premiere-libtorrent/include/libtorrent/piece_picker.hpp

/*

Copyright (c) 2003-2018, Arvid Norberg
All rights reserved.

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modification, are permitted provided that the following conditions
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    * Redistributions of source code must retain the above copyright
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      from this software without specific prior written permission.

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AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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*/
#ifndef TORRENT_PIECE_PICKER_HPP_INCLUDED
#define TORRENT_PIECE_PICKER_HPP_INCLUDED

// heavy weight reference counting invariant checks
//#define TORRENT_DEBUG_REFCOUNTS

#include "libtorrent/config.hpp"

#include <algorithm>
#include <vector>
#include <utility>
#include <cstdint>
#include <tuple>
#include <set>
#include <unordered_map>

#include "libtorrent/peer_id.hpp"
#include "libtorrent/assert.hpp"
#include "libtorrent/time.hpp"
#include "libtorrent/piece_block.hpp"
#include "libtorrent/aux_/vector.hpp"
#include "libtorrent/aux_/array.hpp"
#include "libtorrent/span.hpp"
#include "libtorrent/alert_types.hpp" // for picker_flags_t
#include "libtorrent/download_priority.hpp"
#include "libtorrent/flags.hpp"
#include "libtorrent/units.hpp"

namespace libtorrent {

	class torrent;
	class peer_connection;
	template <typename Index>
	struct typed_bitfield;
	struct counters;
	struct torrent_peer;

	using prio_index_t = aux::strong_typedef<int, struct prio_index_tag_t>;
	using picker_options_t = flags::bitfield_flag<std::uint16_t, struct picker_options_tag>;
	using download_queue_t = aux::strong_typedef<std::uint8_t, struct dl_queue_tag>;

	struct piece_count
	{
		// the number of pieces included in the "set"
		int num_pieces;
		// the number of blocks, out of those pieces, that are pad
		// blocks (i.e. entirely part of pad files)
		int pad_blocks;
		// true if the last piece is part of the set
		bool last_piece;
	};

	class TORRENT_EXTRA_EXPORT piece_picker
	{
		// only defined when TORRENT_PICKER_LOG is defined, used for debugging
		// unit tests
		friend void print_pieces(piece_picker const& p);

	public:

		enum
		{
			// the number of priority levels
			priority_levels = 8,
			// priority factor
			prio_factor = 3
		};

		struct block_info
		{
			block_info(): num_peers(0), state(state_none) {}
			// the peer this block was requested or
			// downloaded from.
			torrent_peer* peer = nullptr;
			// the number of peers that has this block in their
			// download or request queues
			unsigned num_peers:14;
			// the state of this block
			enum { state_none, state_requested, state_writing, state_finished };
			unsigned state:2;
#if TORRENT_USE_ASSERTS
			// to allow verifying the invariant of blocks belonging to the right piece
			piece_index_t piece_index{-1};
			std::set<torrent_peer*> peers;
#endif
		};

		// pick rarest first
		static constexpr picker_options_t rarest_first = 0_bit;

		// pick the most common first, or the last pieces if sequential
		static constexpr picker_options_t reverse = 1_bit;

		// only pick pieces exclusively requested from this peer
		static constexpr picker_options_t on_parole = 2_bit;

		// always pick partial pieces before any other piece
		static constexpr picker_options_t prioritize_partials = 3_bit;

		// pick pieces in sequential order
		static constexpr picker_options_t sequential = 4_bit;

		// treat pieces with priority 6 and below as filtered
		// to trigger end-game mode until all prio 7 pieces are
		// completed
		static constexpr picker_options_t time_critical_mode = 5_bit;

		// only expands pieces (when prefer contiguous blocks is set)
		// within properly aligned ranges, not the largest possible
		// range of pieces.
		static constexpr picker_options_t align_expanded_pieces = 6_bit;

		struct downloading_piece
		{
			downloading_piece()
				: finished(0)
				, passed_hash_check(0)
				, writing(0)
				, locked(0)
				, requested(0)
				, outstanding_hash_check(0) {}

			bool operator<(downloading_piece const& rhs) const { return index < rhs.index; }

			// the index of the piece
			piece_index_t index{(std::numeric_limits<std::int32_t>::max)()};

