// Package mux provides stream multiplexing over a single net.Conn.
//
// Wire format per frame:
//
//	StreamID  (4B, big-endian)
//	Flags     (1B)
//	Length    (2B, big-endian, max 65535)
//	Data      (Length bytes)
//
// Total header = 7 bytes.
//
// Flags:
//
//	0x01  SYN   — open a new stream
//	0x02  FIN   — close a stream
//	0x04  DATA  — payload data
//	0x08  PING  — keepalive (StreamID=0)
//	0x10  PONG  — keepalive response (StreamID=0)
//	0x20  RST   — reset/abort a stream
package mux

import (
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"log"
	"net"
	"sync"
	"sync/atomic"
	"time"
)

const (
	headerSize = 7
	maxPayload = 65535

	FlagSYN  byte = 0x01
	FlagFIN  byte = 0x02
	FlagDATA byte = 0x04
	FlagPING byte = 0x08
	FlagPONG byte = 0x10
	FlagRST  byte = 0x20

	defaultWindowSize  = 256 * 1024 // 256KB per stream receive buffer
	pingInterval       = 15 * time.Second
	pingTimeout        = 10 * time.Second
	acceptBacklog      = 64
)

var (
	ErrSessionClosed = errors.New("mux: session closed")
	ErrStreamClosed  = errors.New("mux: stream closed")
	ErrStreamReset   = errors.New("mux: stream reset by peer")
	ErrTimeout       = errors.New("mux: timeout")
	ErrAcceptBacklog = errors.New("mux: accept backlog full")
)

// ─── Session ───
// A Session multiplexes many Streams over a single underlying net.Conn.

type Session struct {
	conn     net.Conn
	streams  map[uint32]*Stream
	mu       sync.RWMutex
	nextID   uint32 // client uses odd, server uses even
	isServer bool
	acceptCh chan *Stream
	writeMu  sync.Mutex // serialize frame writes
	closed   int32
	quit     chan struct{}
	once     sync.Once

	// stats
	BytesSent     int64
	BytesReceived int64
}

// NewSession wraps a net.Conn as a mux session.
// isServer determines stream ID allocation: server=even, client=odd.
func NewSession(conn net.Conn, isServer bool) *Session {
	s := &Session{
		conn:     conn,
		streams:  make(map[uint32]*Stream),
		acceptCh: make(chan *Stream, acceptBacklog),
		quit:     make(chan struct{}),
		isServer: isServer,
	}
	if isServer {
		s.nextID = 2
	} else {
		s.nextID = 1
	}
	go s.readLoop()
	go s.pingLoop()
	return s
}

// Open creates a new outbound stream.
func (s *Session) Open() (*Stream, error) {
	if s.IsClosed() {
		return nil, ErrSessionClosed
	}

	id := atomic.AddUint32(&s.nextID, 2) - 2 // increment by 2 to keep odd/even
	st := newStream(id, s)

	s.mu.Lock()
	s.streams[id] = st
	s.mu.Unlock()

	// Send SYN
	if err := s.writeFrame(id, FlagSYN, nil); err != nil {
		s.mu.Lock()
		delete(s.streams, id)
		s.mu.Unlock()
		return nil, err
	}

	return st, nil
}

// Accept waits for an inbound stream opened by the remote side.
func (s *Session) Accept() (*Stream, error) {
	select {
	case st := <-s.acceptCh:
		return st, nil
	case <-s.quit:
		return nil, ErrSessionClosed
	}
}

// Close shuts down the session and all streams.
func (s *Session) Close() error {
	s.once.Do(func() {
		atomic.StoreInt32(&s.closed, 1)
		close(s.quit)

		s.mu.Lock()
		for _, st := range s.streams {
			st.closeLocal()
		}
		s.streams = make(map[uint32]*Stream)
		s.mu.Unlock()

		s.conn.Close()
	})
	return nil
}

// IsClosed reports if the session is closed.
func (s *Session) IsClosed() bool {
	return atomic.LoadInt32(&s.closed) == 1
}

// NumStreams returns active stream count.
func (s *Session) NumStreams() int {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return len(s.streams)
}

