package ping

import (
	"context"
	"net"
	"net/netip"
	"os"
	"sync"
	"time"

	"github.com/sagernet/sing-tun/gtcpip/header"
	"github.com/sagernet/sing/common/buf"
	"github.com/sagernet/sing/common/control"
	M "github.com/sagernet/sing/common/metadata"
	"github.com/sagernet/sing/common/pipe"
)

type UnprivilegedConn struct {
	ctx           context.Context
	cancel        context.CancelFunc
	controlFunc   control.Func
	destination   netip.Addr
	idleTimeout   time.Duration
	receiveChan   chan *unprivilegedResponse
	readDeadline  pipe.Deadline
	mappingAccess sync.Mutex
	mapping       map[uint16]net.Conn
}

type unprivilegedResponse struct {
	Buffer *buf.Buffer
	Cmsg   *buf.Buffer
	Addr   netip.Addr
}

func newUnprivilegedConn(ctx context.Context, controlFunc control.Func, destination netip.Addr, idleTimeout time.Duration) (net.Conn, error) {
	conn, err := connect(false, controlFunc, destination)
	if err != nil {
		return nil, err
	}
	conn.Close()
	ctx, cancel := context.WithCancel(ctx)
	return &UnprivilegedConn{
		ctx:          ctx,
		cancel:       cancel,
		controlFunc:  controlFunc,
		destination:  destination,
		idleTimeout:  idleTimeout,
		receiveChan:  make(chan *unprivilegedResponse),
		readDeadline: pipe.MakeDeadline(),
		mapping:      make(map[uint16]net.Conn),
	}, nil
}

func (c *UnprivilegedConn) Read(b []byte) (n int, err error) {
	select {
	case packet := <-c.receiveChan:
		n = copy(b, packet.Buffer.Bytes())
		packet.Buffer.Release()
		packet.Cmsg.Release()
		return
	case <-c.readDeadline.Wait():
		return 0, os.ErrDeadlineExceeded
	case <-c.ctx.Done():
		return 0, os.ErrClosed
	}
}

func (c *UnprivilegedConn) ReadMsg(b []byte, oob []byte) (n, oobn int, addr netip.Addr, err error) {
	select {
	case packet := <-c.receiveChan:
		n = copy(b, packet.Buffer.Bytes())
		oobn = copy(oob, packet.Cmsg.Bytes())
		addr = packet.Addr
		packet.Buffer.Release()
		packet.Cmsg.Release()
		return
	case <-c.readDeadline.Wait():
		return 0, 0, netip.Addr{}, os.ErrDeadlineExceeded
	case <-c.ctx.Done():
		return 0, 0, netip.Addr{}, os.ErrClosed
	}
}

func (c *UnprivilegedConn) Write(b []byte) (n int, err error) {
	var identifier uint16
	if !c.destination.Is6() {
		icmpHdr := header.ICMPv4(b)
		identifier = icmpHdr.Ident()
	} else {
		icmpHdr := header.ICMPv6(b)
		identifier = icmpHdr.Ident()
	}

	c.mappingAccess.Lock()
	if err = c.ctx.Err(); err != nil {
		c.mappingAccess.Unlock()
		return 0, err
	}
	conn, loaded := c.mapping[identifier]
	if !loaded {
		conn, err = connect(false, c.controlFunc, c.destination)
		if err != nil {
			c.mappingAccess.Unlock()
			return
		}
		go c.fetchResponse(conn.(*net.UDPConn), identifier)
		c.mapping[identifier] = conn
	}
	c.mappingAccess.Unlock()
	n, err = conn.Write(b)
	if err != nil {
		c.removeConn(conn.(*net.UDPConn), identifier)
	}
	return
}

func (c *UnprivilegedConn) fetchResponse(conn *net.UDPConn, identifier uint16) {
	defer c.removeConn(conn, identifier)
	for {
		if c.idleTimeout > 0 {
			err := conn.SetReadDeadline(time.Now().Add(c.idleTimeout))
			if err != nil {
				return
			}
		}
		buffer := buf.NewSize(maxICMPPacketSize)
		cmsgBuffer := buf.NewSize(1024)
		n, oobN, _, addr, err := conn.ReadMsgUDPAddrPort(buffer.FreeBytes(), cmsgBuffer.FreeBytes())
		if err != nil {
			buffer.Release()
			cmsgBuffer.Release()
			return
		}
		buffer.Truncate(n)
		cmsgBuffer.Truncate(oobN)
		if !c.destination.Is6() {
			icmpHdr := header.ICMPv4(buffer.Bytes())
			icmpHdr.SetIdent(identifier)
			icmpHdr.SetChecksum(header.ICMPv4Checksum(icmpHdr, 0))
		} else {
			icmpHdr := header.ICMPv6(buffer.Bytes())
			icmpHdr.SetIdent(identifier)
			// offload checksum here since we don't have source address here
		}
		select {
		case c.receiveChan <- &unprivilegedResponse{
			Buffer: buffer,
			Cmsg:   cmsgBuffer,
			Addr:   addr.Addr(),
		}:
		case <-c.ctx.Done():
			buffer.Release()
			cmsgBuffer.Release()
			return
		}
	}
}

func (c *UnprivilegedConn) removeConn(conn *net.UDPConn, identifier uint16) {
	c.mappingAccess.Lock()
	mappedConn, loaded := c.mapping[identifier]
	if loaded && mappedConn == conn {
		delete(c.mapping, identifier)
	}
	c.mappingAccess.Unlock()
	_ = conn.Close()
}

func (c *UnprivilegedConn) Close() error {
	c.mappingAccess.Lock()
	defer c.mappingAccess.Unlock()
	c.cancel()
	for _, conn := range c.mapping {
		_ = conn.Close()
	}
	clear(c.mapping)
	return nil
}

func (c *UnprivilegedConn) LocalAddr() net.Addr {
	return M.Socksaddr{}
}

func (c *UnprivilegedConn) RemoteAddr() net.Addr {
	return M.SocksaddrFrom(c.destination, 0).UDPAddr()
}

func (c *UnprivilegedConn) SetDeadline(t time.Time) error {
	return os.ErrInvalid
}

func (c *UnprivilegedConn) SetReadDeadline(t time.Time) error {
	c.readDeadline.Set(t)
	return nil
}

func (c *UnprivilegedConn) SetWriteDeadline(t time.Time) error {
	return os.ErrInvalid
}
