package utp

import (
	"context"
	"io"
	"net"
	"time"
)

const (
	initRTO      = 1000 // ms
	retransCheck = 500  // ms
	defaultWnd   = 64 * 1024
)

type Conn struct {
	sock   *Socket
	addr   *net.UDPAddr
	recvID uint16
	sendID uint16

	in       chan packet
	reads    chan readReq
	writes   chan writeReq
	closeReq chan struct{}
	ready    chan struct{}
	ctx      context.Context
	cancel   context.CancelFunc

	seq     uint16
	ack     uint16
	unacked []sent
	reorder map[uint16][]byte
	rest    []byte
	pending *readReq
	rto     uint32
}

type readReq struct {
	p     []byte
	reply chan readResp
}

type readResp struct {
	n   int
	err error
}

type writeReq struct {
	data  []byte
	reply chan error
}

type sent struct {
	seq  uint16
	data []byte
	at   time.Time
}

func (c *Conn) flush() {
	if c.pending == nil || len(c.rest) == 0 {
		return
	}
	n := copy(c.pending.p, c.rest)
	c.rest = c.rest[n:]
	c.pending.reply <- readResp{n, nil}
	c.pending = nil
}

func (c *Conn) deliver(payload []byte) {
	c.rest = append(c.rest, payload...)
	c.flush()
}

func (c *Conn) send(typ uint8, payload []byte) error {
	c.seq++
	h := &header{
		typ:       typ,
		ver:       1,
		connID:    c.sendID,
		timestamp: timestamp(),
		wnd:       defaultWnd,
		seq:       c.seq,
		ack:       c.ack,
	}
	buf := make([]byte, headerSize+len(payload))
	encode(h, buf)
	copy(buf[headerSize:], payload)
	if _, err := c.sock.conn.WriteToUDP(buf, c.addr); err != nil {
		return err
	}
	if typ == Data || typ == Syn || typ == Fin {
		c.unacked = append(c.unacked, sent{c.seq, buf, time.Now()})
	}
	return nil
}

func (c *Conn) sendState() error {
	h := &header{
		typ:       State,
		ver:       1,
		connID:    c.sendID,
		timestamp: timestamp(),
		wnd:       defaultWnd,
		seq:       c.seq,
		ack:       c.ack,
	}
	buf := make([]byte, headerSize)
	encode(h, buf)
	_, err := c.sock.conn.WriteToUDP(buf, c.addr)
	return err
}

func (c *Conn) sendSyn() error {
	c.seq++
	h := &header{
		typ:       Syn,
		ver:       1,
		connID:    c.recvID,
		timestamp: timestamp(),
		wnd:       defaultWnd,
		seq:       c.seq,
		ack:       c.ack,
	}
	buf := make([]byte, headerSize)
	encode(h, buf)
	if _, err := c.sock.conn.WriteToUDP(buf, c.addr); err != nil {
		return err
	}
	c.unacked = append(c.unacked, sent{c.seq, buf, time.Now()})
	return nil
}

func (c *Conn) retrans() error {
	select {
	case <-c.ctx.Done():
		return nil
	default:
	}
	now := time.Now()
	for i := range c.unacked {
		p := &c.unacked[i]
		if now.Sub(p.at) > time.Duration(c.rto)*time.Millisecond {
			if _, err := c.sock.conn.WriteToUDP(p.data, c.addr); err != nil {
				return err
			}
			p.at = now
		}
	}
	return nil
}

func (c *Conn) recv(p packet) (bool, error) {
	switch p.hdr.typ {
	case Syn:
		c.ack = p.hdr.seq
		if err := c.sendState(); err != nil {
			return false, err
		}
	case State:
		select {
		case <-c.ready:
		default:
			c.ack = p.hdr.seq - 1
			close(c.ready)
		}
		var remaining []sent
		for _, s := range c.unacked {
			if seqLess(p.hdr.ack, s.seq) {
				remaining = append(remaining, s)
			}
		}
		c.unacked = remaining
	case Data:
		seq, payload := p.hdr.seq, p.payload
		if seq == c.ack+1 {
			c.deliver(payload)
			c.ack++
			for {
				if p, ok := c.reorder[c.ack+1]; ok {
					c.deliver(p)
					delete(c.reorder, c.ack+1)
					c.ack++
				} else {
					break
				}
			}
		} else if seqLess(c.ack+1, seq) {
			c.reorder[seq] = payload
		}
		if err := c.sendState(); err != nil {
			return false, err
		}
	case Fin, Reset:
		return true, nil
	}
	return false, nil
}

func (c *Conn) shutdown() {
	if c.pending != nil {
		c.pending.reply <- readResp{0, io.EOF}
	}
	select {
	case <-c.ready:
		c.send(Fin, nil)
	default:
	}
	c.unacked = nil
	c.cancel()
	c.sock.removeConn(c.recvID)
}

func (c *Conn) run() {
	c.reorder = make(map[uint16][]byte)
	c.rto = initRTO

	if c.recvID < c.sendID {
		c.seq = 1
		if err := c.sendSyn(); err != nil {
			c.shutdown()
			return
		}
	}

	ticker := time.NewTicker(retransCheck * time.Millisecond)
	defer ticker.Stop()

	for {
		select {
		case p := <-c.in:
			done, err := c.recv(p)
			if err != nil {
				c.shutdown()
				return
			}
			if done {
				c.shutdown()
				return
			}
		case req := <-c.reads:
			if c.pending != nil {
				req.reply <- readResp{0, io.ErrNoProgress}
				continue
			}
			c.pending = &req
			c.flush()
		case req := <-c.writes:
			if err := c.send(Data, req.data); err != nil {
				req.reply <- err
				c.shutdown()
				return
			}
			req.reply <- nil
		case <-c.closeReq:
			c.shutdown()
			return
		case <-ticker.C:
			c.retrans()
		case <-c.ctx.Done():
			return
		}
	}
}

func (c *Conn) Read(p []byte) (int, error) {
	reply := make(chan readResp, 1)
	select {
	case c.reads <- readReq{p, reply}:
		r := <-reply
		return r.n, r.err
	case <-c.ctx.Done():
		return 0, io.EOF
	}
}

func (c *Conn) Write(p []byte) (int, error) {
	reply := make(chan error, 1)
	select {
	case c.writes <- writeReq{p, reply}:
		err := <-reply
		if err != nil {
			return 0, err
		}
		return len(p), nil
	case <-c.ctx.Done():
		return 0, io.ErrClosedPipe
	}
}

func (c *Conn) Close() error {
	select {
	case c.closeReq <- struct{}{}:
	case <-c.ctx.Done():
	}
	return nil
}

func timestamp() uint32 {
	return uint32(time.Now().UnixMicro() & 0xFFFFFFFF)
}

func seqLess(a, b uint16) bool {
	return int16(a-b) < 0
}