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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 - 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
-- * Construction
, selector, strategyClass
-- * Strategies
, strictFirst, strictLast
, rarestFirst, randomFirst, endGame
) where
import Data.Bitfield
import Data.Ratio
import Network.BitTorrent.PeerWire.Block
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.
selector :: Selector -- ^ Selector to use at the start.
-> Ratio PieceCount
-> Selector -- ^ Selector to use after the client have the C pieces.
-> Selector -- ^ Selector that changes behaviour based on completeness.
selector start pt ready h a xs =
case strategyClass pt h of
SCBeginning -> start h a xs
SCReady -> ready h a xs
SCEnd -> endGame h a xs
data StartegyClass
= SCBeginning
| SCReady
| SCEnd
deriving (Show, Eq, Ord, Enum, Bounded)
strategyClass :: Ratio PieceCount -> Bitfield -> StartegyClass
strategyClass threshold = classify . completeness
where
classify have
| have < threshold = SCBeginning
| have + 1 % numerator have < 1 = SCReady -- FIXME numerator have is not total count
| otherwise = SCEnd
-- | 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 (map (intersection want) xs)
where
want = difference h a
-- | In average random first is faster than rarest first strategy but
-- only if all pieces are available.
randomFirst :: Selector
randomFirst = do
-- randomIO
error "randomFirst"
endGame :: Selector
endGame = strictLast
|
