actor-0.1: Actor/QList.hs

{-
--------------------------------------------------------------------------------
--
-- Copyright (C) 2008 Martin Sulzmann, Edmund Lam. All rights reserved.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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-}

module Actor.QList where

import IO
import Monad
import Data.IORef

import Actor.SList

-- a QList is a queue (represented as a list) of elements pointing to SList elements

data QItem a = QNode { valQ :: IORef (Item a)
                     , nextQ :: IORef (QItem a) }
            | QNull
            | QHead { nextQ :: IORef (QItem a) }

data QList a = QList { headQ :: IORef (IORef (QItem a)), 
                       tailQ :: IORef (IORef (QItem a)) }

-- we assume a static head pointer, pointing to the first node which must be Head
-- tail points to the last node which must be Null

type QIterator a = IORef (IORef (QItem a))


-- basic api

-- we create a new Qlist
newQList :: IO (QList a)
newQList = 
   do null <- newIORef QNull
      hd <- newIORef (QHead null)
      hdPtr <- newIORef hd
      tailPtr <- newIORef null
      return (QList {headQ = hdPtr, tailQ = tailPtr})


-- we add a new node, by overwriting the null tail node
-- we only need to adjust tail but not head because
-- of the static Head
-- we return the location of the newly added node
enQueue :: QList a -> IORef (Item a) -> IO ()
enQueue (QList {tailQ = tailPtrPtr}) x =
   do null <- newIORef QNull
      tailPtr <- readIORef tailPtrPtr
      writeIORef tailPtr (QNode {valQ = x, nextQ = null})
      writeIORef tailPtrPtr null


-- the iterator always points to the PREVIOUS node,
-- recall that there's a static dummy new Head
newQIterator :: QList a -> IO (QIterator a)
newQIterator (QList {headQ = hd}) =
  do hdPtr <- readIORef hd
     it <- newIORef hdPtr
     return it



-- we iterate through the list and return the first "not deleted" node
-- we delink deleted nodes
-- there's no need to adjust head, tail
-- cause head has a static Head and
-- tail points to Null
iterateQList :: QIterator a -> IO (Maybe (IORef (Item a)))
iterateQList itPtrPtr = 
  let go prevPtr =
        do prevNode <- readIORef prevPtr
           let curPtr = nextQ prevNode -- the prev node has always a next field
                                      -- recall the static dummy head node
           curNode <- readIORef curPtr
           case curNode of
             QNull -> return Nothing -- reached end of the list
             QNode { valQ = itemPtr, nextQ = nextNode } ->
                do  Node { deleted = del } <- readIORef itemPtr   -- check element in store
                    if not del                                    -- if deleted
                     then -- node available
                       do writeIORef itPtrPtr curPtr  -- adjust iterator
                          return (Just itemPtr) 
                     else -- delete curNode by setting the next of prevNode to next of curNode
                     do case prevNode of
                          QHead {} -> writeIORef prevPtr (QHead {nextQ = nextNode})
                          QNode {} -> writeIORef prevPtr 
                                       (QNode {valQ = valQ prevNode, nextQ = nextNode})
                        go prevPtr 
  in do startPtr <- readIORef itPtrPtr
        go startPtr