/*

Copyright (c) 2019, Amir Abrams

Copyright (c) 2014-2020, Arvid Norberg

Copyright (c) 2016, 2018, 2020-2021, Alden Torres

Copyright (c) 2016, Steven Siloti

Copyright (c) 2019, Monson Shao

All rights reserved.

You may use, distribute and modify this code under the terms of the BSD license,

see LICENSE file.

*/

#include "libtorrent/aux_/choker.hpp"

#include "libtorrent/aux_/peer_connection.hpp"

#include "libtorrent/aux_/session_settings.hpp"

#include "libtorrent/aux_/time.hpp"

#include "libtorrent/aux_/torrent.hpp"

#include <functional>

using namespace std::placeholders;

namespace libtorrent::aux {

namespace {

  int compare_peers(peer_connection const* lhs, peer_connection const* rhs)

  {

    int const prio1 = lhs->get_priority(peer_connection::upload_channel);

    int const prio2 = rhs->get_priority(peer_connection::upload_channel);

    if (prio1 != prio2) return prio1 > prio2 ? 1 : -1;

    // compare how many bytes they've sent us

    std::int64_t const c1 = lhs->downloaded_in_last_round();

    std::int64_t const c2 = rhs->downloaded_in_last_round();

    if (c1 != c2) return c1 > c2 ? 1 : -1;

    return 0;

  }

  // return true if 'lhs' peer should be preferred to be unchoke over 'rhs'

  bool unchoke_compare_rr(peer_connection const* lhs

    , peer_connection const* rhs, int pieces)

  {

    int const cmp = compare_peers(lhs, rhs);

    if (cmp != 0) return cmp > 0;

    // when seeding, rotate which peer is unchoked in a round-robin fashion

    // the amount uploaded since unchoked (not just in the last round)

    std::int64_t const u1 = lhs->uploaded_since_unchoked();

    std::int64_t const u2 = rhs->uploaded_since_unchoked();

    // the way the round-robin unchoker works is that it,

    // by default, prioritizes any peer that is already unchoked.

    // this maintain the status quo across unchoke rounds. However,

    // peers that are unchoked, but have sent more than one quota

    // since they were unchoked, they get de-prioritized.

    auto const t1 = lhs->associated_torrent().lock();

    auto const t2 = rhs->associated_torrent().lock();

    TORRENT_ASSERT(t1);

    TORRENT_ASSERT(t2);

    // if a peer is already unchoked, the number of bytes sent since it was unchoked

    // is greater than the send quanta, and it has been unchoked for at least one minute

    // then it's done with its upload slot, and we can de-prioritize it

    bool const c1_quota_complete = !lhs->is_choked()

      && u1 > std::int64_t(t1->torrent_file().piece_length()) * pieces

      && aux::time_now() - lhs->time_of_last_unchoke() > minutes(1);

    bool const c2_quota_complete = !rhs->is_choked()

      && u2 > std::int64_t(t2->torrent_file().piece_length()) * pieces

      && aux::time_now() - rhs->time_of_last_unchoke() > minutes(1);

    // if c2 has completed a quanta, it should be de-prioritized

    // and vice versa

    if (c1_quota_complete != c2_quota_complete)

      return int(c1_quota_complete) < int(c2_quota_complete);

    // when seeding, prefer the peer we're uploading the fastest to

    // force the upload rate to zero for choked peers because

    // if the peers just got choked the previous round

    // there may have been a residual transfer which was already

    // in-flight at the time and we don't want that to cause the peer

    // to be ranked at the top of the choked peers

    std::int64_t const c1 = lhs->is_choked() ? 0 : lhs->uploaded_in_last_round();

    std::int64_t const c2 = rhs->is_choked() ? 0 : rhs->uploaded_in_last_round();

    if (c1 != c2) return c1 > c2;

    // if the peers are still identical (say, they're both waiting to be unchoked)

    // prioritize the one that has waited the longest to be unchoked

    // the round-robin unchoker relies on this logic. Don't change it

    // without moving this into that unchoker logic

    return lhs->time_of_last_unchoke() < rhs->time_of_last_unchoke();

  }

  // return true if 'lhs' peer should be preferred to be unchoke over 'rhs'

  bool unchoke_compare_fastest_upload(peer_connection const* lhs

    , peer_connection const* rhs)

  {

    int const cmp = compare_peers(lhs, rhs);

    if (cmp != 0) return cmp > 0;

    // when seeding, prefer the peer we're uploading the fastest to

    std::int64_t const c1 = lhs->uploaded_in_last_round();

    std::int64_t const c2 = rhs->uploaded_in_last_round();

    if (c1 != c2) return c1 > c2;

    // prioritize the one that has waited the longest to be unchoked

    // the round-robin unchoker relies on this logic. Don't change it

    // without moving this into that unchoker logic

    return lhs->time_of_last_unchoke() < rhs->time_of_last_unchoke();

  }

  int anti_leech_score(peer_connection const* peer)

  {

    // the anti-leech seeding algorithm is based on the paper "Improving

    // BitTorrent: A Simple Approach" from Chow et. al. and ranks peers based

    // on how many pieces they have, preferring to unchoke peers that just

    // started and peers that are close to completing. Like this:

    //   ^

    //   | \                       / |

    //   |  \                     /  |

    //   |   \                   /   |

    // s |    \                 /    |

    // c |     \               /     |

    // o |      \             /      |

    // r |       \           /       |

    // e |        \         /        |

    //   |         \       /         |

    //   |          \     /          |

    //   |           \   /           |

    //   |            \ /            |

    //   |             V             |

    //   +---------------------------+

    //   0%    num have pieces     100%

    std::shared_ptr<torrent> const t = peer->associated_torrent().lock();