			// info about each block in this piece. this is an index into the
			// m_block_info array, when multiplied by m_blocks_per_piece.
			// The m_blocks_per_piece following entries contain information about
			// all blocks in this piece.
			std::uint16_t info_idx{(std::numeric_limits<std::uint16_t>::max)()};

			// the number of blocks in the finished state
			std::uint16_t finished:15;

			// set to true when the hash check job
			// returns with a valid hash for this piece.
			// we might not 'have' the piece yet though,
			// since it might not have been written to
			// disk. This is not set of locked is
			// set.
			std::uint16_t passed_hash_check:1;

			// the number of blocks in the writing state
			std::uint16_t writing:15;

			// when this is set, blocks from this piece may
			// not be picked. This is used when the hash check
			// fails or writing to the disk fails, while waiting
			// to synchronize the disk thread and clear out any
			// remaining state. Once this synchronization is
			// done, restore_piece() is called to clear the
			// locked flag.
			std::uint16_t locked:1;

			// the number of blocks in the requested state
			std::uint16_t requested:15;

			// set to true while there is an outstanding
			// hash check for this piece
			std::uint16_t outstanding_hash_check:1;
		};

		piece_picker(int blocks_per_piece, int blocks_in_last_piece, int total_num_pieces);

		void get_availability(aux::vector<int, piece_index_t>& avail) const;
		int get_availability(piece_index_t piece) const;

		// increases the peer count for the given piece
		// (is used when a HAVE message is received)
		void inc_refcount(piece_index_t index, const torrent_peer* peer);
		void dec_refcount(piece_index_t index, const torrent_peer* peer);

		// increases the peer count for the given piece
		// (is used when a BITFIELD message is received)
		void inc_refcount(typed_bitfield<piece_index_t> const& bitmask
			, const torrent_peer* peer);
		// decreases the peer count for the given piece
		// (used when a peer disconnects)
		void dec_refcount(typed_bitfield<piece_index_t> const& bitmask
			, const torrent_peer* peer);

		// these will increase and decrease the peer count
		// of all pieces. They are used when seeds join
		// or leave the swarm.
		void inc_refcount_all(const torrent_peer* peer);
		void dec_refcount_all(const torrent_peer* peer);

		// This indicates that we just received this piece
		// it means that the refcounter will indicate that
		// we are not interested in this piece anymore
		// (i.e. we don't have to maintain a refcount)
		void we_have(piece_index_t index);
		void we_dont_have(piece_index_t index);

		// the lowest piece index we do not have
		piece_index_t cursor() const { return m_cursor; }

		// one past the last piece we do not have.
		piece_index_t reverse_cursor() const { return m_reverse_cursor; }

		// sets all pieces to dont-have
		void resize(int blocks_per_piece, int blocks_in_last_piece, int total_num_pieces);
		int num_pieces() const { return int(m_piece_map.size()); }

		bool have_piece(piece_index_t index) const;

		bool is_downloading(piece_index_t index) const
		{
			TORRENT_ASSERT(index >= piece_index_t(0));
			TORRENT_ASSERT(index < m_piece_map.end_index());

			piece_pos const& p = m_piece_map[index];
			return p.downloading();
		}

		// sets the priority of a piece.
		// returns true if the priority was changed from 0 to non-0
		// or vice versa
		bool set_piece_priority(piece_index_t index, download_priority_t prio);

		// returns the priority for the piece at 'index'
		download_priority_t piece_priority(piece_index_t index) const;

		// returns the current piece priorities for all pieces
		void piece_priorities(std::vector<download_priority_t>& pieces) const;

