/// @file dpf/net/mux_sink.hpp
/// @brief RoundSink multiplexed on one framed trio channel.
#ifndef LIBDPF_INCLUDE_DPF_NET_MUX_SINK_HPP__
#define LIBDPF_INCLUDE_DPF_NET_MUX_SINK_HPP__

#include <array>
#include <cstddef>
#include <cstdint>
#include <cstring>
#include <memory>
#include <stdexcept>
#include <utility>
#include <vector>

#include "dpf/net/channel.hpp"
#include "dpf/net/comm_hook.hpp"
#include "dpf/net/round_sink.hpp"
#include "dpf/net/trio.hpp"

namespace dpf
{
namespace net
{

#pragma pack(push, 1)
struct round_batch_hdr
{
    std::uint16_t round = 0;
    std::uint32_t begin = 0;
    std::uint32_t count = 0;
};
#pragma pack(pop)

/// @brief Prefix-flush RoundSink over a single duplex `channel`.
/// @details Each flush sends one `msg::round_batch` frame per round that has
///          a new contiguous prefix: header `(round, begin, count)` then
///          `count * slot_bytes` payload. The peer's matching frames are
///          read in the same flush (lower role sends first).
class mux_sink : public RoundSink
{
public:
    mux_sink(channel & link, unsigned self_id, unsigned peer_id,
        std::size_t count, std::vector<std::size_t> slot_bytes)
      : link_(link),
        self_id_(self_id),
        peer_id_(peer_id),
        count_(count),
        slot_bytes_(std::move(slot_bytes))
    {
        windows_.reserve(slot_bytes_.size());
        for (std::size_t sb : slot_bytes_)
            windows_.emplace_back(count_, sb);
    }

    std::size_t count() const noexcept override { return count_; }
    std::size_t rounds() const noexcept override { return slot_bytes_.size(); }

    std::size_t slot_bytes(std::uint16_t round) const override
    {
        if (round >= slot_bytes_.size())
            throw std::out_of_range("mux_sink round");
        return slot_bytes_[round];
    }

    void submit(std::uint16_t round, std::size_t index,
        const std::uint8_t * bytes, std::size_t n) override
    {
        window(round).submit(index, bytes, n);
    }

    bool peer_ready(std::uint16_t round, std::size_t index) const override
    {
        return window(round).peer_ready(index);
    }

    void read_peer(std::uint16_t round, std::size_t index, std::uint8_t * out,
        std::size_t n) const override
    {
        window(round).read_peer(index, out, n);
    }

    void flush() override
    {
        std::vector<std::uint8_t> payload;
        for (std::uint16_t r = 0; r < slot_bytes_.size(); ++r)
            append_pending(r, payload);
        exchange_payload(payload);
    }

    void flush_round(std::uint16_t round) override
    {
        if (round >= slot_bytes_.size())
            throw std::out_of_range("mux_sink flush_round");
        std::vector<std::uint8_t> payload;
        append_pending(round, payload);
        exchange_payload(payload);
    }

    void poll() override {}

    std::uint64_t exchanges() const noexcept { return link_exchanges_; }

private:
    round_window & window(std::uint16_t round)
    {
        if (round >= windows_.size())
            throw std::out_of_range("mux_sink round");
        return windows_[round];
    }

    const round_window & window(std::uint16_t round) const
    {
        if (round >= windows_.size())
            throw std::out_of_range("mux_sink round");
        return windows_[round];
    }

    void append_pending(std::uint16_t r, std::vector<std::uint8_t> & payload)
    {
        std::size_t begin = 0;
        std::size_t nslots = 0;
        const std::uint8_t * pending = windows_[r].pending_out(begin, nslots);
        if (nslots == 0)
            return;
        round_batch_hdr hdr{};
        hdr.round = r;
        hdr.begin = static_cast<std::uint32_t>(begin);
        hdr.count = static_cast<std::uint32_t>(nslots);
        const std::size_t body = nslots * slot_bytes_[r];
        const std::size_t old = payload.size();
        payload.resize(old + sizeof(hdr) + body);
        std::memcpy(payload.data() + old, &hdr, sizeof(hdr));
        if (body != 0)
            std::memcpy(payload.data() + old + sizeof(hdr), pending, body);
        windows_[r].mark_flushed(nslots);
    }

