{- | Module : Data.Set.BKTree Copyright : (c) Josef Svenningsson 2007 License : BSD-style Maintainer : josef.svenningsson@gmail.com Stability : Alpha quality. Interface may change without notice. Portability : portable Burhard-Keller trees provide an implementation of sets which apart from the ordinary operations also has an approximate member search, allowing you to search for elements that are of a distance @n@ from the element you are searching for. The distance is determined using a metric on the type of elements. Therefore all elements must implement the 'Metric' type class, rather than the more usual 'Ord'. Useful metrics include the manhattan distance between two points, the Levenshtein edit distance between two strings, the number of edges in the shortest path between two nodes in a undirected graph and the Hamming distance between two binary strings. Any euclidean space also has a metric. However, in this module we use int-valued metrics and that doesn't quite with the metrics of euclidean spaces which are real-values. The worst case complexity of many of these operations is quite bad, but the expected behavior varies greatly with the metric. For example, the discrete metric (@distance x y | y == x = 0 | otherwise = 1@) makes BK-trees behave abysmally. The metrics mentioned above should give good performance characteristics. -} module Data.Set.BKTree (-- The main type BKTree -- Metric ,Metric(..) -- ,Manhattan(..) -- ,null,empty ,fromList,singleton ,insert ,member,memberDistance ,delete ,union,unions ,elems,elemsDistance ,closest #ifdef DEBUG ,runTests #endif )where import qualified Data.IntMap as M import qualified Data.List as L hiding (null) import Prelude hiding (null) #ifdef DEBUG import qualified Prelude import Test.QuickCheck import Text.Printf #endif data BKTree a = Node a (M.IntMap (BKTree a)) | Empty #ifdef DEBUG deriving Show #endif -- | A type is 'Metric' if is has a function 'distance' which has the following -- properties: -- -- * @'distance' x y >= 0@ -- -- * @'distance' x y == 0@ if and only if @x == y@ -- -- * @'distance' x y == 'distance' y x@ -- -- * @'distance' x z <= 'distance' x y + 'distance' y z@ -- -- All types of elements to 'BKTree' must implement 'Metric'. -- -- This definition of a metric deviates from the mathematical one in that it -- returns an integer instead of a real number. The reason for choosing -- integers is that I wanted to avoid the rather unpredictable rounding -- of floating point numbers. class Eq a => Metric a where distance :: a -> a -> Int instance Metric Int where distance i j = abs (i - j) -- Fishy instance. Maybe I shouldn't have it. -- Or generalize Metric to use integer? instance Metric Integer where distance i j = fromInteger (abs (i - j)) instance Metric Char where distance i j = abs (fromEnum i - fromEnum j) -- | Test if the tree is empty. null :: BKTree a -> Bool null (Empty) = True null (Node _ _) = False -- | The empty tree. empty :: BKTree a empty = Empty -- | The tree with a single element singleton :: a -> BKTree a singleton a = Node a M.empty -- | Inserts an element into the tree. If an element is inserted several times -- it will be stored several times. insert :: Metric a => a -> BKTree a -> BKTree a insert a Empty = Node a M.empty insert a (Node b map) = Node b map' where map' = M.insertWith recurse d (Node a M.empty) map d = distance a b recurse _ tree = insert a tree -- | Checks whether an element is in the tree. member :: Metric a => a -> BKTree a -> Bool member a Empty = False member a (Node b map) | d == 0 = True | otherwise = case M.lookup d map of Nothing -> False Just tree -> member a tree where d = distance a b -- | Approximate searching. @'memberDistance' n a tree@ will return true if -- there is an element in @tree@ which has a 'distance' less than or equal to -- @n@ from @a@. memberDistance :: Metric a => Int -> a -> BKTree a -> Bool memberDistance n a Empty = False memberDistance n a (Node b map) | d <= n = True | otherwise = any (memberDistance n a) (M.elems subMap) where d = distance a b subMap = case M.split (d-n-1) map of (_,mapRight) -> case M.split (d+n+1) mapRight of (mapCenter,_) -> mapCenter -- | Removes an element from the tree. If an element occurs several times in -- the only the first occurrence will be deleted. delete :: Metric a => a -> BKTree a -> BKTree a delete a Empty = Empty delete a t@(Node b