package dhcpd

import (
	"context"
	"fmt"
	"net/netip"
	"time"

	"steward/internal/dhcp4"
	"steward/internal/model"
)

// Outbound is a reply and where to send it.
type Outbound struct {
	Packet *dhcp4.Packet
	// Dst is the client's address for unicast replies; invalid for a
	// broadcast on the interface.
	Dst netip.Addr
}

var broadcast = netip.AddrFrom4([4]byte{255, 255, 255, 255})

// Handle answers one request that arrived on an interface. It returns nil
// when there is nothing to send.
func (s *Server) Handle(ctx context.Context, iface string, p *dhcp4.Packet) *Outbound {
	if p.Op != dhcp4.BootRequest {
		return nil
	}
	if p.HType != 1 || p.HLen != 6 {
		s.warnOnce("htype "+iface, time.Hour, "ignoring a DHCP request for a hardware type other than Ethernet", "interface", iface, "htype", p.HType, "hlen", p.HLen)
		return nil
	}
	cfg := s.cfg.Load()
	subs := cfg.byIface[iface]
	if len(subs) == 0 {
		return nil
	}
	if !dhcp4.IsZero(p.GIAddr) {
		s.warnOnce("relay "+iface, time.Hour, "ignoring a relayed DHCP request: Steward 0.1 doesn't serve relays yet", "interface", iface, "relay", p.GIAddr)
		return nil
	}
	cl := client{mac: p.MAC().String(), id: p.ClientID()}
	t := p.Type()
	ev := Event{Time: s.now(), Interface: iface, Type: t.String(), MAC: cl.mac, Hostname: p.HostName()}
	if a, ok := p.IPOption(dhcp4.OptRequestedIP); ok {
		ev.IP = a.String()
	} else if !dhcp4.IsZero(p.CIAddr) {
		ev.IP = p.CIAddr.String()
	}
	s.events.add(ev)
	var out *Outbound
	switch t {
	case dhcp4.Discover:
		out = s.discover(ctx, cfg, iface, subs, p, cl)
	case dhcp4.Request:
		out = s.request(ctx, cfg, iface, p, cl)
	case dhcp4.Decline:
		s.decline(ctx, cfg, iface, p, cl)
	case dhcp4.Release:
		s.release(ctx, iface, p, cl)
	case dhcp4.Inform:
		out = s.inform(cfg, iface, p, cl)
	default:
		return nil
	}
	if out != nil {
		ev := Event{Time: s.now(), Interface: iface, Type: out.Packet.Type().String(), MAC: cl.mac, Hostname: p.HostName()}
		if !dhcp4.IsZero(out.Packet.YIAddr) {
			ev.IP = out.Packet.YIAddr.String()
		}
		if m := out.Packet.Get(dhcp4.OptMessage); m != nil {
			ev.Detail = string(m)
		}
		s.events.add(ev)
	}
	return out
}

// ---- DISCOVER

func (s *Server) discover(ctx context.Context, cfg *config, iface string, subs []*subnet, p *dhcp4.Packet, cl client) *Outbound {
	requested, _ := p.IPOption(dhcp4.OptRequestedIP)
	for try := 0; try < 8; try++ {
		ip, sub, fresh := s.choose(cfg, iface, subs, cl, requested)
		if !ip.IsValid() {
			s.warnOnce("full "+iface, 10*time.Minute, "no free address for a client: every pool on the interface is in use", "interface", iface, "client", cl.String())
			s.events.add(Event{Time: s.now(), Interface: iface, Type: "NO ADDRESS", MAC: cl.mac, Detail: "every pool on " + iface + " is in use"})
			return nil
		}
		// A fresh address from a pool is pinged first: something that
		// answers is using it without a lease.
		if fresh && cfg.settings.PingCheck && s.pinger != nil {
			if s.pinger.Ping(ctx, ip, cfg.settings.EffectivePingTimeout()) {
				s.conflict(ctx, cfg, iface, sub, ip)
				continue
			}
		}
		res := cfg.reservationFor(sub.ID, cl)
		if res != nil && res.IP != ip {
			res = nil
		}
		sid, err := s.serverAddr(iface, sub)
		if err != nil {
			s.warnOnce("sid "+iface, 10*time.Minute, "can't answer DHCP: "+err.Error(), "interface", iface)
			return nil
		}
		r := p.Reply(dhcp4.Offer)
		r.YIAddr = ip
		s.addOptions(r, p, sub, res, sid, true)
		return &Outbound{Packet: r}
	}
	return nil
}