// ─── Frame I/O ───

func (s *Session) writeFrame(streamID uint32, flags byte, data []byte) error {
	if len(data) > maxPayload {
		return fmt.Errorf("mux: payload too large: %d > %d", len(data), maxPayload)
	}

	hdr := make([]byte, headerSize)
	binary.BigEndian.PutUint32(hdr[0:4], streamID)
	hdr[4] = flags
	binary.BigEndian.PutUint16(hdr[5:7], uint16(len(data)))

	s.writeMu.Lock()
	defer s.writeMu.Unlock()

	s.conn.SetWriteDeadline(time.Now().Add(10 * time.Second))

	if _, err := s.conn.Write(hdr); err != nil {
		return err
	}
	if len(data) > 0 {
		if _, err := s.conn.Write(data); err != nil {
			return err
		}
	}

	atomic.AddInt64(&s.BytesSent, int64(headerSize+len(data)))
	return nil
}

func (s *Session) readLoop() {
	hdr := make([]byte, headerSize)
	for {
		if _, err := io.ReadFull(s.conn, hdr); err != nil {
			if !s.IsClosed() {
				log.Printf("[mux] read header error: %v", err)
			}
			s.Close()
			return
		}

		streamID := binary.BigEndian.Uint32(hdr[0:4])
		flags := hdr[4]
		length := binary.BigEndian.Uint16(hdr[5:7])

		var data []byte
		if length > 0 {
			data = make([]byte, length)
			if _, err := io.ReadFull(s.conn, data); err != nil {
				if !s.IsClosed() {
					log.Printf("[mux] read data error: %v", err)
				}
				s.Close()
				return
			}
		}

		atomic.AddInt64(&s.BytesReceived, int64(headerSize+int(length)))
		s.handleFrame(streamID, flags, data)
	}
}

func (s *Session) handleFrame(streamID uint32, flags byte, data []byte) {
	// Ping/Pong on StreamID 0
	if flags&FlagPING != 0 {
		s.writeFrame(0, FlagPONG, nil)
		return
	}
	if flags&FlagPONG != 0 {
		return // pong received, connection alive
	}

	// SYN — new inbound stream
	if flags&FlagSYN != 0 {
		st := newStream(streamID, s)
		s.mu.Lock()
		s.streams[streamID] = st
		s.mu.Unlock()

		select {
		case s.acceptCh <- st:
		default:
			log.Printf("[mux] accept backlog full, dropping stream %d", streamID)
			s.writeFrame(streamID, FlagRST, nil)
			s.mu.Lock()
			delete(s.streams, streamID)
			s.mu.Unlock()
		}
		return
	}

	// Find the stream
	s.mu.RLock()
	st, ok := s.streams[streamID]
	s.mu.RUnlock()

	if !ok {
		if flags&FlagRST == 0 {
			s.writeFrame(streamID, FlagRST, nil)
		}
		return
	}

	// RST
	if flags&FlagRST != 0 {
		st.resetByPeer()
		s.mu.Lock()
		delete(s.streams, streamID)
		s.mu.Unlock()
		return
	}

	// DATA
	if flags&FlagDATA != 0 && len(data) > 0 {
		st.pushData(data)
	}

	// FIN
	if flags&FlagFIN != 0 {
		st.finByPeer()
	}
}

func (s *Session) removeStream(id uint32) {
	s.mu.Lock()
	delete(s.streams, id)
	s.mu.Unlock()
}

func (s *Session) pingLoop() {
	ticker := time.NewTicker(pingInterval)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			if err := s.writeFrame(0, FlagPING, nil); err != nil {
				return
			}
		case <-s.quit:
			return
		}
	}
}

// ─── Stream ───
// A Stream is a virtual connection within a Session, implementing net.Conn.

type Stream struct {
	id      uint32
	sess    *Session
	readBuf *ringBuffer
	readCh  chan struct{} // signaled when data arrives
	closed  int32
	finRecv int32 // remote sent FIN
	finSent int32 // we sent FIN
	reset   int32
	mu      sync.Mutex
}

func newStream(id uint32, sess *Session) *Stream {
	return &Stream{
		id:      id,
		sess:    sess,
		readBuf: newRingBuffer(defaultWindowSize),
		readCh:  make(chan struct{}, 1),
	}
}

