package dhcpd

import (
	"context"
	"log/slog"
	"net"
	"net/netip"
	"os"
	"sync"
	"sync/atomic"
	"time"

	"golang.org/x/net/icmp"
	"golang.org/x/net/ipv4"
)

// ICMPPinger pings with an unprivileged ICMP socket (Linux "ping sockets",
// allowed for the groups in net.ipv4.ping_group_range, which systemd opens
// to everyone). When the socket can't be opened, the check is skipped and
// Unavailable says why.
type ICMPPinger struct {
	Log  *slog.Logger
	seq  atomic.Uint32
	once sync.Once
	// Unavailable is why pings can't be sent ("" when they can).
	Unavailable atomic.Value
}

// Ping reports whether a answers an echo request within timeout.
func (p *ICMPPinger) Ping(ctx context.Context, a netip.Addr, timeout time.Duration) bool {
	conn, err := icmp.ListenPacket("udp4", "0.0.0.0")
	if err != nil {
		p.once.Do(func() {
			msg := "can't send pings, so addresses aren't checked before they are offered and clients' last-seen times aren't known: " + err.Error() + " (check net.ipv4.ping_group_range)"
			p.Unavailable.Store(msg)
			if p.Log != nil {
				p.Log.Warn(msg)
			}
		})
		return false
	}
	defer conn.Close()
	seq := int(p.seq.Add(1) & 0xffff)
	msg := icmp.Message{Type: ipv4.ICMPTypeEcho, Body: &icmp.Echo{ID: os.Getpid() & 0xffff, Seq: seq, Data: []byte("steward")}}
	b, err := msg.Marshal(nil)
	if err != nil {
		return false
	}
	dst := &net.UDPAddr{IP: a.AsSlice()}
	if _, err := conn.WriteTo(b, dst); err != nil {
		return false
	}
	deadline := time.Now().Add(timeout)
	if d, ok := ctx.Deadline(); ok && d.Before(deadline) {
		deadline = d
	}
	conn.SetReadDeadline(deadline)
	buf := make([]byte, 1500)
	for {
		n, from, err := conn.ReadFrom(buf)
		if err != nil {
			return false
		}
		ua, ok := from.(*net.UDPAddr)
		if !ok || !ua.IP.Equal(dst.IP) {
			continue
		}
		m, err := icmp.ParseMessage(1, buf[:n])
		if err != nil {
			continue
		}
		if e, ok := m.Body.(*icmp.Echo); ok && m.Type == ipv4.ICMPTypeEchoReply && e.Seq == seq {
			return true
		}
	}
}

// Problem says why pings can't be sent ("" when they can).
func (p *ICMPPinger) Problem() string {
	if v, ok := p.Unavailable.Load().(string); ok {
		return v
	}
	return ""
}