		// pieces should be the vector that represents the pieces a
		// client has. It returns a list of all pieces that this client
		// has and that are interesting to download. It returns them in
		// priority order. It doesn't care about the download flag.
		// The user of this function must lookup if any piece is
		// marked as being downloaded. If the user of this function
		// decides to download a piece, it must mark it as being downloaded
		// itself, by using the mark_as_downloading() member function.
		// THIS IS DONE BY THE peer_connection::send_request() MEMBER FUNCTION!
		// The last argument is the torrent_peer pointer for the peer that
		// we'll download from.
		// prefer_contiguous_blocks indicates how many blocks we would like
		// to request contiguously. The blocks are not merged by the piece
		// picker, but may be coalesced later by the peer_connection.
		// this feature is used by web_peer_connection to request larger blocks
		// at a time to mitigate limited pipelining and lack of keep-alive
		// (i.e. higher overhead per request).
		picker_flags_t pick_pieces(typed_bitfield<piece_index_t> const& pieces
			, std::vector<piece_block>& interesting_blocks, int num_blocks
			, int prefer_contiguous_blocks, torrent_peer* peer
			, picker_options_t options, std::vector<piece_index_t> const& suggested_pieces
			, int num_peers
			, counters& pc
			) const;

		// picks blocks from each of the pieces in the piece_list
		// vector that is also in the piece bitmask. The blocks
		// are added to interesting_blocks, and busy blocks are
		// added to backup_blocks. num blocks is the number of
		// blocks to be picked. Blocks are not picked from pieces
		// that are being downloaded
		int add_blocks(piece_index_t piece, typed_bitfield<piece_index_t> const& pieces
			, std::vector<piece_block>& interesting_blocks
			, std::vector<piece_block>& backup_blocks
			, std::vector<piece_block>& backup_blocks2
			, int num_blocks, int prefer_contiguous_blocks
			, torrent_peer* peer, std::vector<piece_index_t> const& ignore
			, picker_options_t options) const;

		// picks blocks only from downloading pieces
		int add_blocks_downloading(downloading_piece const& dp
			, typed_bitfield<piece_index_t> const& pieces
			, std::vector<piece_block>& interesting_blocks
			, std::vector<piece_block>& backup_blocks
			, std::vector<piece_block>& backup_blocks2
			, int num_blocks, int prefer_contiguous_blocks
			, torrent_peer* peer
			, picker_options_t options) const;

		// clears the peer pointer in all downloading pieces with this
		// peer pointer
		void clear_peer(torrent_peer* peer);

#if TORRENT_USE_INVARIANT_CHECKS
		// this is an invariant check
		void check_peers();
#endif

		// returns true if any client is currently downloading this
		// piece-block, or if it's queued for downloading by some client
		// or if it already has been successfully downloaded
		bool is_requested(piece_block block) const;
		// returns true if the block has been downloaded
		bool is_downloaded(piece_block block) const;
		// returns true if the block has been downloaded and written to disk
		bool is_finished(piece_block block) const;

		// marks this piece-block as queued for downloading
		// options are flags from options_t.
		bool mark_as_downloading(piece_block block, torrent_peer* peer
			, picker_options_t options = {});

		// returns true if the block was marked as writing,
		// and false if the block is already finished or writing
		bool mark_as_writing(piece_block block, torrent_peer* peer);

		void mark_as_canceled(piece_block block, torrent_peer* peer);
		void mark_as_finished(piece_block block, torrent_peer* peer);

		void mark_as_pad(piece_block block);

		// prevent blocks from being picked from this piece.
		// to unlock the piece, call restore_piece() on it
		void lock_piece(piece_index_t piece);

		void write_failed(piece_block block);
		int num_peers(piece_block block) const;

		void piece_passed(piece_index_t index);

		// returns information about the given piece
		void piece_info(piece_index_t index, piece_picker::downloading_piece& st) const;

		struct piece_stats_t
		{
			int peer_count;
			int priority;
			bool have;
			bool downloading;
		};

		piece_stats_t piece_stats(piece_index_t index) const;

		// if a piece had a hash-failure, it must be restored and
		// made available for redownloading
		void restore_piece(piece_index_t index);

		// clears the given piece's download flag
		// this means that this piece-block can be picked again
		void abort_download(piece_block block, torrent_peer* peer = nullptr);

		// returns true if all blocks in this piece are finished
		// or if we have the piece
		bool is_piece_finished(piece_index_t index) const;

		// returns true if we have the piece or if the piece
		// has passed the hash check
		bool has_piece_passed(piece_index_t index) const;

