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|
-- TODO tests
-- |
-- Copyright : (c) Sam T. 2013
-- License : MIT
-- Maintainer : pxqr.sta@gmail.com
-- Stability : experimental
-- Portability : portable
--
-- This module provides commonly used piece seletion algorithms
-- which used to find out which one next piece to download.
-- Selectors considered to be used in the following order:
--
-- * Random first or rarest first selection - at the start.
--
-- * Rarest first selection - performed to avoid situation when
-- rarest piece is unaccessible.
--
-- * _End game_ seletion - performed after a peer has requested all
-- the subpieces of the content.
--
-- Note that BitTorrent applies the strict priority policy for
-- _subpiece_ or _blocks_ selection.
--
module Network.BitTorrent.PeerWire.Selection
( Selector
, strictFirst, strictLast
, rarestFirst, randomFirst, endGame
, autoSelector
) where
import Data.Bitfield
import Network.BitTorrent.PeerWire.Block
import Network.BitTorrent.PeerWire.Message
type Selector = Bitfield -- ^ Indices of client "have" pieces.
-> Bitfield -- ^ Indices of peer "have" pieces.
-> [Bitfield] -- ^ Indices of other peers "have" pieces.
-> Maybe PieceIx -- ^ Zero-based index of piece to request
-- to, if any.
-- | Select the first available piece.
strictFirst :: Selector
strictFirst h a _ = findMin (difference a h)
-- | Select the last available piece.
strictLast :: Selector
strictLast h a _ = findMax (difference a h)
-- |
rarestFirst :: Selector
rarestFirst h a xs = rarest (frequencies (map (intersection want) xs))
where
want = difference h a
rarest = Just . head
-- | In general random first is faster than rarest first strategy but
-- only if all pieces are available.
randomFirst :: IO Selector
randomFirst = do
-- randomIO
error "randomFirst"
endGame :: Selector
endGame = strictLast
autoSelector :: Selector
autoSelector = undefined
|
