-- |
--   Copyright   :  (c) Sam Truzjan 2013
--   License     :  BSD3
--   Maintainer  :  pxqr.sta@gmail.com
--   Stability   :  experimental
--   Portability :  portable
--
--   Blocks are used to transfer pieces.
--
{-# LANGUAGE BangPatterns               #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE DeriveFunctor              #-}
{-# LANGUAGE DeriveDataTypeable         #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
module Network.BitTorrent.Exchange.Block
       ( -- * Block attributes
         BlockOffset
       , BlockCount
       , BlockSize
       , defaultTransferSize

         -- * Block index
       , BlockIx(..)
       , blockIxRange

         -- * Block data
       , Block(..)
       , blockIx
       , blockSize
       , blockRange
       , isPiece
       , leadingBlock

         -- * Block bucket
       , Bucket

         -- ** Query
       , Network.BitTorrent.Exchange.Block.null
       , Network.BitTorrent.Exchange.Block.full
       , Network.BitTorrent.Exchange.Block.size
       , Network.BitTorrent.Exchange.Block.spans

         -- ** Construction
       , Network.BitTorrent.Exchange.Block.empty
       , Network.BitTorrent.Exchange.Block.insert
       , Network.BitTorrent.Exchange.Block.insertLazy
       , Network.BitTorrent.Exchange.Block.merge
       , Network.BitTorrent.Exchange.Block.fromList

         -- ** Rendering
       , Network.BitTorrent.Exchange.Block.toPiece

         -- ** Debug
       , Network.BitTorrent.Exchange.Block.valid
       ) where

import Prelude hiding (span)
import Control.Applicative
import Data.ByteString as BS hiding (span)
import Data.ByteString.Lazy as BL hiding (span)
import Data.ByteString.Lazy.Builder as BS
import Data.Default
import Data.Monoid
import Data.List as L hiding (span)
import Data.Serialize as S
import Data.Typeable
import Numeric
import Text.PrettyPrint as PP hiding ((<>))
import Text.PrettyPrint.Class

import Data.Torrent

{-----------------------------------------------------------------------
--  Block attributes
-----------------------------------------------------------------------}

-- | Offset of a block in a piece in bytes. Should be multiple of
-- the choosen block size.
type BlockOffset = Int

-- | Size of a block in bytes. Should be power of 2.
--
--   Normally block size is equal to 'defaultTransferSize'.
--
type BlockSize   = Int

-- | Number of block in a piece of a torrent. Used to distinguish
-- block count from piece count.
type BlockCount  = Int

-- | Widely used semi-official block size. Some clients can ignore if
-- block size of BlockIx in Request message is not equal to this
-- value.
--
defaultTransferSize :: BlockSize
defaultTransferSize = 16 * 1024

{-----------------------------------------------------------------------
    Block Index
-----------------------------------------------------------------------}

-- | BlockIx correspond.
data BlockIx = BlockIx {
    -- | Zero-based piece index.
    ixPiece  :: {-# UNPACK #-} !PieceIx

    -- | Zero-based byte offset within the piece.
  , ixOffset :: {-# UNPACK #-} !BlockOffset

    -- | Block size starting from offset.
  , ixLength :: {-# UNPACK #-} !BlockSize
  } deriving (Show, Eq, Typeable)

-- | First block in torrent. Useful for debugging.
instance Default BlockIx where
  def = BlockIx 0 0 defaultTransferSize

getInt :: S.Get Int
getInt = fromIntegral <$> S.getWord32be
{-# INLINE getInt #-}

putInt :: S.Putter Int
putInt = S.putWord32be . fromIntegral
{-# INLINE putInt #-}

instance Serialize BlockIx where
  {-# SPECIALIZE instance Serialize BlockIx #-}
  get = BlockIx <$> getInt
                <*> getInt
                <*> getInt
  {-# INLINE get #-}

  put BlockIx {..} = do
    putInt ixPiece
    putInt ixOffset
    putInt ixLength
  {-# INLINE put #-}

instance Pretty BlockIx where
  pretty BlockIx {..} =
    ("piece  = " <> int ixPiece  <> ",") <+>
    ("offset = " <> int ixOffset <> ",") <+>
    ("length = " <> int ixLength)

