// This file Copyright © Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.

#include <algorithm>
#include <array>
#include <cstddef>
#include <cstdint>
#include <initializer_list>
#include <limits>
#include <memory>
#include <ranges>
#include <utility> // for std::swap()
#include <vector>

#include <fmt/format.h>

#include "libtransmission/transmission.h"

#include "libtransmission/bandwidth.h"
#include "libtransmission/crypto-utils.h"
#include "libtransmission/log.h"
#include "libtransmission/peer-io.h"
#include "libtransmission/tr-assert.h"
#include "libtransmission/utils.h" // tr_time_msec()
#include "libtransmission/values.h"

using namespace tr::Values;

Speed tr_bandwidth::get_speed(RateControl& r, unsigned int interval_msec, uint64_t now)
{
    if (now == 0U)
    {
        now = tr_time_msec();
    }

    if (now != r.cache_time_)
    {
        uint64_t bytes = 0U;
        uint64_t const cutoff = now - interval_msec;

        for (auto i = r.newest_; r.date_[i] > cutoff;)
        {
            bytes += r.size_[i];

            if (--i >= HistorySize) // overflowed below zero
            {
                i = HistorySize - 1U; // circular history
            }

            if (i == r.newest_)
            {
                break; // we've come all the way around
            }
        }

        r.cache_val_ = Speed{ bytes * 1000U / interval_msec, Speed::Units::Byps };
        r.cache_time_ = now;
    }

    return r.cache_val_;
}

void tr_bandwidth::notify_bandwidth_consumed_bytes(uint64_t const now, RateControl& r, size_t size)
{
    if (r.date_[r.newest_] + GranularityMSec >= now)
    {
        r.size_[r.newest_] += size;
    }
    else
    {
        if (++r.newest_ == HistorySize)
        {
            r.newest_ = 0U;
        }

        r.date_[r.newest_] = now;
        r.size_[r.newest_] = size;
    }

    /* invalidate cache_val*/
    r.cache_time_ = 0U;
}

// ---

tr_bandwidth::tr_bandwidth(tr_bandwidth* parent, bool is_group)
    : priority_(is_group ? std::numeric_limits<tr_priority_t>::max() : TR_PRI_NORMAL)
{
    set_parent(parent);
}

// ---

namespace
{
namespace deparent_helpers
{
void remove_child(std::vector<tr_bandwidth*>& v, tr_bandwidth* remove_me) noexcept
{
    // the list isn't sorted -- so instead of erase()ing `it`,
    // do the cheaper option of overwriting it with the final item
    if (auto it = std::ranges::find(v, remove_me); it != std::ranges::end(v))
    {
        std::swap(*it, v.back());
        v.pop_back();
    }
}
} // namespace deparent_helpers
} // namespace

void tr_bandwidth::deparent() noexcept
{
    using namespace deparent_helpers;

    if (parent_ == nullptr)
    {
        return;
    }

    remove_child(parent_->children_, this);
    parent_ = nullptr;
}

void tr_bandwidth::set_parent(tr_bandwidth* new_parent)
{
    TR_ASSERT(this != new_parent);

    deparent();

    if (new_parent != nullptr)
    {
#ifdef TR_ENABLE_ASSERTS
        TR_ASSERT(new_parent->parent_ != this);
        auto& children = new_parent->children_;
        TR_ASSERT(std::ranges::find(children, this) == std::ranges::end(children)); // not already there
#endif

        new_parent->children_.push_back(this);
        parent_ = new_parent;
    }
}

// ---

void tr_bandwidth::allocate_bandwidth(
    tr_priority_t parent_priority,
    uint64_t period_msec,
    std::vector<std::shared_ptr<tr_peerIo>>& peer_pool)
{
    auto const priority = std::min(parent_priority, priority_);

    // set the available bandwidth
    for (auto const dir : { tr_direction::Up, tr_direction::Down })
    {
        if (auto& bandwidth = band_[static_cast<uint8_t>(dir)]; bandwidth.is_limited_)
        {
            auto const next_pulse_speed = bandwidth.desired_speed_;
            bandwidth.bytes_left_ = next_pulse_speed.base_quantity() * period_msec / 1000U;
        }
    }

    // add this bandwidth's peer, if any, to the peer pool
    if (auto shared = peer_.lock(); shared)
    {
        TR_ASSERT(tr_isPriority(priority));
        shared->set_priority(priority);
        peer_pool.push_back(std::move(shared));
    }

    // traverse & repeat for the subtree
    for (auto* child : children_)
    {
        child->allocate_bandwidth(priority, period_msec, peer_pool);
    }
}

void tr_bandwidth::phase_one(std::vector<tr_peerIo*>& peers, tr_direction dir)
{
    // First phase of IO. Tries to distribute bandwidth fairly to keep faster
    // peers from starving the others.
    tr_logAddTrace(
        fmt::format("{} peers to go round-robin for {}", peers.size(), dir == tr_direction::Up ? "upload" : "download"));

    // Shuffle the peers so they all have equal chance to be first in line.
    static thread_local auto urbg = tr_urbg<size_t>{};
    std::shuffle(std::begin(peers), std::end(peers), urbg);

