/// @file dpf/net/policy.hpp
/// @brief Explicit transport, framing, socket, window, and deadline policy.
/// @details Library code never reads the environment. Every knob here is a
///          field the caller sets; `app::run_config::from_env` is the one place
///          that turns `DPF_*` variables into these values.
#ifndef LIBDPF_INCLUDE_DPF_NET_POLICY_HPP__
#define LIBDPF_INCLUDE_DPF_NET_POLICY_HPP__

#include <chrono>
#include <cstddef>
#include <cstdint>
#include <stdexcept>
#include <string>

namespace dpf
{
namespace net
{

/// @brief Which byte transport carries a party-to-party edge.
enum class transport : unsigned char
{
    memory_sink,    ///< in-process paired `memory_sink` (no stream arrays)
    memory_stream,  ///< in-process synchronous `stream_array`
    async_memory,   ///< in-process event-driven stream array
    mux,            ///< all lanes on one TCP connection
    parallel,       ///< one TCP connection per lane
    sctp,           ///< one SCTP association, lane i = SCTP stream i (Linux)
    local           ///< one unix-domain socket per lane (same host)
};

inline const char * transport_name(transport t) noexcept
{
    switch (t)
    {
    case transport::memory_sink:
        return "memory";
    case transport::memory_stream:
        return "stream";
    case transport::async_memory:
        return "async";
    case transport::mux:
        return "mux";
    case transport::parallel:
        return "parallel";
    case transport::sctp:
        return "sctp";
    case transport::local:
        return "local";
    }
    return "async";
}

/// @brief Parse a transport name. Unknown names throw.
inline transport parse_transport(const std::string & s)
{
    if (s == "memory")
        return transport::memory_sink;
    if (s == "stream")
        return transport::memory_stream;
    if (s == "async" || s == "async_memory")
        return transport::async_memory;
    if (s == "mux")
        return transport::mux;
    if (s == "parallel")
        return transport::parallel;
    if (s == "sctp")
        return transport::sctp;
    if (s == "local")
        return transport::local;
    throw std::invalid_argument("unknown transport: " + s
        + " (memory|stream|async|mux|parallel|sctp|local)");
}

/// @brief True for transports that cross a kernel socket.
inline bool is_socket_transport(transport t) noexcept
{
    return t == transport::mux || t == transport::parallel
        || t == transport::sctp || t == transport::local;
}

/// @brief Whether round payloads on a lane carry a `{round,nbytes}` header.
/// @details `automatic` frames only when rounds outnumber lanes. `always`
///          frames even with one lane per round (to measure the header, or to
///          let partial prefixes and reconnect resume work on every round).
///          `never` requires at least one lane per round.
enum class framing_mode : unsigned char
{
    automatic,
    always,
    never
};

inline const char * framing_name(framing_mode m) noexcept
{
    switch (m)
    {
    case framing_mode::automatic:
        return "auto";
    case framing_mode::always:
        return "always";
    case framing_mode::never:
        return "never";
    }
    return "auto";
}

inline framing_mode parse_framing(const std::string & s)
{
    if (s == "auto" || s == "automatic")
        return framing_mode::automatic;
    if (s == "always" || s == "on")
        return framing_mode::always;
    if (s == "never" || s == "off")
        return framing_mode::never;
    throw std::invalid_argument("unknown framing: " + s + " (auto|always|never)");
}

/// @brief Options applied to every TCP socket a backend adopts.
/// @details Zero means "leave the kernel default".
struct socket_options
{
    bool no_delay = true;
    /// Re-armed after every read and write: Linux clears it after one ACK.
    bool quickack = true;
    bool keepalive = true;
    int keepalive_idle_s = 0;
    int keepalive_interval_s = 0;
    int keepalive_count = 0;
    int send_buffer = 0;
    int recv_buffer = 0;
};

/// @brief Per-connection wire policy for the async stream backends.
struct wire_policy
{
    /// Outstanding-byte high-water mark per lane (per connection on mux and
    /// SCTP, per socket on parallel). `0` is unlimited.
    std::size_t window_bytes = std::size_t{1} << 20;
    /// Largest frame accepted from the peer.
    std::size_t max_frame = std::size_t{16} << 20;
    /// Split writes into frames of at most this many payload bytes and
    /// round-robin them across lanes. `0` sends each write as one frame.
    std::size_t chunk_bytes = std::size_t{64} << 10;
    /// Frames and bytes gathered into one write syscall.
    std::size_t coalesce_frames = 16;
    std::size_t coalesce_bytes = std::size_t{256} << 10;
    /// Drop the consumed prefix of an inbox once it passes this many bytes.
    std::size_t compact_bytes = std::size_t{64} << 10;
    socket_options socket{};

    void validate() const
    {
        if (max_frame == 0)
            throw std::invalid_argument("wire_policy: max_frame is 0");
        if (chunk_bytes > max_frame)
            throw std::invalid_argument(
                "wire_policy: chunk_bytes exceeds max_frame");
        if (coalesce_frames == 0)
            throw std::invalid_argument("wire_policy: coalesce_frames is 0");
    }
};

/// @brief Bounds for session setup and for draining a write window.
struct deadlines
{
    std::chrono::milliseconds join{30000};
    std::chrono::milliseconds connect{30000};
    std::chrono::milliseconds accept{30000};
    std::chrono::milliseconds handshake{30000};
    std::chrono::milliseconds drain{30000};
};

}  // namespace net
}  // namespace dpf

#endif  // LIBDPF_INCLUDE_DPF_NET_POLICY_HPP__