EpiChord: create a stub cache-invariant checker

rough logic is implemented, but only for successors and not
predecessors.
Additionally it just returns empty IO actions instead of lookup
operations, until those are implemented.
part of #1

Untested!

Hash2Pub/Hash2Pub.cabal

@@ -58,7 +58,7 @@ library
   exposed-modules:     Hash2Pub.FediChord
 
   -- Modules included in this library but not exported.
-  -- other-modules:
+  other-modules: Hash2Pub.Utils
 
   -- LANGUAGE extensions used by modules in this package.
   other-extensions:    GeneralizedNewtypeDeriving, DataKinds, OverloadedStrings

Hash2Pub/src/Hash2Pub/FediChord.hs

@@ -36,6 +36,8 @@ import qualified Data.ByteString.Lazy as BL
 import qualified Data.ByteString.UTF8 as BSU
 import qualified Data.ByteArray as BA
 
+import Hash2Pub.Utils
+
 -- define protocol constants
 -- | static definition of ID length in bits
 idBits :: Integer
@@ -94,7 +96,7 @@ data NodeState = NodeState {
   , apPort :: Maybe PortNumber
   -- ^ port of the ActivityPub relay and storage service
   -- might have to be queried first
-  , nodeCache :: Map.Map NodeID CacheEntry
+  , nodeCache :: NodeCache
   -- ^ EpiChord node cache with expiry times for nodes
   -- as the map is ordered, lookups for the closes preceding node can be done using @lookupLT@
   , successors :: [NodeID]
@@ -110,8 +112,13 @@ data NodeState = NodeState {
   , pNumParallelQueries :: Int
   -- ^ number of parallel sent queries
   -- needs to be parameterisable for simulation purposes
+  , jEntriesPerSlice :: Int
+  -- ^ number of desired entries per cache slice
+  -- needs to be parameterisable for simulation purposes
   } deriving (Show)
 
+type NodeCache = Map.Map NodeID CacheEntry
+
 -- |an entry of the 'nodeCache'
 type CacheEntry = (
     NodeState
@@ -168,6 +175,44 @@ byteStringToUInteger bs = sum $ parsedBytes 0 bs
 
 
 
+-- TODO: needs testing
+-- |checks wether the cache entries fulfill the logarithmic EpiChord invariant
+-- of having j entries per slice, and creates a list of necessary lookup actions.
+-- Should be invoked periodically.
+checkCacheSlices :: NodeState -> [IO ()]
+checkCacheSlices state =
+    checkSlice jEntries (nid state) startBound lastSucc cache'
+    -- TODO: do the same for predecessors
+    where
+        jEntries = jEntriesPerSlice state
+        cache' = nodeCache state
+        lastSucc = last <$> maybeEmpty (successors state)
+        startBound = NodeID 2^(255::Integer) + nid state
+        checkSlice :: Int -> NodeID -> NodeID -> Maybe NodeID -> NodeCache -> [IO ()]
+        checkSlice _ _ _ Nothing _ = []
+        checkSlice j ownID upperBound (Just lastSuccNode) cache
+            | upperBound < lastSuccNode = []
+            | otherwise = 
+                -- continuously half the DHT namespace, take the upper part as a slice,
+                -- check for existing entries in that slice and create a lookup action
+                -- and recursively do this on the lower half.
+                -- recursion edge case: all successors/ predecessors need to be in the
+                -- first slice.
+                let
+                  diff = getNodeID $ upperBound - ownID
+                  lowerBound = ownID + NodeID (diff `div` 2)
+                in
+                   -- TODO: replace empty IO actions with actual lookups
+                   -- TODO: validate ID before adding to cache
+                   case Map.lookupLT upperBound cache of
+                       Nothing -> return () : checkSlice j ownID lowerBound (Just lastSuccNode) cache
+                       Just (matchID, _) ->
+                           if
+                               matchID <= lowerBound then return () : checkSlice j ownID lowerBound (Just lastSuccNode) cache
+                           else
+                               checkSlice j ownID lowerBound (Just lastSuccNode) cache
+
+
 -- Todo: DHT backend can learn potential initial bootstrapping points through the instances mentioned in the received AP-relay messages
 -- needs to know its own domain anyways for ID generation
 -- persist them on disk so they can be used for all following bootstraps

Hash2Pub/src/Hash2Pub/Utils.hs

@@ -0,0 +1,6 @@
+module Hash2Pub.Utils where
+
+-- |wraps a list into a Maybe, by replacing empty lists with Nothing
+maybeEmpty :: [a] -> Maybe [a]
+maybeEmpty [] = Nothing
+maybeEmpty nonemptyList = Just nonemptyList