    void exchange_payload(const std::vector<std::uint8_t> & payload)
    {
        // Always exchange so a party with nothing new still receives.
        std::vector<std::uint8_t> theirs;
        if (self_id_ < peer_id_)
        {
            link_.send_bytes(msg::round_batch, payload);
            theirs = link_.recv_bytes(msg::round_batch);
        }
        else if (self_id_ > peer_id_)
        {
            theirs = link_.recv_bytes(msg::round_batch);
            link_.send_bytes(msg::round_batch, payload);
        }
        else
            throw std::invalid_argument("mux_sink flush with self");
        ++link_exchanges_;
        ingest(theirs);
    }

    void ingest(const std::vector<std::uint8_t> & bytes)
    {
        std::size_t off = 0;
        while (off < bytes.size())
        {
            if (off + sizeof(round_batch_hdr) > bytes.size())
                throw std::runtime_error("mux_sink truncated header");
            round_batch_hdr hdr{};
            std::memcpy(&hdr, bytes.data() + off, sizeof(hdr));
            off += sizeof(hdr);
            if (hdr.round >= slot_bytes_.size())
                throw std::runtime_error("mux_sink bad round");
            const std::size_t body =
                static_cast<std::size_t>(hdr.count) * slot_bytes_[hdr.round];
            if (off + body > bytes.size())
                throw std::runtime_error("mux_sink truncated body");
            windows_[hdr.round].accept_peer_at(hdr.begin, bytes.data() + off,
                hdr.count);
            off += body;
        }
    }

    channel & link_;
    unsigned self_id_ = 0;
    unsigned peer_id_ = 0;
    std::size_t count_ = 0;
    std::vector<std::size_t> slot_bytes_;
    std::vector<round_window> windows_;
    std::uint64_t link_exchanges_ = 0;
};

/// @brief Bind a mux sink to the p0–p1 link of an existing trio.
inline mux_sink make_mux_sink(trio & net, std::size_t count,
    std::vector<std::size_t> slot_bytes)
{
    const role self = net.self();
    if (self == role::p2)
        throw std::logic_error("mux_sink is for computing parties");
    const role peer = self == role::p0 ? role::p1 : role::p0;
    return mux_sink(net.to(peer), to_u(self), to_u(peer), count,
        std::move(slot_bytes));
}

/// @brief Default `comm_hook`: framed mesh channels and a mux batch sink.
/// @details Subclass to reroute selected calls; leave the rest to these
///          defaults. Installing this hook is equivalent to a null hook for
///          framing, but lets overrides replace individual methods.
class mesh_comm_hook : public comm_hook
{
public:
    void send_bytes(trio & net, unsigned peer_id, msg tag, const void * data,
        std::size_t n) override
    {
        net.to(static_cast<role>(peer_id)).send_bytes(tag, data, n);
    }

    std::vector<std::uint8_t> recv_bytes(trio & net, unsigned peer_id,
        msg tag) override
    {
        return net.to(static_cast<role>(peer_id)).recv_bytes(tag);
    }

    std::vector<std::uint8_t> exchange_bytes(trio & net, unsigned peer_id,
        msg tag, const void * data, std::size_t n) override
    {
        auto & link = net.to(static_cast<role>(peer_id));
        const unsigned self_id = to_u(net.self());
        std::vector<std::uint8_t> theirs(n);
        if (self_id < peer_id)
        {
            link.send_bytes(tag, data, n);
            theirs = link.recv_bytes(tag);
        }
        else if (self_id > peer_id)
        {
            theirs = link.recv_bytes(tag);
            link.send_bytes(tag, data, n);
        }
        else
            throw std::invalid_argument("mesh_comm_hook exchange with self");
        if (theirs.size() != n)
            throw std::runtime_error("mesh_comm_hook exchange size");
        return theirs;
    }

    std::vector<std::uint8_t> exchange_vec_bytes(trio & net, unsigned peer_id,
        msg tag, const void * data, std::size_t nbytes) override
    {
        return exchange_bytes(net, peer_id, tag, data, nbytes);
    }

    std::unique_ptr<RoundSink> batch(trio & net, std::size_t count,
        std::vector<std::size_t> slot_bytes) override
    {
        return std::make_unique<mux_sink>(make_mux_sink(net, count,
            std::move(slot_bytes)));
    }
};

inline std::unique_ptr<RoundSink> trio::batch(std::size_t count,
    std::vector<std::size_t> slot_bytes)
{
    if (hook_)
        return hook_->batch(*this, count, std::move(slot_bytes));
    return std::make_unique<mux_sink>(make_mux_sink(*this, count,
        std::move(slot_bytes)));
}

}  // namespace net
}  // namespace dpf

#endif  // LIBDPF_INCLUDE_DPF_NET_MUX_SINK_HPP__