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{-|

  Module abstracting the peer map implementation.

This is abstracted separately since the speed of peermap updates can

be a significant part of the total runtime, and as such changing the

implementation should be easy in case it's needed.

-}

{-

Copyright (C) 2009 Google Inc.

This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or

(at your option) any later version.

This program is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with this program; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

02110-1301, USA.

-}

module Ganeti.HTools.PeerMap

    ( PeerMap

    , Key

    , Elem

    , empty

    , accumArray

    , Ganeti.HTools.PeerMap.find

    , add

    , remove

    , maxElem

    ) where

import Data.Maybe (fromMaybe)

import Data.List

import Data.Ord (comparing)

import Ganeti.HTools.Types

type Key = Ndx

type Elem = Int

type PeerMap = [(Key, Elem)]

-- * Initialization functions

-- | Create a new empty map.

empty :: PeerMap

empty = []

-- | Our reverse-compare function.

pmCompare :: (Key, Elem) -> (Key, Elem) -> Ordering

pmCompare a b = comparing snd b a

-- | Add or update (via a custom function) an element.

addWith :: (Elem -> Elem -> Elem) -> Key -> Elem -> PeerMap -> PeerMap

addWith fn k v lst =

    case lookup k lst of

      Nothing -> insertBy pmCompare (k, v) lst

      Just o -> insertBy pmCompare (k, fn o v) (remove k lst)

-- | Create a PeerMap from an association list, with possible duplicates

accumArray :: (Elem -> Elem -> Elem) -- ^ function used to merge the elements

              -> [(Key, Elem)]       -- ^ source data

              -> PeerMap             -- ^ results

accumArray _  [] = empty

accumArray fn ((k, v):xs) = addWith fn k v $ accumArray fn xs

-- * Basic operations

-- | Returns either the value for a key or zero if not found

find :: Key -> PeerMap -> Elem

find k = fromMaybe 0 . lookup k

-- | Add an element to a peermap, overwriting the previous value

add :: Key -> Elem -> PeerMap -> PeerMap

add = addWith (flip const)

-- | Remove an element from a peermap

remove :: Key -> PeerMap -> PeerMap

remove _ [] = []

remove k ((x@(x', _)):xs) = if k == x'

                            then xs

                            else x:remove k xs

-- | Find the maximum element.

--

-- Since this is a sorted list, we just get the value at the head of

-- the list, or zero for a null list

maxElem :: PeerMap -> Elem

maxElem (x:_) = snd x

maxElem _ = 0