local cache query lookup + some validation considerations

no unit tests done so far

Hash2Pub/Hash2Pub.cabal

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

Hash2Pub/src/Hash2Pub/DHTProtocol.hs

@@ -0,0 +1,49 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Hash2Pub.DHTProtocol
+    ( QueryResponse (..)
+    , incomingQuery
+    )
+        where
+
+import Data.Maybe (catMaybes)
+import qualified Data.Map.Strict as M
+import Hash2Pub.FediChord
+  ( NodeID
+  , NodeState (..)
+  , cacheGetNodeStateUnvalidated
+  , NodeCache
+  , CacheEntry
+  )
+
+data QueryResponse = FORWARD [CacheEntry] -- ^return closest nodes from local cache.
+                                          -- whole cache entry is returned for making
+                                          -- the entry time stamp available to the
+                                          -- protocol serialiser
+                   | FOUND NodeState     -- ^node is the responsible node for queried ID
+
+-- TODO: evaluate more fine-grained argument passing to allow granular locking
+-- | look up an ID to either claim responsibility for it or return the closest l nodes from the local cache
+incomingQuery :: NodeState -> NodeCache -> Int -> NodeID -> QueryResponse
+incomingQuery ownState nCache lBestNodes targetID
+    -- as target ID falls between own ID and first predecessor, it is handled by this node
+    | targetID <= nid ownState && targetID > (head . predecessors) ownState = FOUND ownState
+    -- my interpretation: the "l best next hops" are the l-1 closest preceding nodes and
+    -- the closest succeeding node (like with the p initiated parallel queries
+    | otherwise = FORWARD . catMaybes $ closestSuccessor : closestPredecessors
+  where
+    closestSuccessor :: Maybe CacheEntry
+    closestSuccessor = snd <$> M.lookupGT targetID nCache
+
+    closestPredecessors :: [Maybe CacheEntry]
+    closestPredecessors = closestPredecessor (lBestNodes-1) $ nid ownState
+    closestPredecessor :: (Integral n) => n -> NodeID -> [Maybe CacheEntry]
+    closestPredecessor 0 _ = []
+    closestPredecessor remainingLookups lastID =
+        let result = predecessorLookup nCache lastID
+      in
+        case result of
+          Nothing -> []
+          Just nPred -> result:closestPredecessor (remainingLookups-1) (nid . cacheGetNodeStateUnvalidated $ nPred)
+    predecessorLookup :: NodeCache -> NodeID -> Maybe CacheEntry
+    predecessorLookup nCache' lastID = snd <$> M.lookupLT lastID nCache' 

Hash2Pub/src/Hash2Pub/FediChord.hs

@@ -14,7 +14,9 @@ module Hash2Pub.FediChord (
   , getNodeID
   , toNodeID
   , NodeState (..)
+  , NodeCache
   , CacheEntry
+  , cacheGetNodeStateUnvalidated
   , genNodeID
   , genNodeIDBS
   , genKeyID
@@ -100,7 +102,9 @@ data NodeState = NodeState {
   -- ^ 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] -- could be a set instead as these are ordered as well
+  -- ^ successor nodes in ascending order by distance
   , predecessors :: [NodeID]
+  -- ^ predecessor nodes in ascending order by distance
   ----- protocol parameters -----
   -- TODO: evaluate moving these somewhere else
   , kNeighbours :: Int
@@ -126,6 +130,10 @@ type CacheEntry = (
   , SystemTime
                   )   -- ^ ( a node's validation status, data, timestamp for cache entry expiration )
 
+-- | return the @NodeState@ data from a cache entry without checking its validation status
+cacheGetNodeStateUnvalidated :: CacheEntry -> NodeState
+cacheGetNodeStateUnvalidated (_, nState, _) = nState
+
 -- | generates a 256 bit long NodeID using SHAKE128, represented as ByteString
 genNodeIDBS :: HostAddress6     -- ^a node's IPv6 address
             -> String           -- ^a node's 1st and 2nd level domain name