-- | Get location of payload bytes in the torrent content.
blockIxRange :: (Num a, Integral a) => PieceSize -> BlockIx -> (a, a)
blockIxRange piSize BlockIx {..} = (offset, offset + len)
  where
    offset = fromIntegral piSize * fromIntegral ixPiece
           + fromIntegral ixOffset
    len    = fromIntegral ixLength
{-# INLINE blockIxRange #-}

{-----------------------------------------------------------------------
    Block
-----------------------------------------------------------------------}

data Block payload = Block {
    -- | Zero-based piece index.
    blkPiece  :: {-# UNPACK #-} !PieceIx

    -- | Zero-based byte offset within the piece.
  , blkOffset :: {-# UNPACK #-} !BlockOffset

    -- | Payload bytes.
  , blkData   :: !payload
  } deriving (Show, Eq, Functor, Typeable)

-- | Payload is ommitted.
instance Pretty (Block BL.ByteString) where
  pretty = pretty . blockIx
  {-# INLINE pretty #-}

-- | Get size of block /payload/ in bytes.
blockSize :: Block BL.ByteString -> BlockSize
blockSize = fromIntegral . BL.length . blkData
{-# INLINE blockSize #-}

-- | Get block index of a block.
blockIx :: Block BL.ByteString -> BlockIx
blockIx = BlockIx <$> blkPiece <*> blkOffset <*> blockSize

-- | Get location of payload bytes in the torrent content.
blockRange :: (Num a, Integral a)
           => PieceSize -> Block BL.ByteString -> (a, a)
blockRange piSize = blockIxRange piSize . blockIx
{-# INLINE blockRange #-}

-- | Test if a block can be safely turned into a piece.
isPiece :: PieceSize -> Block BL.ByteString -> Bool
isPiece pieceLen blk @ (Block i offset _) =
     offset == 0 && blockSize blk == pieceLen && i >= 0
{-# INLINE isPiece #-}

-- | First block in the piece.
leadingBlock :: PieceIx -> BlockSize -> BlockIx
leadingBlock pix blockSize = BlockIx
  { ixPiece  = pix
  , ixOffset = 0
  , ixLength = blockSize
  }
{-# INLINE leadingBlock #-}

{-----------------------------------------------------------------------
--  Bucket
-----------------------------------------------------------------------}

type Pos  = Int
type ChunkSize = Int

-- | A sparse set of blocks used to represent an /in progress/ piece.
data Bucket
  = Nil
  | Span {-# UNPACK #-} !ChunkSize          !Bucket
  | Fill {-# UNPACK #-} !ChunkSize !Builder !Bucket

instance Show Bucket where
  showsPrec i  Nil          = showString ""
  showsPrec i (Span s   xs) = showString "Span " <> showInt s
                           <> showString " "     <> showsPrec i xs
  showsPrec i (Fill s _ xs) = showString "Fill " <> showInt s
                           <> showString " "     <> showsPrec i xs

-- | INVARIANT: 'Nil' should appear only after 'Span' of 'Fill'.
nilInvFailed :: a
nilInvFailed = error "Nil: bucket invariant failed"

valid :: Bucket -> Bool
valid = check Nothing
  where
    check Nothing     Nil           = False -- see 'nilInvFailed'
    check (Just _)    _             = True
    check prevIsSpan (Span sz   xs) =
      prevIsSpan /= Just True &&    -- Span n (NotSpan .. ) invariant
      sz > 0 &&                     -- Span is always non-empty
      check (Just True) xs
    check prevIsSpan (Fill sz b xs) =
      prevIsSpan /= Just True &&    -- Fill n (NotFill .. ) invariant
      sz > 0 &&                     -- Fill is always non-empty
      check (Just False) xs

instance Pretty Bucket where
  pretty Nil = nilInvFailed
  pretty bkt = go bkt
    where
      go  Nil           = PP.empty
      go (Span sz   xs) = "Span" <+> PP.int sz <+> go xs
      go (Fill sz b xs) = "Fill" <+> PP.int sz <+> go xs

-- | Smart constructor: use it when some block is /deleted/ from
-- bucket.
span :: ChunkSize -> Bucket -> Bucket
span sz (Span sz'   xs) = Span (sz + sz') xs
span sz  xxs            = Span  sz        xxs
{-# INLINE span #-}