		// returns the number of blocks there is in the given piece
		int blocks_in_piece(piece_index_t index) const;

		// return the peer pointers to all peers that participated in
		// this piece
		void get_downloaders(std::vector<torrent_peer*>& d, piece_index_t index) const;

		std::vector<piece_picker::downloading_piece> get_download_queue() const;
		int get_download_queue_size() const;

		void get_download_queue_sizes(int* partial
			, int* full, int* finished, int* zero_prio) const;

		torrent_peer* get_downloader(piece_block block) const;


		// piece states
		//
		//       have: -----------
		//     pieces: # # # # # # # # # # #
		//   filtered:         -------
		//   pads blk: ^       ^         ^
		//
		//  want-have: * * * *
		//       want: * * * *         * * *
		// total-have: * * * * * *
		//
		// we only care about:
		// 1. pieces we have (less pad blocks we have)
		// 2. pieces we have AND want (less pad blocks we have and want)
		// 3. pieces we want (less pad blocks we want)

		// number of pieces not filtered, as well as the number of
		// blocks out of those pieces that are pad blocks.
		// ``last_piece`` is set if the last piece is one of the
		// pieces.
		piece_count want() const;

		// number of pieces we have out of the ones we have not filtered
		piece_count have_want() const;

		// number of pieces we have (regardless of whether they are filtered)
		piece_count have() const;

		piece_count all_pieces() const;

		int pad_blocks_in_piece(piece_index_t const index) const;

		// number of pieces whose hash has passed (but haven't necessarily
		// been flushed to disk yet)
		int num_passed() const { return m_num_passed; }

		// return true if we have all the pieces we wanted
		bool is_finished() const { return m_num_have - m_num_have_filtered == num_pieces() - m_num_filtered; }

		bool is_seeding() const { return m_num_have == num_pieces(); }

		// the number of pieces we want and don't have
		int num_want_left() const { return num_pieces() - m_num_have - m_num_filtered + m_num_have_filtered; }

#if TORRENT_USE_INVARIANT_CHECKS
		void check_piece_state() const;
		// used in debug mode
		void verify_priority(prio_index_t start, prio_index_t end, int prio) const;
		void verify_pick(std::vector<piece_block> const& picked
			, typed_bitfield<piece_index_t> const& bits) const;

		void check_peer_invariant(typed_bitfield<piece_index_t> const& have
			, torrent_peer const* p) const;
		void check_invariant(const torrent* t = nullptr) const;
#endif

		// functor that compares indices on downloading_pieces
		struct has_index
		{
			explicit has_index(piece_index_t const i) : index(i)
			{ TORRENT_ASSERT(i >= piece_index_t(0)); }
			bool operator()(downloading_piece const& p) const
			{ return p.index == index; }
			piece_index_t const index;
		};

		int blocks_in_last_piece() const
		{ return m_blocks_in_last_piece; }

		std::pair<int, int> distributed_copies() const;

		// return the array of block_info objects for a given downloading_piece.
		// this array has m_blocks_per_piece elements in it
		span<block_info const> blocks_for_piece(downloading_piece const& dp) const;

	private:

		int num_pad_blocks() const { return m_num_pad_blocks; }

		span<block_info> mutable_blocks_for_piece(downloading_piece const& dp);

		std::tuple<bool, bool, int, int> requested_from(
			piece_picker::downloading_piece const& p
			, int num_blocks_in_piece, torrent_peer* peer) const;

		bool can_pick(piece_index_t piece, typed_bitfield<piece_index_t> const& bitmask) const;
		bool is_piece_free(piece_index_t piece, typed_bitfield<piece_index_t> const& bitmask) const;
		std::pair<piece_index_t, piece_index_t>
		expand_piece(piece_index_t piece, int whole_pieces
			, typed_bitfield<piece_index_t> const& have
			, picker_options_t options) const;

		struct piece_pos
		{
			piece_pos() {}
			piece_pos(int const peer_count_, int const index_)
				: peer_count(static_cast<std::uint16_t>(peer_count_))
				, download_state(static_cast<uint8_t>(piece_pos::piece_open))
				, piece_priority(static_cast<std::uint8_t>(default_priority))
				, index(index_)
			{
				TORRENT_ASSERT(peer_count_ >= 0);
				TORRENT_ASSERT(peer_count_ < (std::numeric_limits<std::uint16_t>::max)());
				TORRENT_ASSERT(index_ >= 0);
			}