// choose picks the address to offer: the client's reservation, the
// address it had, the one it asks for, or a free one from the pools. fresh
// says the address comes from a pool and the client never had it. The
// address is held for the client for a minute.
func (s *Server) choose(cfg *config, iface string, subs []*subnet, cl client, requested netip.Addr) (netip.Addr, *subnet, bool) {
	now := s.now()
	own := s.ownAddrs(iface)
	s.mu.Lock()
	defer s.mu.Unlock()
	hold := func(ip netip.Addr, sub *subnet, fresh bool) (netip.Addr, *subnet, bool) {
		s.offers[ip] = offer{cl: cl, expires: now.Add(offerHold)}
		return ip, sub, fresh
	}
	// 1. A reservation.
	for _, sub := range subs {
		if r := cfg.reservationFor(sub.ID, cl); r != nil {
			if s.usable(cfg, own, sub, r.IP, cl, now) {
				return hold(r.IP, sub, false)
			}
			s.warnOnce("resbusy "+r.IP.String(), 10*time.Minute, "a client's reserved address is held by another client; handing out another one until it is free", "ip", r.IP, "client", cl.String())
		}
	}
	// 2. The address the client had (an active lease first, then the most
	// recent one).
	var prev *model.Lease
	for _, l := range s.leases {
		if !l.Owns(cl.mac, cl.id) || l.State == model.LeaseDeclined {
			continue
		}
		if prev == nil || l.ActiveAt(now) && !prev.ActiveAt(now) || l.ActiveAt(now) == prev.ActiveAt(now) && l.End.After(prev.End) {
			prev = l
		}
	}
	if prev != nil {
		if sub := cfg.subnetFor(iface, prev.IP); sub != nil && s.usable(cfg, own, sub, prev.IP, cl, now) {
			return hold(prev.IP, sub, false)
		}
	}
	// 3. The address the client asks for.
	if requested.IsValid() {
		if sub := cfg.subnetFor(iface, requested); sub != nil && s.usable(cfg, own, sub, requested, cl, now) {
			return hold(requested, sub, true)
		}
	}
	// 4. A free address: one never handed out, in the subnet's order
	// (order.go), else the one free the longest.
	for _, sub := range subs {
		if ip, ok := s.neverUsed(cfg, own, sub, cl, now); ok {
			return hold(ip, sub, true)
		}
	}
	var oldest netip.Addr
	var oldestSub *subnet
	var oldestEnd time.Time
	for _, sub := range subs {
		for _, pool := range sub.Pools {
			eachIn(pool, false, func(ip netip.Addr) bool {
				if l := s.leases[ip]; l != nil && s.usable(cfg, own, sub, ip, cl, now) && (!oldest.IsValid() || l.End.Before(oldestEnd)) {
					oldest, oldestSub, oldestEnd = ip, sub, l.End
				}
				return false
			})
		}
	}
	if oldest.IsValid() {
		return hold(oldest, oldestSub, true)
	}
	return netip.Addr{}, nil, false
}

// usable reports whether ip can go to the client (s.mu held): inside the
// subnet but not its network, broadcast or server address; reserved for
// this client or in a pool and not reserved for anyone else; not leased
// to, declined by or offered to someone else.
func (s *Server) usable(cfg *config, own map[netip.Addr]bool, sub *subnet, ip netip.Addr, cl client, now time.Time) bool {
	if !sub.Prefix.Contains(ip) || ip == sub.network || ip == sub.broadcast || own[ip] {
		return false
	}
	if r := cfg.resByIP[ip]; r != nil {
		if !cl.matches(r) {
			return false
		}
	} else {
		inPool := false
		for _, p := range sub.Pools {
			if p.Contains(ip) {
				inPool = true
				break
			}
		}
		if !inPool {
			return false
		}
		// A client with a reservation in this subnet gets that address,
		// not a pool address.
		if r := cfg.reservationFor(sub.ID, cl); r != nil {
			return false
		}
	}
	if l := s.leases[ip]; l != nil && l.HeldAt(now) && (l.State == model.LeaseDeclined || !l.Owns(cl.mac, cl.id)) {
		return false
	}
	if o, ok := s.offers[ip]; ok && now.Before(o.expires) && o.cl != cl {
		return false
	}
	return true
}

// conflict holds back an address that answered a ping.
func (s *Server) conflict(ctx context.Context, cfg *config, iface string, sub *subnet, ip netip.Addr) {
	now := s.now()
	s.mu.Lock()
	defer s.mu.Unlock()
	delete(s.offers, ip)
	l := &model.Lease{IP: ip, SubnetID: sub.ID, State: model.LeaseDeclined, Start: now, End: now.Add(cfg.settings.EffectiveDeclineHold()), Interface: iface, Hostname: "(answered a ping)"}
	if err := s.putLease(ctx, l); err != nil {
		s.log.Error("saving a conflict failed", "ip", ip, "err", err)
	}
	s.log.Warn("an address answered a ping before it was offered: something uses it without a lease; holding it back", "ip", ip, "interface", iface, "until", l.End.Format(time.RFC3339))
	s.events.add(Event{Time: now, Interface: iface, Type: "CONFLICT", IP: ip.String(), Detail: "answered a ping; held back"})
}

