{-# LANGUAGE MultiParamTypeClasses #-}
{-# OPTIONS_GHC -fno-warn-orphans #-}

{- |
Module    : Data.SkeletalSet
Copyright : (c) Global Access Internet Services GmbH
License   : BSD3
Maintainer: Pavlo Kerestey <pavlo@kerestey.net>

A Haskell implementation of skeletal set - a set equipped with an 
equivalence relation. SkeletalSet is a useful data structure when
equivalence is chosen not to be equality. This allows to influence the
membership of the elements in a set.

Here we have chosen to use a specific variant of equivalence of
transforming the elements to comparable intermediaries. Although it
does not make every equivalence relation possible, it is a practical
choice for a lot of computations.

== Usage

When manipulating collections of objects in the real world, we often
use lists/arrays. Sometimes we need to represent some properties of
the relation between the elements though, and the lists do not provide
such possibility. This library not only provides the guarantee that a
skeletal set is correct by construction, but also that the manipulations
will not change its structure.

We use it to run computations over time series of sampling data,
collections of users (who are unique by username or email) - to keep
the same structure as the one which would be used in the database with
unique indexes.

To implement equivalence we chose to use a data class `EquivalenceBy`
which provides a method of mapping an element to an intermediary,
which is then used for comparison and ultimately lead to a choice
of the members.

The type is `SkeletalSet e a` where `a` is the member type and
e is the type of equivalence intermediary. To chose the members of the
skeletal set we compare the e(quivalences) of the elements with each other.

The definition of `EquivalenceBy e a` is

class EquivalenceBy e a where
  eqRel :: a -> e

To give a simple example of how the library could be used we will
combine apples and oranges to a SkeletalSet of fruit names by colour. We
want one fruit per colour as a result and don't care if its apple or
an orange.

import Data.SkeletalSet (SkeletalSet)
import qualified Data.SkeletalSet as SkeletalSet

data Colour = Red | Green | Blue deriving (Eq,Ord)

instance EquivalenceBy Colour (Colour,String) where
  eqRel = fst

apples, organges, fruits :: SkeletalSet Int (Int,String)
apples  = SkeletalSet.fromList [(Green,"golden delicious"), (Orange,"honeycrunch")]
oranges = SkeletalSet.fromList [(Orange,"seville"), (Red,"blood orange")]

fruits = apples `SkeletalSet.union` oranges
-- > [(Green,"golden delicious"), (Orange,"seville"), (Red,"blood orange")]

One can see the benefit of using a `SkeletalSet` instead of "Data.List"
because with the latter, we would have to use 'Data.List.nubBy' every
time the data is transformed.

When performing a `union`, our implementation would use `max` between
two equivalent elements to resolve the conflict. Bear in mind, that
the elements, though equivalent, might not be equal. In the example
above, ordering of @ "seville" @ is bigger than @ "golden delicious" @
thus @ ("Orange", "seville") @ is chosen in the result.

=== Friends of friends and computation on union

For another example, lets get all the users of two different services
F and G. We are not interested in the different details, but want the
instance of the users to be unique.

type Email = String
data User = User {
  email :: Email,
  contacts :: Int
  } deriving (Eq,Show)

instance EquivalenceBy Email User where
eqRel u = email u

usersF, usersG, allUsers :: SkeletalSet Email User
usersF <- getUsers F
usersG <- getUsers G

allUsers = SkeletalSet.unionWith mergeContactDetails usersF usersG

mergeContactDetails :: User -> User -> User
mergeContactDetails a b = User (email a) (contacts a + contacts b)
-- ... --

We assume that here are equivalent elements in both sets - in this
case they have the same email address. Thus we use `unionWith` to merge
the other details of the contact. Here, we could also do computations
and, for example, sum the number of friends/contacts from both

Here is also one of the shortcomings of the
library. mergeContactDetails choses the email of the first
argument. Since in the context of unionWith, the emails of the first
and the second users are the same. It is not nice from the perspective
of the function itself though.

@ SkeletalSet.size allUsers @ Would give us the amount of all unique users
in both services together.

== Future Work

- There is an unproven hypothesis about a relation between skeletal sets and
  Quotient Sets. It seems, that a `SkeletalSet (a,b) (a,b,c)` is equivalent
  to a `QuotientSet a (SkeletalSet b (a,b,c))`. This means that every
  QuotientSet can actually be represented as a skeletal set.

- Performance is another issue. Current implementation uses the
  `newtype SkeletalSet x y = SkeletalSet (Map x y)` which may be inefficient.

module Data.SkeletalSet
  ( -- * Type
    -- * Class
  , EquivalenceBy(..)
    -- * Operators
  , (=~=)
  , (\\)
  , ()
    -- * Construction
  , empty
  , ø
  , singleton
  , union
  , unions
  , unionWith
    -- * Difference
  , difference
    -- * Filter
  , filter
    -- * Query
  , null
  , size
  , member
  , equivalence
    -- * Traversal
    -- ** map
  , map
  , mapResolve
  , mapM
    -- * Conversion
  , fromList
  , fromListWith
  , toList
  ) where

import qualified Data.List               as List
import qualified Data.Map.Strict         as Map
import           Data.SkeletalSet.Equivalence
import           Data.SkeletalSet.Types
import           Prelude                 hiding (filter, lookup, map, mapM,
                                          mapM_, null, zip)
import qualified Prelude                 as P

-- | Instance Show, used for debugging
instance (Show a) =>
         Show (SkeletalSet e a) where
  show s = "{{ " ++ P.unlines (P.map show (toList s)) ++ " }}"