-- | Smart constructor: use it when some block is /inserted/ to
-- bucket.
fill :: ChunkSize -> Builder -> Bucket -> Bucket
fill sz b (Fill sz' b' xs) = Fill (sz + sz') (b <> b') xs
fill sz b  xxs             = Fill  sz         b        xxs
{-# INLINE fill #-}

{-----------------------------------------------------------------------
--  Bucket queries
-----------------------------------------------------------------------}

-- | /O(1)/. Test if this bucket is empty.
null :: Bucket -> Bool
null  Nil         = nilInvFailed
null (Span _ Nil) = True
null  _           = False
{-# INLINE null #-}

-- | /O(1)/. Test if this bucket is complete.
full :: Bucket -> Bool
full  Nil           = nilInvFailed
full (Fill _ _ Nil) = True
full  _             = False
{-# INLINE full #-}

-- | /O(n)/. Total size of the incompleted piece.
size :: Bucket -> PieceSize
size Nil = nilInvFailed
size bkt = go bkt
  where
    go  Nil           = 0
    go (Span sz   xs) = sz + go xs
    go (Fill sz _ xs) = sz + go xs

-- | /O(n)/. List incomplete blocks to download. If some block have
-- size more than the specified 'BlockSize' then block is split into
-- smaller blocks to satisfy given 'BlockSize'. Small (for
-- e.g. trailing) blocks is not ignored, but returned in-order.
spans :: BlockSize -> Bucket -> [(BlockOffset, BlockSize)]
spans expectedSize = go 0
  where
    go _    Nil           = []
    go off (Span sz   xs) = listChunks off sz ++ go (off + sz) xs
    go off (Fill sz _ xs) = go (off + sz) xs

    listChunks off restSize
      | restSize <= 0 = []
      |   otherwise   = (off, blkSize)
                      : listChunks (off + blkSize) (restSize - blkSize)
      where
        blkSize = min expectedSize restSize

{-----------------------------------------------------------------------
--  Bucket contstruction
-----------------------------------------------------------------------}

-- | /O(1)/. A new empty bucket capable to alloof specified size.
empty :: PieceSize -> Bucket
empty sz
  |   sz < 0  = error "empty: Bucket size must be a non-negative value"
  | otherwise = Span sz Nil
{-# INLINE empty #-}

insertSpan :: Pos -> BS.ByteString -> ChunkSize -> Bucket -> Bucket
insertSpan !pos !bs !span_sz !xs =
  let pref_len = pos
      fill_len = span_sz - pos `min` BS.length bs
      suff_len = (span_sz - pos) - fill_len
  in mkSpan pref_len $
     fill   fill_len (byteString (BS.take fill_len bs)) $
     mkSpan suff_len $
     xs
  where
    mkSpan 0  xs = xs
    mkSpan sz xs = Span sz xs

-- | /O(n)/. Insert a strict bytestring at specified position.
--
--   Best case: if blocks are inserted in sequential order, then this
--   operation should take /O(1)/.
--
insert :: Pos -> BS.ByteString -> Bucket -> Bucket
insert _      _  Nil    = nilInvFailed
insert dstPos bs bucket = go 0 bucket
  where
    intersects curPos sz = dstPos >= curPos && dstPos <= curPos + sz

    go _             Nil     = Nil
    go curPos       (Span sz xs)
      | intersects curPos sz = insertSpan (dstPos - curPos) bs sz xs
      |       otherwise      = span sz    (go (curPos + sz) xs)
    go curPos bkt @ (Fill sz br xs)
      | intersects curPos sz = bkt
      |       otherwise      = fill sz br (go (curPos + sz) xs)

fromList :: PieceSize -> [(Pos, BS.ByteString)] -> Bucket
fromList s = L.foldr (uncurry Network.BitTorrent.Exchange.Block.insert)
                     (Network.BitTorrent.Exchange.Block.empty s)

-- TODO zero-copy
insertLazy :: Pos -> BL.ByteString -> Bucket -> Bucket
insertLazy pos bl = Network.BitTorrent.Exchange.Block.insert pos (BL.toStrict bl)

-- | /O(n)/.
merge :: Bucket -> Bucket -> Bucket
merge = error "Bucket.merge: not implemented"

-- | /O(1)/.
toPiece :: Bucket -> Maybe BL.ByteString
toPiece  Nil           = nilInvFailed
toPiece (Fill _ b Nil) = Just (toLazyByteString b)
toPiece  _             = Nothing