			// the piece is partially downloaded or requested
			static constexpr download_queue_t piece_downloading{0};

			// partial pieces where all blocks in the piece have been requested
			static constexpr download_queue_t piece_full{1};
			// partial pieces where all blocks in the piece have been received
			// and are either finished or writing
			static constexpr download_queue_t piece_finished{2};
			// partial pieces whose priority is 0
			static constexpr download_queue_t piece_zero_prio{3};

			// the states up to this point indicate the piece is being
			// downloaded (or at least has a partially downloaded piece
			// in one of the m_downloads buckets).
			static constexpr download_queue_t num_download_categories{4};

			// the piece is open to be picked
			static constexpr download_queue_t piece_open{4};

			// this is not a new download category/download list bucket.
			// it still goes into the piece_downloading bucket. However,
			// it indicates that this piece only has outstanding requests
			// from reverse peers. This is to de-prioritize it somewhat
			static constexpr download_queue_t piece_downloading_reverse{5};
			static constexpr download_queue_t piece_full_reverse{6};

			// returns one of the valid download categories of state_t or
			// piece_open if this piece is not being downloaded
			download_queue_t download_queue() const
			{
				if (state() == piece_downloading_reverse)
					return piece_downloading;
				if (state() == piece_full_reverse)
					return piece_full;
				return state();
			}

			bool reverse() const
			{
				return state() == piece_downloading_reverse
					|| state() == piece_full_reverse;
			}

			void unreverse()
			{
				if (state() == piece_downloading_reverse)
						state(piece_downloading);
				else if (state() == piece_full_reverse)
						state(piece_full);
			}

			void make_reverse()
			{
				if (state() == piece_downloading)
						state(piece_downloading_reverse);
				else if (state() == piece_full)
						state(piece_full_reverse);
			}

			// the number of peers that has this piece
			// (availability)
			std::uint32_t peer_count : 26;

			// one of the enums from state_t. This indicates whether this piece
			// is currently being downloaded or not, and what state it's in if
			// it is. Specifically, as an optimization, pieces that have all blocks
			// requested from them are separated out into separate lists to make
			// lookups quicker. The main oddity is that whether a downloading piece
			// has only been requested from peers that are reverse, that's
			// recorded as piece_downloading_reverse, which really means the same
			// as piece_downloading, it just saves space to also indicate that it
			// has a bit lower priority. The reverse bit is only relevant if the
			// state is piece_downloading.
			std::uint32_t download_state : 3;

			// TODO: 2 having 8 priority levels is probably excessive. It should
			// probably be changed to 3 levels + dont-download

			// is 0 if the piece is filtered (not to be downloaded)
			// 1 is low priority
			// 2 is low priority
			// 3 is mid priority
			// 4 is default priority
			// 5 is mid priority
			// 6 is high priority
			// 7 is high priority
			std::uint32_t piece_priority : 3;

			// index in to the piece_info vector
			prio_index_t index;

#ifdef TORRENT_DEBUG_REFCOUNTS
			// all the peers that have this piece
			std::set<const torrent_peer*> have_peers;
#endif

			// index is set to this to indicate that we have the
			// piece. There is no entry for the piece in the
			// buckets if this is the case.
			static constexpr prio_index_t we_have_index{-1};

			// the priority value that means the piece is filtered
			static constexpr std::uint32_t filter_priority = 0;