// ---- REQUEST

func (s *Server) request(ctx context.Context, cfg *config, iface string, p *dhcp4.Packet, cl client) *Outbound {
	sidOpt, hasSID := p.IPOption(dhcp4.OptServerID)
	reqIP, hasReq := p.IPOption(dhcp4.OptRequestedIP)
	switch {
	case hasSID:
		// SELECTING: the client answers an offer, ours or another server's.
		if !s.isOwnAddr(iface, sidOpt) {
			s.mu.Lock()
			for ip, o := range s.offers {
				if o.cl == cl {
					delete(s.offers, ip)
				}
			}
			s.mu.Unlock()
			return nil
		}
		if !hasReq {
			return s.nak(cfg, iface, p, nil, "no requested address")
		}
		return s.ack(ctx, cfg, iface, p, cl, reqIP)
	case hasReq:
		// INIT-REBOOT: the client wants to keep the address it had.
		return s.ack(ctx, cfg, iface, p, cl, reqIP)
	case !dhcp4.IsZero(p.CIAddr):
		// RENEWING (unicast to us) or REBINDING (broadcast).
		return s.ack(ctx, cfg, iface, p, cl, p.CIAddr)
	}
	return s.nak(cfg, iface, p, nil, "no address in the request")
}

// ack gives the client ip if it may have it, saving the lease first;
// otherwise a NAK, so the client starts over.
func (s *Server) ack(ctx context.Context, cfg *config, iface string, p *dhcp4.Packet, cl client, ip netip.Addr) *Outbound {
	sub := cfg.subnetFor(iface, ip)
	if sub == nil {
		return s.nak(cfg, iface, p, nil, fmt.Sprintf("%s does not belong on %s", ip, iface))
	}
	now := s.now()
	own := s.ownAddrs(iface)
	s.mu.Lock()
	if !s.usable(cfg, own, sub, ip, cl, now) {
		s.mu.Unlock()
		why := "address not available"
		if r := cfg.reservationFor(sub.ID, cl); r != nil && r.IP != ip {
			why = "this client has a reservation for " + r.IP.String()
		}
		return s.nak(cfg, iface, p, sub, why)
	}
	start, host := now, p.HostName()
	var seen *time.Time
	if l := s.leases[ip]; l != nil && l.Owns(cl.mac, cl.id) {
		seen = l.LastSeen
		if l.ActiveAt(now) {
			start = l.Start
		}
		// Renewals often leave the host name out.
		if host == "" {
			host = l.Hostname
		}
	}
	l := &model.Lease{IP: ip, SubnetID: sub.ID, MAC: cl.mac, ClientID: cl.id, Hostname: host, State: model.LeaseActive,
		Start: start, End: now.Add(sub.leaseTime), Interface: iface, LastSeen: seen}
	err := s.putLease(ctx, l)
	delete(s.offers, ip)
	s.mu.Unlock()
	if err != nil {
		// Without the lease saved, the address could go to someone else
		// after a restart: don't hand it out.
		s.log.Error("saving a lease failed; not answering", "ip", ip, "err", err)
		return nil
	}
	sid, err := s.serverAddr(iface, sub)
	if err != nil {
		s.warnOnce("sid "+iface, 10*time.Minute, "can't answer DHCP: "+err.Error(), "interface", iface)
		return nil
	}
	res := cfg.reservationFor(sub.ID, cl)
	if res != nil && res.IP != ip {
		res = nil
	}
	r := p.Reply(dhcp4.Ack)
	r.YIAddr = ip
	r.CIAddr = p.CIAddr
	if !r.CIAddr.IsValid() {
		r.CIAddr = netip.AddrFrom4([4]byte{})
	}
	s.addOptions(r, p, sub, res, sid, true)
	return &Outbound{Packet: r, Dst: unicastTo(p)}
}

// nak refuses a request. Without authority (settings.not_authoritative),
// requests for addresses Steward doesn't know get no answer instead.
func (s *Server) nak(cfg *config, iface string, p *dhcp4.Packet, sub *subnet, why string) *Outbound {
	if cfg.settings.NotAuthoritative && sub == nil {
		return nil
	}
	sid, err := s.serverAddr(iface, sub)
	if err != nil {
		return nil
	}
	r := p.Reply(dhcp4.Nak)
	r.Flags |= dhcp4.FlagBroadcast
	r.Set(dhcp4.OptServerID, dhcp4.IP(sid))
	r.Set(dhcp4.OptMessage, []byte(why))
	return &Outbound{Packet: r}
}

// unicastTo is where a reply goes: the client's own address when it has
// one, else a broadcast.
func unicastTo(p *dhcp4.Packet) netip.Addr {
	if !dhcp4.IsZero(p.CIAddr) {
		return p.CIAddr
	}
	return netip.Addr{}
}