-- | Monoid instance for SkeletalSet
instance (Ord e, Ord a) =>
         Monoid (SkeletalSet e a) where
  mempty = empty
  mappend = union
  mconcat = unions

-- * Operators
infix 4 =~=

-- | Same as `equivalence`
  :: (Eq e)
  => SkeletalSet e a -> SkeletalSet e a -> Bool
(=~=) = equivalence

infix 5 \\

-- | Same as `difference`
  :: (Ord e)
  => SkeletalSet e a -> SkeletalSet e a -> SkeletalSet e a
(\\) = difference

-- | Same as `union`
  :: (Ord e, Ord a)
  => SkeletalSet e a -> SkeletalSet e a -> SkeletalSet e a
() = union

-- * Construction
-- | An empty SkeletalSet
empty :: SkeletalSet e a
empty = SkeletalSet Map.empty

-- | Same as `empty`
ø :: SkeletalSet e a
ø = empty

-- | A SkeletalSet with a single element
  :: (EquivalenceBy e a)
  => a -> SkeletalSet e a
singleton a = SkeletalSet (Map.singleton (eqRel a) a)

-- ** Combining
-- | Combine two SkeletalSets resolving conflicts with `max` by
-- default. This makes the union operation commutative and
-- associative.
  :: (Ord e, Ord a)
  => SkeletalSet e a -> SkeletalSet e a -> SkeletalSet e a
union = unionWith max

-- | A generalized variant of union which accepts a function that will
-- be used when two equivalent elements are found an the conflict
-- needs to be resolved. Note that the elements are not necessarily
-- equal
  :: (Ord e)
  => (a -> a -> a) -> SkeletalSet e a -> SkeletalSet e a -> SkeletalSet e a
unionWith f (SkeletalSet x1) (SkeletalSet x2) = SkeletalSet (Map.unionWith f x1 x2)

-- | Union several SkeletalSets into one. This uses de default union
-- variant
  :: (Ord e, Ord a)
  => [SkeletalSet e a] -> SkeletalSet e a
unions = List.foldl' union empty

-- ** Difference
-- | Difference of two skeletal sets. Return elements of the first skeletal sets
-- not existing in the second set.
  :: (Ord e)
  => SkeletalSet e a -> SkeletalSet e a -> SkeletalSet e a
difference (SkeletalSet x) (SkeletalSet y) = SkeletalSet (Map.difference x y)

-- ** Filter
-- | Filter a skeletal set. Return a skeletal set with elements that statisfy the
-- predicate
  :: (Ord e)
  => (a -> Bool) -> SkeletalSet e a -> SkeletalSet e a
filter p (SkeletalSet s) = SkeletalSet (Map.filter p s)

-- * Query
-- | Test if SkeletalSet is empty
null :: SkeletalSet e a -> Bool
null (SkeletalSet x) = Map.null x

-- | Get the size of a skeletal set
size :: SkeletalSet e a -> Int
size (SkeletalSet x) = Map.size x

-- | Test if an element is a member of a skeletal set
  :: (EquivalenceBy e a, Ord e)
  => a -> SkeletalSet e a -> Bool
member e (SkeletalSet x) = Map.member (eqRel e) x

-- | Test if two SkeletalSets are equivalent i.e. if all the elements are
-- equivalent
  :: (Eq e)
  => SkeletalSet e a -> SkeletalSet e a -> Bool
equivalence (SkeletalSet x) (SkeletalSet y) = Map.keys x == Map.keys y

-- * Traversal
-- | Map a function over elements of a skeletal set. It resolves conflict in
-- the result by chosing the maximum one
  :: (EquivalenceBy eb b, Ord eb, Ord b)
  => (a -> b) -> SkeletalSet ea a -> SkeletalSet eb b
map f a = fromList (P.map f (toList a))

-- | Generalized version of map, allowing to use custom function to
-- resolve a conflict if two equivalent elements are found in the
-- result
  :: (EquivalenceBy eb b, Ord eb)
  => (b -> b -> b) -- ^ conflict resolution function
  -> (a -> b)      -- ^ map function
  -> SkeletalSet ea a   -- ^ input
  -> SkeletalSet eb b   -- ^ result
mapResolve r f a = fromListWith r (P.map f (toList a))

-- | Monadic variant of a map
  :: (Monad m, EquivalenceBy eb b, Ord eb, Ord b)
  => (a -> m b) -> SkeletalSet ea a -> m (SkeletalSet eb b)
mapM f xs = fromList <$> P.mapM f (toList xs)

-- * Conversion
-- ** Lists
-- | Convert skeletal set into a List
toList :: SkeletalSet e a -> [a]
toList (SkeletalSet a) = Map.elems a

-- | A default variant of fromList using `max` to resolve a conflict
-- if two equivalent elements are found. Therefore it depends on Ord
-- instance of the element
  :: (EquivalenceBy e a, Ord e, Ord a)
  => [a] -> SkeletalSet e a
fromList = fromListWith max

-- | A generalized version of fromList, which will use a supplied
-- funtion if two equivalent elements are found in the input list
  :: (EquivalenceBy e a, Ord e)
  => (a -> a -> a) -> [a] -> SkeletalSet e a
fromListWith f = SkeletalSet . Map.fromListWith f . P.map (\x -> (eqRel x, x))
-- O(n+(n*log n))
-- An implementation of List.foldl' (\a b -> union a (singleton b)) empty
-- would be O(n*2n)