			// the max number the peer count can hold
			static constexpr std::uint32_t max_peer_count = 0xffff;

			bool have() const { return index == we_have_index; }
			void set_have() { index = we_have_index; TORRENT_ASSERT(have()); }
			void set_not_have() { index = prio_index_t(0); TORRENT_ASSERT(!have()); }
			bool downloading() const { return state() != piece_open; }

			bool filtered() const { return piece_priority == filter_priority; }

			// this function returns the effective priority of the piece. It's
			// actually the sort order of this piece compared to other pieces. A
			// lower index means it will be picked before a piece with a higher
			// index.
			// The availability of the piece (the number of peers that have this
			// piece) is fundamentally controlling the priority. It's multiplied
			// by 3 to form 3 levels of priority for each availability.
			//
			//  downloading pieces (not reverse)
			//   |   open pieces (not downloading)
			//   |   |   downloading pieces (reverse peers)
			//   |   |   |
			// +---+---+---+
			// | 0 | 1 | 2 |
			// +---+---+---+
			// this '3' is called prio_factor
			//
			// the manually set priority takes precedence over the availability
			// by multiplying availability by priority.

			int priority(piece_picker const* picker) const
			{
				// filtered pieces (prio = 0), pieces we have or pieces with
				// availability = 0 should not be present in the piece list
				// returning -1 indicates that they shouldn't.
				if (filtered() || have() || peer_count + picker->m_seeds == 0
					|| state() == piece_full
					|| state() == piece_finished)
					return -1;

				TORRENT_ASSERT(piece_priority > 0);

				// this is to keep downloading pieces at higher priority than
				// pieces that are not being downloaded, and to make reverse
				// downloading pieces to be lower priority
				int adjustment = -2;
				if (reverse()) adjustment = -1;
				else if (state() != piece_open) adjustment = -3;

				// the + 1 here is because peer_count count be 0, it m_seeds
				// is > 0. We don't actually care about seeds (except for the
				// first one) since the order of the pieces is unaffected.
				int availability = int(peer_count) + 1;
				TORRENT_ASSERT(availability > 0);
				TORRENT_ASSERT(int(priority_levels - piece_priority) > 0);

				return availability * int(priority_levels - piece_priority)
					* prio_factor + adjustment;
			}

			bool operator!=(piece_pos const& p) const
			{ return index != p.index || peer_count != p.peer_count; }

			bool operator==(piece_pos const& p) const
			{ return index == p.index && peer_count == p.peer_count; }

			download_queue_t state() const { return download_queue_t(download_state); }
			void state(download_queue_t q) { download_state = static_cast<std::uint8_t>(q); }
		};

#ifndef TORRENT_DEBUG_REFCOUNTS
		static_assert(sizeof(piece_pos) == sizeof(char) * 8, "unexpected struct size");
#endif

		bool partial_compare_rarest_first(downloading_piece const* lhs
			, downloading_piece const* rhs) const;

		void break_one_seed();

		void update_pieces() const;

		prio_index_t priority_begin(int prio) const;
		prio_index_t priority_end(int prio) const;

		// fills in the range [start, end) of pieces in
		// m_pieces that have priority 'prio'
		std::pair<prio_index_t, prio_index_t> priority_range(int prio) const;

		// adds the piece 'index' to m_pieces
		void add(piece_index_t index);
		// removes the piece with the given priority and the
		// elem_index in the m_pieces vector
		void remove(int priority, prio_index_t elem_index);
		// updates the position of the piece with the given
		// priority and the elem_index in the m_pieces vector
		void update(int priority, prio_index_t elem_index);
		// shuffles the given piece inside it's priority range
		void shuffle(int priority, prio_index_t elem_index);

		std::vector<downloading_piece>::iterator add_download_piece(piece_index_t index);
		void erase_download_piece(std::vector<downloading_piece>::iterator i);

		std::vector<downloading_piece>::const_iterator find_dl_piece(download_queue_t, piece_index_t) const;
		std::vector<downloading_piece>::iterator find_dl_piece(download_queue_t, piece_index_t);

		// returns an iterator to the downloading piece, whichever
		// download list it may live in now
		std::vector<downloading_piece>::iterator update_piece_state(
			std::vector<downloading_piece>::iterator dp);

	private:

#if TORRENT_USE_ASSERTS || TORRENT_USE_INVARIANT_CHECKS
		index_range<download_queue_t> categories() const
		{ return {{}, piece_picker::piece_pos::num_download_categories}; }
#endif

		// the following vectors are mutable because they sometimes may
		// be updated lazily, triggered by const functions