// ---- DECLINE, RELEASE, INFORM

func (s *Server) decline(ctx context.Context, cfg *config, iface string, p *dhcp4.Packet, cl client) {
	ip, ok := p.IPOption(dhcp4.OptRequestedIP)
	if !ok {
		return
	}
	now := s.now()
	s.mu.Lock()
	defer s.mu.Unlock()
	l := s.leases[ip]
	if l == nil || !l.Owns(cl.mac, cl.id) {
		return
	}
	l2 := *l
	l2.State, l2.End = model.LeaseDeclined, now.Add(cfg.settings.EffectiveDeclineHold())
	if err := s.putLease(ctx, &l2); err != nil {
		s.log.Error("saving a declined lease failed", "ip", ip, "err", err)
		return
	}
	s.log.Warn("a client declined its address: another device uses it; holding it back", "ip", ip, "client", cl.String(), "interface", iface, "until", l2.End.Format(time.RFC3339))
}

func (s *Server) release(ctx context.Context, iface string, p *dhcp4.Packet, cl client) {
	ip := p.CIAddr
	now := s.now()
	s.mu.Lock()
	defer s.mu.Unlock()
	l := s.leases[ip]
	if l == nil || !l.Owns(cl.mac, cl.id) || !l.ActiveAt(now) {
		return
	}
	l2 := *l
	l2.State, l2.End = model.LeaseReleased, now
	if err := s.putLease(ctx, &l2); err != nil {
		s.log.Error("saving a released lease failed", "ip", ip, "err", err)
	}
}

func (s *Server) inform(cfg *config, iface string, p *dhcp4.Packet, cl client) *Outbound {
	if dhcp4.IsZero(p.CIAddr) {
		return nil
	}
	sub := cfg.subnetFor(iface, p.CIAddr)
	if sub == nil {
		return nil
	}
	sid, err := s.serverAddr(iface, sub)
	if err != nil {
		return nil
	}
	var res *model.Reservation
	if r := cfg.resByIP[p.CIAddr]; r != nil && cl.matches(r) {
		res = r
	}
	r := p.Reply(dhcp4.Ack)
	r.CIAddr = p.CIAddr
	s.addOptions(r, p, sub, res, sid, false)
	return &Outbound{Packet: r, Dst: p.CIAddr}
}

// ---- options

// Options always sent when configured, even when the client doesn't ask.
var essential = []byte{dhcp4.OptSubnetMask, dhcp4.OptRouters, dhcp4.OptDNSServers, dhcp4.OptDomainName, dhcp4.OptHostName}

// addOptions adds the server identifier, the lease times (lease true) and
// the configured options: the reservation's over the subnet's over the
// settings'. With a parameter request list only what the client asked
// for (and the essentials) is sent, in its order.
func (s *Server) addOptions(r, req *dhcp4.Packet, sub *subnet, res *model.Reservation, sid netip.Addr, lease bool) {
	r.Set(dhcp4.OptServerID, dhcp4.IP(sid))
	if lease {
		lt := uint32(sub.leaseTime / time.Second)
		r.Set(dhcp4.OptLeaseTime, dhcp4.U32(lt))
		r.Set(dhcp4.OptRenewalTime, dhcp4.U32(lt/2))
		r.Set(dhcp4.OptRebindingTime, dhcp4.U32(lt*7/8))
	}
	opts := sub.options
	if res != nil && (len(res.Options) > 0 || res.Hostname != "") {
		opts = encodeOptions(res.Options, sub.options)
		if res.Hostname != "" {
			opts[dhcp4.OptHostName] = []byte(res.Hostname)
		}
	}
	all := map[byte][]byte{dhcp4.OptSubnetMask: dhcp4.IP(model.Mask(sub.Prefix))}
	for k, v := range opts {
		all[k] = v
	}
	var order []byte
	added := map[byte]bool{}
	addCode := func(c byte) {
		if _, ok := all[c]; ok && !added[c] {
			added[c] = true
			order = append(order, c)
		}
	}
	if prl := req.ParamRequest(); len(prl) > 0 {
		for _, c := range prl {
			addCode(c)
		}
		for _, c := range essential {
			addCode(c)
		}
	} else {
		for c := byte(1); c < 255; c++ {
			addCode(c)
		}
	}
	limit := req.MaxMessageSize() - 28 // IP and UDP headers
	for _, c := range order {
		r.Set(c, all[c])
		if r.Len() > limit {
			r.Options = r.Options[:len(r.Options)-1]
			s.warnOnce(fmt.Sprintf("size %d", c), time.Hour, "a reply would be too large for the client; leaving out options", "first_left_out", dhcp4.ByCode(c).Name, "max_message_size", req.MaxMessageSize())
			break
		}
	}
}