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diff --git a/src/Data/Kademlia/Routing/Bucket.hs b/src/Data/Kademlia/Routing/Bucket.hs
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+-- |
+--   Copyright   :  (c) Sam T. 2013
+--   License     :  MIT
+--   Maintainer  :  pxqr.sta@gmail.com
+--   Stability   :  experimental
+--   Portability :  portable
+--
+--   Bucket is used to
+--
+--   Bucket is kept sorted by time last seen — least-recently seen
+--   node at the head, most-recently seen at the tail. Reason: when we
+--   insert a node into the bucket we first filter nodes with smaller
+--   lifetime since they more likely leave network and we more likely
+--   don't reach list end. This should reduce list traversal, we don't
+--   need to reverse list in insertion routines.
+--
+--   Bucket is also limited in its length — thus it's called k-bucket.
+--   When bucket becomes full we should split it in two lists by
+--   current span bit. Span bit is defined by depth in the routing
+--   table tree. Size of the bucket should be choosen such that it's
+--   very unlikely that all nodes in bucket fail within an hour of
+--   each other.
+--
+{-# LANGUAGE RecordWildCards #-}
+module Data.Kademlia.Routing.Bucket
+       ( Bucket(maxSize, kvs)
+
+         -- * Query
+       , size, isFull, member
+
+         -- * Construction
+       , empty, singleton
+
+         -- * Modification
+       , enlarge, split, insert
+
+         -- * Defaults
+       , defaultBucketSize
+       ) where
+
+import Control.Applicative hiding (empty)
+import Data.Bits
+import Data.List as L hiding (insert)
+
+
+type Size = Int
+
+data Bucket k v = Bucket {
+    -- | We usually use equally sized buckets in the all routing table
+    -- so keeping max size in each bucket lead to redundancy. Altrough
+    -- it allow us to use some interesting schemes in route tree.
+    maxSize :: Size
+
+    -- | Key -> value pairs as described above.
+    --   Each key in a given bucket should be unique.
+  , kvs     :: [(k, v)]
+  }
+
+-- | Gives /current/ size of bucket.
+--
+--   forall bucket. size bucket <= maxSize bucket
+--
+size :: Bucket k v -> Size
+size = L.length . kvs
+
+isFull :: Bucket k v -> Bool
+isFull Bucket {..} = L.length kvs == maxSize
+
+member :: Eq k => k -> Bucket k v -> Bool
+member k = elem k . map fst . kvs
+
+empty :: Size -> Bucket k v
+empty s = Bucket (max 0 s) []
+
+singleton :: Size -> k -> v -> Bucket k v
+singleton s k v = Bucket (max 1 s) [(k, v)]
+
+
+-- | Increase size of a given bucket.
+enlarge :: Size -> Bucket k v -> Bucket k v
+enlarge additional b = b { maxSize = maxSize b + additional }
+
+split :: Bits k => Int -> Bucket k v -> (Bucket k v, Bucket k v)
+split index Bucket {..} =
+    let (far, near) = partition spanBit kvs
+    in (Bucket maxSize near, Bucket maxSize far)
+  where
+    spanBit = (`testBit` index) . fst
+
+
+-- move elem to the end in one traversal
+moveToEnd :: Eq k => (k, v) -> Bucket k v -> Bucket k v
+moveToEnd kv@(k, _) b = b { kvs = go (kvs b) }
+  where
+    go [] = []
+    go (x : xs)
+      | fst x == k = xs ++ [kv]
+      | otherwise  = x : go xs
+
+insertToEnd :: (k, v) -> Bucket k v -> Bucket k v
+insertToEnd kv b = b { kvs = kvs b ++ [kv] }
+
+-- | * If the info already exists in bucket then move it to the end.
+--
+--   * If bucket is not full then insert the info to the end.
+--
+--   * If bucket is full then ping the least recently seen node.
+--     Here we have a choice:
+--
+--         If node respond then move it the end and discard node
+--         we  want to insert.
+--
+--         If not remove it from the bucket and add the
+--         (we want to insert) node to the end.
+--
+insert :: Applicative f => Eq k
+       => (v ->  f Bool)  -- ^ Ping RPC
+       -> (k, v) -> Bucket k v -> f (Bucket k v)
+
+insert ping new bucket@(Bucket {..})
+    | fst new `member` bucket = pure (new `moveToEnd` bucket)
+    | size bucket < maxSize   = pure (new `insertToEnd` bucket)
+    | least : rest <- kvs     =
+      let select alive = if alive then least else new
+          mk most = Bucket maxSize (rest ++ [most])
+      in mk . select <$> ping (snd least)
+      where
+--    | otherwise                 = pure bucket
+     -- WARN: or maybe error "insertBucket: max size should not be 0" ?
+
+lookup :: k -> Bucket k v -> Maybe v
+lookup = undefined
+
+closest :: Int -> k -> Bucket k v -> [(k, v)]
+closest = undefined
+
+-- | Most clients use this value for maximum bucket size.
+defaultBucketSize :: Int
+defaultBucketSize = 20