		// this maps indices to number of peers that has this piece and
		// index into the m_piece_info vectors.
		// piece_pos::we_have_index means that we have the piece, so it
		// doesn't exist in the piece_info buckets
		// pieces with the filtered flag set doesn't have entries in
		// the m_piece_info buckets either
		// TODO: should this be allocated lazily?
		mutable aux::vector<piece_pos, piece_index_t> m_piece_map;

		// this indicates whether a block has been marked as a pad
		// block or not. It's indexed by block index, i.e. piece_index
		// * blocks_per_piece + block. These blocks should not be
		// picked and are considered to be had
		// TODO: this could be a much more efficient data structure
		bitfield m_pad_blocks;

		// tracks the number of blocks in a specific piece that are pad blocks
		std::unordered_map<piece_index_t, int> m_pads_in_piece;

		// the number of bits set in the m_pad_blocks bitfield, i.e.
		// the number of blocks marked as pads
		int m_num_pad_blocks = 0;

		// the number of pad blocks that we already have
		int m_have_pad_blocks = 0;

		// the number of pad blocks part of filtered pieces we don't have
		int m_filtered_pad_blocks = 0;

		// the number of pad blocks we have that are also filtered
		int m_have_filtered_pad_blocks = 0;

		// the number of seeds. These are not added to
		// the availability counters of the pieces
		int m_seeds = 0;

		// the number of pieces that have passed the hash check
		int m_num_passed = 0;

		// this vector contains all piece indices that are pickable
		// sorted by priority. Pieces are in random random order
		// among pieces with the same priority
		mutable aux::vector<piece_index_t, prio_index_t> m_pieces;

		// these are indices to the priority boundaries inside
		// the m_pieces vector. priority 0 always start at
		// 0, priority 1 starts at m_priority_boundaries[0] etc.
		mutable aux::vector<prio_index_t> m_priority_boundaries;

		// each piece that's currently being downloaded has an entry in this list
		// with block allocations. i.e. it says which parts of the piece that is
		// being downloaded. This list is ordered by piece index to make lookups
		// efficient there are as many buckets as there are piece states. See
		// piece_pos::state_t. The only download state that does not have a
		// corresponding downloading_piece vector is piece_open and
		// piece_downloading_reverse (the latter uses the same as
		// piece_downloading).
		aux::array<aux::vector<downloading_piece>
			, static_cast<std::uint8_t>(piece_pos::num_download_categories)
			, download_queue_t> m_downloads;

		// this holds the information of the blocks in partially downloaded
		// pieces. the downloading_piece::info index point into this vector for
		// its storage
		aux::vector<block_info> m_block_info;

		// these are block ranges in m_block_info that are free. The numbers
		// in here, when multiplied by m_blocks_per_piece is the index to the
		// first block in the range that's free to use by a new downloading_piece.
		// this is a free-list.
		std::vector<std::uint16_t> m_free_block_infos;

		std::uint16_t m_blocks_per_piece = 0;
		std::uint16_t m_blocks_in_last_piece = 0;

		// the number of filtered pieces that we don't already
		// have. total_number_of_pieces - number_of_pieces_we_have
		// - num_filtered is supposed to the number of pieces
		// we still want to download
		// TODO: it would be more intuitive to account "wanted" pieces
		// instead of filtered
		int m_num_filtered = 0;

		// the number of pieces we have that also are filtered
		int m_num_have_filtered = 0;

		// we have all pieces in the range [0, m_cursor)
		// m_cursor is the first piece we don't have
		piece_index_t m_cursor{0};

		// we have all pieces in the range [m_reverse_cursor, end)
		// m_reverse_cursor is the first piece where we also have
		// all the subsequent pieces
		piece_index_t m_reverse_cursor{0};

		// the number of pieces we have (i.e. passed + flushed).
		// This includes pieces that we have filtered but still have
		int m_num_have = 0;

		// if this is set to true, it means update_pieces()
		// has to be called before accessing m_pieces.
		mutable bool m_dirty = false;
	public:

		enum { max_pieces = (std::numeric_limits<int>::max)() - 1 };

	};
}

#endif // TORRENT_PIECE_PICKER_HPP_INCLUDED