// Read implements io.Reader.
func (st *Stream) Read(p []byte) (int, error) {
	for {
		if atomic.LoadInt32(&st.reset) == 1 {
			return 0, ErrStreamReset
		}

		n := st.readBuf.Read(p)
		if n > 0 {
			return n, nil
		}

		// Buffer empty — check if FIN received
		if atomic.LoadInt32(&st.finRecv) == 1 {
			return 0, io.EOF
		}

		if atomic.LoadInt32(&st.closed) == 1 {
			return 0, ErrStreamClosed
		}

		// Wait for data
		select {
		case <-st.readCh:
		case <-st.sess.quit:
			return 0, ErrSessionClosed
		}
	}
}

// Write implements io.Writer.
func (st *Stream) Write(p []byte) (int, error) {
	if atomic.LoadInt32(&st.closed) == 1 || atomic.LoadInt32(&st.reset) == 1 {
		return 0, ErrStreamClosed
	}

	total := 0
	for len(p) > 0 {
		chunk := p
		if len(chunk) > maxPayload {
			chunk = p[:maxPayload]
		}
		if err := st.sess.writeFrame(st.id, FlagDATA, chunk); err != nil {
			return total, err
		}
		total += len(chunk)
		p = p[len(chunk):]
	}
	return total, nil
}

// Close sends FIN and closes the stream.
func (st *Stream) Close() error {
	if !atomic.CompareAndSwapInt32(&st.closed, 0, 1) {
		return nil
	}
	if atomic.CompareAndSwapInt32(&st.finSent, 0, 1) {
		st.sess.writeFrame(st.id, FlagFIN, nil)
	}
	st.sess.removeStream(st.id)
	st.notify()
	return nil
}

// LocalAddr implements net.Conn.
func (st *Stream) LocalAddr() net.Addr  { return st.sess.conn.LocalAddr() }
func (st *Stream) RemoteAddr() net.Addr { return st.sess.conn.RemoteAddr() }
func (st *Stream) SetDeadline(t time.Time) error {
	return nil // TODO: implement per-stream deadlines
}
func (st *Stream) SetReadDeadline(t time.Time) error  { return nil }
func (st *Stream) SetWriteDeadline(t time.Time) error { return nil }

func (st *Stream) pushData(data []byte) {
	st.readBuf.Write(data)
	st.notify()
}

func (st *Stream) finByPeer() {
	atomic.StoreInt32(&st.finRecv, 1)
	st.notify()
}

func (st *Stream) resetByPeer() {
	atomic.StoreInt32(&st.reset, 1)
	atomic.StoreInt32(&st.closed, 1)
	st.notify()
}

func (st *Stream) closeLocal() {
	atomic.StoreInt32(&st.closed, 1)
	st.notify()
}

func (st *Stream) notify() {
	select {
	case st.readCh <- struct{}{}:
	default:
	}
}

// ─── Ring Buffer ───
// Lock-free-ish ring buffer for stream receive data.

type ringBuffer struct {
	buf  []byte
	r, w int
	mu   sync.Mutex
	size int
}

func newRingBuffer(size int) *ringBuffer {
	return &ringBuffer{
		buf:  make([]byte, size),
		size: size,
	}
}

func (rb *ringBuffer) Write(p []byte) int {
	rb.mu.Lock()
	defer rb.mu.Unlock()

	n := 0
	for _, b := range p {
		next := (rb.w + 1) % rb.size
		if next == rb.r {
			break // full
		}
		rb.buf[rb.w] = b
		rb.w = next
		n++
	}
	return n
}

func (rb *ringBuffer) Read(p []byte) int {
	rb.mu.Lock()
	defer rb.mu.Unlock()

	n := 0
	for n < len(p) && rb.r != rb.w {
		p[n] = rb.buf[rb.r]
		rb.r = (rb.r + 1) % rb.size
		n++
	}
	return n
}

func (rb *ringBuffer) Len() int {
	rb.mu.Lock()
	defer rb.mu.Unlock()
	if rb.w >= rb.r {
		return rb.w - rb.r
	}
	return rb.size - rb.r + rb.w
}