    TORRENT_ASSERT(t);

    std::int64_t const total_size = t->torrent_file().total_size();

    if (total_size == 0) return 0;

    // Cap the given_size so that it never causes the score to increase

    std::int64_t const given_size = std::min(peer->statistics().total_payload_upload()

      , total_size / 2);

    std::int64_t const have_size = std::max(given_size

      , std::int64_t(t->torrent_file().piece_length()) * peer->num_have_pieces());

    return int(std::abs((have_size - total_size / 2) * 2000 / total_size));

  }

  // return true if 'lhs' peer should be preferred to be unchoke over 'rhs'

  bool unchoke_compare_anti_leech(peer_connection const* lhs

    , peer_connection const* rhs)

  {

    int const cmp = compare_peers(lhs, rhs);

    if (cmp != 0) return cmp > 0;

    int const score1 = anti_leech_score(lhs);

    int const score2 = anti_leech_score(rhs);

    if (score1 != score2) return score1 > score2;

    // prioritize the one that has waited the longest to be unchoked

    // the round-robin unchoker relies on this logic. Don't change it

    // without moving this into that unchoker logic

    return lhs->time_of_last_unchoke() < rhs->time_of_last_unchoke();

  }

  bool upload_rate_compare(peer_connection const* lhs

    , peer_connection const* rhs)

  {

    // take torrent priority into account

    std::int64_t const c1 = lhs->uploaded_in_last_round()

      * lhs->get_priority(peer_connection::upload_channel);

    std::int64_t const c2 = rhs->uploaded_in_last_round()

      * rhs->get_priority(peer_connection::upload_channel);

    return c1 > c2;

  }

  } // anonymous namespace

  int unchoke_sort(std::vector<peer_connection*>& peers

    , time_duration const unchoke_interval

    , aux::session_settings const& sett)

  {

#if TORRENT_USE_ASSERTS

    for (auto p : peers)

    {

      TORRENT_ASSERT(p->self());

      TORRENT_ASSERT(p->associated_torrent().lock());

    }

#endif

    int upload_slots = sett.get_int(settings_pack::unchoke_slots_limit);

    if (upload_slots < 0)

      upload_slots = std::numeric_limits<int>::max();

    // ==== rate-based ====

    //

    // The rate based unchoker looks at our upload rate to peers, and find

    // a balance between number of upload slots and the rate we achieve. The

    // intention is to not spread upload bandwidth too thin, but also to not

    // unchoke few enough peers to not be able to saturate the up-link.

    // this is done by traversing the peers sorted by our upload rate to

    // them in decreasing rates. For each peer we increase the threshold by

    // 2 kiB/s. The first peer we get to whom we upload slower than

    // the threshold, we stop and that's the number of unchoke slots we have.

    if (sett.get_int(settings_pack::choking_algorithm)

      == settings_pack::rate_based_choker)

    {

      // first reset the number of unchoke slots, because we'll calculate

      // it purely based on the current state of our peers.

      upload_slots = 0;

      int rate_threshold = sett.get_int(settings_pack::rate_choker_initial_threshold);

      std::sort(peers.begin(), peers.end()

        , [](peer_connection const* lhs, peer_connection const* rhs)

        { return upload_rate_compare(lhs, rhs); });

      for (auto const* p : peers)

      {

        int const rate = int(p->uploaded_in_last_round()

          * 1000 / total_milliseconds(unchoke_interval));

        // always have at least 1 unchoke slot

        if (rate < rate_threshold) break;

        ++upload_slots;

        // TODO: make configurable

        rate_threshold += 2048;

      }

      ++upload_slots;

    }

    // sorts the peers that are eligible for unchoke by download rate and

    // secondary by total upload. The reason for this is, if all torrents are

    // being seeded, the download rate will be 0, and the peers we have sent

    // the least to should be unchoked

    // we use partial sort here, because we only care about the top

    // upload_slots peers.

    int const slots = std::min(upload_slots, int(peers.size()));

    if (sett.get_int(settings_pack::seed_choking_algorithm)

      == settings_pack::round_robin)

    {

      int const pieces = sett.get_int(settings_pack::seeding_piece_quota);

      std::nth_element(peers.begin(), peers.begin()

        + slots, peers.end()

        , [pieces](peer_connection const* lhs, peer_connection const* rhs)

        { return unchoke_compare_rr(lhs, rhs, pieces); });

    }

    else if (sett.get_int(settings_pack::seed_choking_algorithm)

      == settings_pack::fastest_upload)

    {

      std::nth_element(peers.begin(), peers.begin()

        + slots, peers.end()

        , [](peer_connection const* lhs, peer_connection const* rhs)

        { return unchoke_compare_fastest_upload(lhs, rhs); });

    }

    else if (sett.get_int(settings_pack::seed_choking_algorithm)

      == settings_pack::anti_leech)

    {

      std::nth_element(peers.begin(), peers.begin()

        + slots, peers.end()

        , [](peer_connection const* lhs, peer_connection const* rhs)

        { return unchoke_compare_anti_leech(lhs, rhs); });

    }

    else

    {

      int const pieces = sett.get_int(settings_pack::seeding_piece_quota);

      std::nth_element(peers.begin(), peers.begin()

        + slots, peers.end()

        , [pieces](peer_connection const* lhs, peer_connection const* rhs)

        { return unchoke_compare_rr(lhs, rhs, pieces); } );

      TORRENT_ASSERT_FAIL();

    }

    return upload_slots;

  }

}