    // Give each peer `Increment` bandwidth bytes to use. Repeat this
    // process until we run out of bandwidth and/or peers that can use it.
    for (size_t n_unfinished = std::size(peers); n_unfinished > 0U;)
    {
        for (size_t i = 0U; i < n_unfinished;)
        {
            // Value of 3000 bytes chosen so that when using µTP we'll send a full-size
            // frame right away and leave enough buffered data for the next frame to go
            // out in a timely manner.
            static auto constexpr Increment = 3000U;

            auto const bytes_used = peers[i]->flush(dir, Increment);
            tr_logAddTrace(fmt::format("peer #{} of {} used {} bytes in this pass", i, n_unfinished, bytes_used));

            if (bytes_used != Increment)
            {
                // peer is done writing for now; move it to the end of the list
                std::swap(peers[i], peers[n_unfinished - 1]);
                --n_unfinished;
            }
            else
            {
                ++i;
            }
        }
    }
}

void tr_bandwidth::allocate(uint64_t period_msec)
{
    // keep these peers alive for the scope of this function
    auto refs = std::vector<std::shared_ptr<tr_peerIo>>{};

    auto peer_arrays = std::array<std::vector<tr_peerIo*>, 3>{};
    auto& high = peer_arrays[0];
    auto& normal = peer_arrays[1];
    auto& low = peer_arrays[2];

    // allocateBandwidth () is a helper function with two purposes:
    // 1. allocate bandwidth to b and its subtree
    // 2. accumulate an array of all the peerIos from b and its subtree.
    allocate_bandwidth(std::numeric_limits<tr_priority_t>::max(), period_msec, refs);

    for (auto const& io : refs)
    {
        io->flush_outgoing_protocol_msgs();

        switch (io->priority())
        {
        case TR_PRI_HIGH:
            high.push_back(io.get());
            [[fallthrough]];

        case TR_PRI_NORMAL:
            normal.push_back(io.get());
            [[fallthrough]];

        case TR_PRI_LOW:
            low.push_back(io.get());
            break;

        default:
            TR_ASSERT_MSG(false, "invalid priority");
            break;
        }
    }

    // First phase of IO. Tries to distribute bandwidth fairly to keep faster
    // peers from starving the others. Loop through the peers, giving each a
    // small chunk of bandwidth. Keep looping until we run out of bandwidth
    // and/or peers that can use it
    for (auto& peers : peer_arrays)
    {
        phase_one(peers, tr_direction::Up);
        phase_one(peers, tr_direction::Down);
    }

    // Second phase of IO. To help us scale in high bandwidth situations,
    // enable on-demand IO for peers with bandwidth left to burn.
    // This on-demand IO is enabled until (1) the peer runs out of bandwidth,
    // or (2) the next tr_bandwidth::allocate () call, when we start over again.
    for (auto const& io : refs)
    {
        io->set_enabled(tr_direction::Up, io->has_bandwidth_left(tr_direction::Up));
        io->set_enabled(tr_direction::Down, io->has_bandwidth_left(tr_direction::Down));
    }
}

// ---

size_t tr_bandwidth::clamp(tr_direction const dir, size_t byte_count) const noexcept
{
    auto const idx = static_cast<uint8_t>(dir);

    if (band_[idx].is_limited_)
    {
        byte_count = std::min(byte_count, band_[idx].bytes_left_);
    }

    if (parent_ != nullptr && band_[idx].honor_parent_limits_ && byte_count > 0U)
    {
        byte_count = parent_->clamp(dir, byte_count);
    }

    return byte_count;
}

void tr_bandwidth::notify_bandwidth_consumed(tr_direction dir, size_t byte_count, bool is_piece_data, uint64_t now)
{
    auto& band = band_[static_cast<uint8_t>(dir)];

    if (is_piece_data)
    {
        notify_bandwidth_consumed_bytes(now, band.piece_, byte_count);
    }
    else
    {
        notify_bandwidth_consumed_bytes(now, band.raw_, byte_count);

        if (band.is_limited_)
        {
            band.bytes_left_ -= std::min(band.bytes_left_, byte_count);
        }
    }

    if (parent_ != nullptr)
    {
        parent_->notify_bandwidth_consumed(dir, byte_count, is_piece_data, now);
    }
}

// ---

tr_bandwidth_limits tr_bandwidth::get_limits() const
{
    auto limits = tr_bandwidth_limits{};
    limits.up_limit = get_desired_speed(tr_direction::Up);
    limits.down_limit = get_desired_speed(tr_direction::Down);
    limits.up_limited = is_limited(tr_direction::Up);
    limits.down_limited = is_limited(tr_direction::Down);
    return limits;
}

void tr_bandwidth::set_limits(tr_bandwidth_limits const& limits)
{
    set_desired_speed(tr_direction::Up, limits.up_limit);
    set_desired_speed(tr_direction::Down, limits.down_limit);
    set_limited(tr_direction::Up, limits.up_limited);
    set_limited(tr_direction::Down, limits.down_limited);
}
