{-# LANGUAGE CPP #-}

------------------------------------------------------------------------
-- |
-- Module      :  Data.HashSet
-- Copyright   :  2011 Bryan O'Sullivan
-- License     :  BSD-style
-- Maintainer  :  johan.tibell@gmail.com
-- Stability   :  provisional
-- Portability :  portable
--
-- A set of /hashable/ values.  A set cannot contain duplicate items.
-- A 'HashSet' makes no guarantees as to the order of its elements.
--
-- The implementation is based on /big-endian patricia trees/, indexed
-- by a hash of the original value.  A 'HashSet' is often faster than
-- other tree-based set types, especially when value comparison is
-- expensive, as in the case of strings.
--
-- Many operations have a worst-case complexity of /O(min(n,W))/.
-- This means that the operation can become linear in the number of
-- elements with a maximum of /W/ -- the number of bits in an 'Int'
-- (32 or 64).

module Data.HashSet
    (
      HashSet

    -- * Construction
    , empty
    , singleton

    -- * Combine
    , union

    -- * Basic interface
    , null
    , size
    , member
    , insert
    , delete

    -- * Transformations
    , map

    -- * Folds
    , foldl'
    , foldr

    -- * Filter
    , filter

    -- ** Lists
    , toList
    , fromList
    ) where

import Control.DeepSeq (NFData(..))
import Data.HashMap.Common (HashMap, foldrWithKey)
import Data.Hashable (Hashable)
import Data.Monoid (Monoid(..))
import Prelude hiding (filter, foldr, map, null)
import qualified Data.Foldable as Foldable
import qualified Data.HashMap.Lazy as H
import qualified Data.List as List

#if defined(__GLASGOW_HASKELL__)
import GHC.Exts (build)
#endif

-- | A set of values.  A set cannot contain duplicate values.
newtype HashSet a = HashSet {
      asMap :: HashMap a ()
    }

instance (NFData a) => NFData (HashSet a) where
    rnf = rnf . asMap
    {-# INLINE rnf #-}

instance (Hashable a, Eq a) => Eq (HashSet a) where
    -- This performs two passes over the tree.
    a == b = foldr f True b && size a == size b
        where f i = (&& i `member` a)
    {-# INLINE (==) #-}

instance Foldable.Foldable HashSet where
    foldr = Data.HashSet.foldr
    {-# INLINE foldr #-}

instance (Hashable a, Eq a) => Monoid (HashSet a) where
    mempty = empty
    {-# INLINE mempty #-}
    mappend = union
    {-# INLINE mappend #-}

-- | /O(1)/ Construct an empty set.
empty :: HashSet a
empty = HashSet H.empty

-- | /O(1)/ Construct a set with a single element.
singleton :: Hashable a => a -> HashSet a
singleton a = HashSet (H.singleton a ())
#if __GLASGOW_HASKELL__ >= 700
{-# INLINABLE singleton #-}
#endif

-- | /O(n)/ Construct a set containing all elements from both sets.
union :: (Eq a, Hashable a) => HashSet a -> HashSet a -> HashSet a
union s1 s2 = HashSet $ H.union (asMap s1) (asMap s2)
{-# INLINE union #-}

-- | /O(1)/ Return 'True' if this set is empty, 'False' otherwise.
null :: HashSet a -> Bool
null = H.null . asMap
{-# INLINE null #-}

-- | /O(n)/ Return the number of elements in this set.
size :: HashSet a -> Int
size = H.size . asMap
{-# INLINE size #-}

-- | /O(min(n,W))/ Return 'True' if the given value is present in this
-- set, 'False' otherwise.
member :: (Eq a, Hashable a) => a -> HashSet a -> Bool
member a s = case H.lookup a (asMap s) of
               Just _ -> True
               _      -> False
{-# INLINE member #-}

-- | /O(min(n,W))/ Add the specified value to this set.
insert :: (Eq a, Hashable a) => a -> HashSet a -> HashSet a
insert a = HashSet . H.insert a () . asMap
{-# INLINE insert #-}

-- | /O(min(n,W))/ Remove the specified value from this set if
-- present.
delete :: (Eq a, Hashable a) => a -> HashSet a -> HashSet a
delete a = HashSet . H.delete a . asMap
{-# INLINE delete #-}

-- | /O(n)/ Transform this set by applying a function to every value.
-- The resulting set may be smaller than the source.
map :: (Hashable b, Eq b) => (a -> b) -> HashSet a -> HashSet b
map f = fromList . List.map f . toList
#if __GLASGOW_HASKELL__ >= 700
{-# INLINABLE map #-}
#endif

-- | /O(n)/ Reduce this set by applying a binary operator to all
-- elements, using the given starting value (typically the
-- left-identity of the operator).  Each application of the operator
-- is evaluated before before using the result in the next
-- application.  This function is strict in the starting value.
foldl' :: (a -> b -> a) -> a -> HashSet b -> a
foldl' f z0 = H.foldlWithKey' g z0 . asMap
  where g z k _ = f z k
{-# INLINE foldl' #-}

-- | /O(n)/ Reduce this set by applying a binary operator to all
-- elements, using the given starting value (typically the
-- right-identity of the operator).
foldr :: (b -> a -> a) -> a -> HashSet b -> a
foldr f z0 = foldrWithKey g z0 . asMap
  where g k _ z = f k z
{-# INLINE foldr #-}

-- | /O(n)/ Filter this set by retaining only elements satisfying a
-- predicate.
filter :: (a -> Bool) -> HashSet a -> HashSet a
filter p = HashSet . H.filterWithKey q . asMap
  where q k _ = p k
{-# INLINE filter #-}

-- | /O(n)/ Return a list of this set's elements.  The list is
-- produced lazily.
toList :: HashSet a -> [a]
#if defined(__GLASGOW_HASKELL__)
toList t = build (\ c z -> foldrWithKey ((const .) c) z (asMap t))
#else
toList = foldrWithKey (\ k _ xs -> k : xs) [] . asMap
#endif
{-# INLINE toList #-}

-- | /O(n*min(W, n))/ Construct a set from a list of elements.
fromList :: (Eq a, Hashable a) => [a] -> HashSet a
fromList = HashSet . List.foldl' (\ m k -> H.insert k () m) H.empty
{-# INLINE fromList #-}