From 451a6eeec18b80f06378acba82e2b2dc56275188 Mon Sep 17 00:00:00 2001
From: James Crayne <jim.crayne@gmail.com>
Date: Wed, 30 May 2018 06:38:54 +0000
Subject: Data.CyclicBuffer, wip

---
 src/Data/CyclicBuffer.hs | 100 +++++++++++++++++++++++++++++++++++++++++++++++
 src/Data/PacketQueue.hs  |   4 +-
 2 files changed, 102 insertions(+), 2 deletions(-)
 create mode 100644 src/Data/CyclicBuffer.hs

(limited to 'src/Data')

diff --git a/src/Data/CyclicBuffer.hs b/src/Data/CyclicBuffer.hs
new file mode 100644
index 00000000..ab022e3f
--- /dev/null
+++ b/src/Data/CyclicBuffer.hs
@@ -0,0 +1,100 @@
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE FlexibleContexts #-}
+module Data.CyclicBuffer where
+
+import Control.Concurrent.STM
+import Control.Concurrent.STM.TArray
+import Control.Monad
+import Control.Applicative
+import Data.Word
+import Data.Array.MArray
+import Data.Maybe
+
+data CyclicBuffer a = CyclicBuffer
+    { vwflgs  :: TArray Word32 Bool -- TODO: Use TVar. TArray Word32 (TVar Bool)
+                                    --       This would allow updating by external code.
+                                    --       The TVar could be returned from dequeue
+    , pktq    :: TArray Word32 (Maybe a)
+    , seqno   :: TVar Word32
+    , qsize   :: Word32
+    , buffend :: TVar Word32 -- on incoming, highest packet number handled + 1
+    , dropCnt :: TVar Word32
+    }
+
+cyclicBufferViewList :: CyclicBuffer a -> STM [(Word32,a)]
+cyclicBufferViewList p = do
+    let f (n,Nothing) = Nothing
+        f (n,Just x)  = Just (n,x)
+    catMaybes . map f <$> getAssocs (pktq p)
+
+getCapacity :: Applicative m => CyclicBuffer t -> m Word32
+getCapacity (CyclicBuffer { qsize }) = pure qsize
+
+-- | Create a new CyclicBuffer with Overwrite on Wrap.
+new :: Word32 -- ^ Capacity of queue.
+    -> Word32 -- ^ Initial sequence number.
+    -> STM (CyclicBuffer a)
+new capacity seqstart = do
+    let cap = if capacity `mod` 2 == 0 then capacity else capacity + 1
+    q <- newArray (0,cap - 1) Nothing
+    flgs <- newArray (0,cap - 1) False
+    seqv <- newTVar seqstart
+    bufe <- newTVar 0
+    dropped <- newTVar 0
+    return CyclicBuffer
+        { vwflgs  = flgs
+        , pktq    = q
+        , seqno   = seqv
+        , qsize   = cap
+        , buffend = bufe
+        , dropCnt = dropped
+        }
+
+observeOutOfBand :: CyclicBuffer a -> Word32-> STM ()
+observeOutOfBand CyclicBuffer { seqno, qsize, buffend } no = do
+    low <- readTVar seqno
+    let proj = no - low
+    -- Ignore packet if out of range.
+    when ( proj < qsize) $ do
+    modifyTVar' buffend (\be -> if be - low <= proj then no + 1 else be)
+
+
+-- | Retry until the next expected packet is enqueued.  Then return it.
+dequeue :: CyclicBuffer a -> STM a
+dequeue CyclicBuffer { pktq, seqno, qsize } = do
+    i0 <- readTVar seqno
+    let i = i0 `mod` qsize
+    x <- maybe retry return =<< readArray pktq i
+    writeArray pktq i Nothing
+    modifyTVar' seqno   succ
+    return x
+
+-- | Like dequeue, but just marks as viewed rather than removing
+markButNotDequeue :: CyclicBuffer a -> STM a
+markButNotDequeue CyclicBuffer { vwflgs, pktq, seqno, qsize } = do
+    i0 <- readTVar seqno
+    let i = i0 `mod` qsize
+    x <- maybe retry return =<< readArray pktq i
+    writeArray vwflgs i True
+    modifyTVar' seqno   succ
+    return x
+
+-- | Enqueue a packet.  Packets need not be enqueued in order as long as there
+-- is spare capacity in the queue.  If the capacity is exceeded, packets are
+-- dropped and the drop count increased accordingly.
+enqueue :: CyclicBuffer a -- ^ The cyclic buffer(queue)
+        -> Word32        -- ^ Sequence number of the packet.
+        -> a             -- ^ The packet.
+        -> STM ()
+enqueue CyclicBuffer{vwflgs, pktq, seqno, qsize, buffend, dropCnt} no x = do
+    low <- readTVar seqno
+    let proj = no - low
+    let i = no `mod` qsize
+    when (proj >= qsize) $ do
+        viewed <- readArray vwflgs i
+        when (not viewed) $
+            modifyTVar' dropCnt (+1)
+    writeArray pktq i (Just x)
+    writeArray vwflgs i False -- mark as not viewed
+    modifyTVar' buffend (\be -> if be - low <= proj then no + 1 else be)
+    return ()
diff --git a/src/Data/PacketQueue.hs b/src/Data/PacketQueue.hs
index 4f0f04e3..144c8de0 100644
--- a/src/Data/PacketQueue.hs
+++ b/src/Data/PacketQueue.hs
@@ -37,7 +37,7 @@ data PacketQueue a = PacketQueue
     , seqno   :: TVar Word32
     , qsize   :: Word32
     , buffend :: TVar Word32 -- on incoming, highest packet number handled + 1
-    , qOverWriteMode :: Bool
+    , qOverWriteMode :: Bool -- TODO: Remove me, use Data.CyclicBuffer instead.
     }
 
 packetQueueViewList :: PacketQueue a -> STM [(Word32,a)]
@@ -102,7 +102,7 @@ dequeue PacketQueue { pktq, seqno, qsize } = do
     modifyTVar' seqno   succ
     return x
 
--- | Like dequeue, but marks as handled rather than removing
+-- | Like dequeue, but marks as viewed rather than removing
 markButNotDequeue :: PacketQueue (Bool,a) -> STM a
 markButNotDequeue PacketQueue { pktq, seqno, qsize } = do
     i0 <- readTVar seqno
-- 
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