map) | d == 0 = unions (M.elems map) | otherwise = Node b (M.update (Just . delete a) d map) where d = distance a b -- | Returns all the elements of the tree elems :: BKTree a -> [a] elems Empty = [] elems (Node a imap) = a : concatMap elems (M.elems imap) -- | @'elemsDistance' n a tree@ returns all the elements in @tree@ which are -- at a 'distance' less than or equal to @n@ from the element @a@. elemsDistance :: Metric a => Int -> a -> BKTree a -> [a] elemsDistance n a Empty = [] elemsDistance n a (Node b imap) = (if d <= n then (b :) else id) $ concatMap (elemsDistance n a) (M.elems subMap) where d = distance a b subMap = case M.split (d-n-1) imap of (_,mapRight) -> case M.split (d+n+1) mapRight of (mapCenter,_) -> mapCenter -- | Constructs a tree from a list fromList :: Metric a => [a] -> BKTree a fromList [] = Empty fromList (a:as) = Node a $ M.fromAscList $ map recurse $ L.groupBy ((==) `on` fst) $ L.sortBy (compare `on` fst) $ map mkDistance $ as where mkDistance b = (distance a b,b) recurse bs@((k,_):_) = (k,fromList (map snd bs)) -- | Merges several trees unions :: Metric a => [BKTree a] -> BKTree a unions [] = Empty unions (Empty:ts) = unions ts unions (Node piv pmap:ts) = Node piv $ M.fromAscList $ map recurse $ L.groupBy ((==) `on` fst) $ L.sortBy (compare `on` fst) $ (M.toList pmap ++) $ concatMap mkDistance $ ts where mkDistance n@(Node a _) = [(distance piv a,n)] mkDistance _ = [] recurse bs@((k,_):_) = (k,unions (map snd bs)) -- | Merges two trees union :: Metric a => BKTree a -> BKTree a -> BKTree a union t1 t2 = unions [t1,t2] -- | @'closest' a tree@ returns the element in @tree@ which is closest to -- @a@ together with the distance. Returns @Nothing@ if the tree is empty. closest :: Metric a => a -> BKTree a -> Maybe (a,Int) closest a Empty = Nothing closest a tree@(Node b _) = Just (closeLoop a (b,distance a b) tree) closeLoop a candidate Empty = candidate closeLoop a candidate@(b,d) (Node x imap) = L.foldl' (closeLoop a) newCand (M.elems subMap) where newCand = if j >= d then candidate else (x,j) j = distance a x subMap = case M.split (d-j-1) imap of (_,mapRight) -> case M.split (d+j+1) mapRight of (mapCenter,_) -> mapCenter -- Helper functions on rel f x y = rel (f x) (f y) #ifdef DEBUG -- Testing -- Semantics of BKTrees. Just a boring list of integers sem tree = L.sort (elems tree) -- For testing functions that transform trees trans f xs = sem (f (fromList xs)) -- Tests for individual functions prop_empty n = not (member (n::Int) empty) prop_null xs = null (fromList xs) == Prelude.null (xs :: [Int]) prop_singleton n = elems (fromList [n]) == [n :: Int] prop_insert n xs = trans (insert (n::Int)) xs == L.sort (n:xs) prop_member n xs = member n (fromList xs) == L.elem (n::Int) xs prop_memberDistance dist n xs = let d = dist `mod` 5 ref = L.any (\e -> distance n e <= d) xs in collect ref $ memberDistance d n (fromList xs) == L.any (\e -> distance n e <= d) (xs :: [Int]) prop_delete n xs = trans (delete n) xs == L.sort (removeFirst (xs :: [Int])) where removeFirst [] = [] removeFirst (a:as) | a == n = as | otherwise = a : removeFirst as prop_elems xs = L.sort (elems (fromList xs)) == L.sort (xs::[Int]) prop_elemsDistance dist n xs = let d = dist `mod` 5 in L.sort (elemsDistance d n (fromList xs)) == L.sort (filter (\e -> distance n e <= d) (xs::[Int])) prop_unions xss = sem (unions (map fromList xss)) == L.sort (concat (xss::[[Int]])) prop_union xs ys = sem (union (fromList xs) (fromList ys)) == L.sort (xs ++ (ys::[Int])) prop_closest n xs = case (closest n (fromList xs),xs) of (Nothing,[]) -> True (Just (_,d),ys) -> d == minimum (map (distance n) (ys::[Int])) _ -> False -- Testing the relations between operations prop_insertDelete n xs = trans (delete n . insert n) xs == L.sort (xs::[Int]) -- All the tests tests = [("empty", quickCheck prop_empty) ,("null", quickCheck prop_null) ,("singleton", quickCheck prop_singleton) ,("insert", quickCheck prop_insert) ,("member", quickCheck prop_member) ,("memberDistance", quickCheck prop_memberDistance) ,("delete", quickCheck prop_delete) ,("elems", quickCheck prop_elems) ,("elemsDistance", quickCheck prop_elemsDistance) ,("unions", quickCheck prop_unions) ,("union", quickCheck prop_union) ,("closest", quickCheck prop_closest) ,("insert/delete", quickCheck prop_insertDelete) ] runTests = mapM_ (\ (s,a) -> printf "%-25s :" s >> a) tests #endif