construct a list hc(S,C) terms, where C is the Huffman code word for the symbol S. In our example, the result could be Hs = [hc(a,'0'), hc(b,'101'), hc(c,'100'), hc(d,'111'), hc(e,'1101'), hc(f,'1100')] [hc(a,'01'),...etc.]. The task shall be performed by the predicate huffman/2 defined as follows:
% huffman(Fs,Hs) :- Hs is the Huffman code table for the frequency table Fs
import Data.List import Data.Ord (comparing) data HTree a = Leaf a | Branch (HTree a) (HTree a) deriving Show huffman :: (Ord a, Ord w, Num w) => [(a,w)] -> [(a,[Char])] huffman freq = sortBy (comparing fst) $ serialize $ htree $ sortBy (comparing fst) $ [(w, Leaf x) | (x,w) <- freq] where htree [(_, t)] = t htree ((w1,t1):(w2,t2):wts) = htree $ insertBy (comparing fst) (w1 + w2, Branch t1 t2) wts serialize (Branch l r) = [(x, '0':code) | (x, code) <- serialize l] ++ [(x, '1':code) | (x, code) <- serialize r] serialize (Leaf x) = [(x, "")]
The argument to htree is a list of (weight, tree) pairs, in order of increasing weight. The implementation could be made more efficient by using a priority queue instead of an ordered list.
Or, a solution that does not use trees:
import List -- tupleUpdate - a function to record the Huffman codes; add string -- "1" or "0" to element 'c' of tuple array ta -- let ta = [('a',"0"),('b',"1")] -- tupleUpdate ta 'c' "1" => [('c',"1"),('a',"0"),('b',"1")] tupleUpdate :: [(Char,[Char])]->Char->String ->[(Char,[Char])] tupleUpdate ta el val | ((dropWhile(\x -> (fst x)/= el) ta)==)= (el,val):ta | otherwise = (takeWhile (\x -> (fst x)/=el) ta) ++ ((fst(head ha),val ++ snd(head ha)) : (tail (dropWhile (\x -> (fst x)/=el) ta))) where ha = [(xx,yy)|(xx,yy) <- ta,xx ==el] -- tupleUpdater - wrapper for tupleUpdate, use a list decomposition "for loop" -- let ta = [('a',"0"),('b',"1")] -- tupleUpdater ta "fe" "1" => [('e',"1"),('f',"1"),('a',"0"),('b',"1")] tupleUpdater :: [(Char,[Char])]->String->String ->[(Char,[Char])] tupleUpdater a (x:xs) c = tupleUpdater (tupleUpdate a x c) xs c tupleUpdater a  c = a -- huffer - recursively run the encoding algorithm and record the left/right -- codes as they are discovered in argument hc, which starts as  -- let ha =[(45,"a"),(13,"b"),(12,"c"),(16,"d"),(9,"e"),(5,"f")] -- huffer ha  => ([(100,"acbfed")],[('a',"0"),('b',"101"),('c',"100"),('d',"111"),('e',"1101"),('f',"1100")]) huffer :: [(Integer,String)] -> [(Char,[Char])]-> ([(Integer,String)],[(Char,[Char])]) huffer ha hc | ((length ha)==1)=(ha,sort hc) | otherwise = huffer ((num,str): tail(tail(has)) ) hc2 where num = fst (head has) + fst (head (tail has)) left = snd (head has) rght = snd (head (tail has)) str = left ++ rght has = sort ha hc2 = tupleUpdater (tupleUpdater hc rght "1") left "0" -- huffman - wrapper for huffer to convert the input to a format huffer likes -- and extract the output to match the problem specification huffman :: [(Char,Integer)] -> [(Char,[Char])] huffman h = snd(huffer (zip (map snd h) (map (:) (map fst h))) )
A relatively short solution:
import Data.List (sortBy, insertBy) import Data.Ord (comparing) import Control.Arrow (second) huffman :: [(Char, Int)] -> [(Char, String)] huffman = let shrink [(_, ys)] = sortBy (comparing fst) ys shrink (x1:x2:xs) = shrink $ insertBy (comparing fst) (add x1 x2) xs add (p1, xs1) (p2, xs2) = (p1 + p2, map (second ('0':)) xs1 ++ map (second ('1':)) xs2) in shrink . map (\(c, p) -> (p, [(c ,"")])) . sortBy (comparing snd)
Another short solution that's relatively easy to understand (I'll be back to comment later):
import qualified Data.List as L huffman :: [(Char, Int)] -> [(Char, [Char])] huffman x = reformat $ huffman_combine $ resort $ morph x where morph x = [ ([],[c],n) | (c,n) <- x ] resort x = L.sortBy (\(_,_,a) (_,_,b) -> compare a b) x reformat (x,y,_) = L.sortBy (\(a,b) (x,y) -> compare (length b) (length y)) $ zip y x huffman_combine (x:) = x huffman_combine (x:xs) = huffman_combine $ resort ( (combine_elements x (head xs)) : (tail xs) ) where combine_elements (a,b,c) (x,y,z) = ( (map ('0':) a) ++ (map ('1':) x), b ++ y, c+z)