// Package dhcpd is Steward's DHCPv4 server: it listens on the interfaces
// that have subnets, answers requests from those subnets only, and keeps
// the leases (docs/design.md, "Answering DHCP").
package dhcpd

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

	"steward/internal/model"
)

// LeaseStore persists leases.
type LeaseStore interface {
	LoadLeases(ctx context.Context) ([]*model.Lease, error)
	PutLease(ctx context.Context, l *model.Lease) error
	DeleteLease(ctx context.Context, ip netip.Addr) error
	PruneLeases(ctx context.Context, before time.Time) (int64, error)
	SetLastSeen(ctx context.Context, seen map[netip.Addr]time.Time) error
}

// Interfaces reports a network interface's IPv4 addresses with their
// prefix lengths (10.0.10.1/24); an error when it doesn't exist.
type Interfaces interface {
	Addrs(name string) ([]netip.Prefix, error)
}

// Pinger checks whether an address answers (conflict detection, and
// clients' last-seen times).
type Pinger interface {
	Ping(ctx context.Context, a netip.Addr, timeout time.Duration) bool
}

// How long an offer holds its address for the client.
const offerHold = time.Minute

// Leases that ended this long ago are deleted (until then, a client that
// comes back gets its old address if it is free).
const keepEnded = 30 * 24 * time.Hour

// Server answers DHCP requests.
type Server struct {
	log    *slog.Logger
	store  LeaseStore
	ifaces Interfaces
	pinger Pinger
	now    func() time.Time

	// ServerPort and ClientPort are 67 and 68 (tests use others).
	ServerPort, ClientPort int

	cfg atomic.Pointer[config]

	mu     sync.Mutex
	leases map[netip.Addr]*model.Lease
	offers map[netip.Addr]offer

	events  events
	warned  sync.Map // throttled log lines: key -> time.Time
	listen  listeners
	changed chan struct{}
}

type offer struct {
	cl      client
	expires time.Time
}

// New makes a server. ifaces and pinger may be nil (the host's interfaces;
// no ping check).
func New(log *slog.Logger, st LeaseStore, ifaces Interfaces, pinger Pinger) *Server {
	if log == nil {
		log = slog.Default()
	}
	if ifaces == nil {
		ifaces = &HostInterfaces{}
	}
	s := &Server{log: log, store: st, ifaces: ifaces, pinger: pinger, now: time.Now, ServerPort: 67, ClientPort: 68,
		leases: map[netip.Addr]*model.Lease{}, offers: map[netip.Addr]offer{}, changed: make(chan struct{}, 1)}
	s.cfg.Store(compile(&model.State{Settings: model.DefaultSettings()}))
	s.events.max = 1000
	return s
}

// LoadLeases reads the leases from the store (at start).
func (s *Server) LoadLeases(ctx context.Context) error {
	ls, err := s.store.LoadLeases(ctx)
	if err != nil {
		return err
	}
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, l := range ls {
		s.leases[l.IP] = l
	}
	return nil
}

// SetConfig gives the server a new configuration. Listeners follow it.
func (s *Server) SetConfig(st *model.State) {
	s.cfg.Store(compile(st))
	select {
	case s.changed <- struct{}{}:
	default:
	}
}

// Leases returns a copy of every lease, by address.
func (s *Server) Leases() []*model.Lease {
	s.mu.Lock()
	defer s.mu.Unlock()
	out := make([]*model.Lease, 0, len(s.leases))
	for _, l := range s.leases {
		c := *l
		out = append(out, &c)
	}
	sort.Slice(out, func(i, j int) bool { return out[i].IP.Less(out[j].IP) })
	return out
}

// Lease returns one lease.
func (s *Server) Lease(ip netip.Addr) *model.Lease {
	s.mu.Lock()
	defer s.mu.Unlock()
	if l := s.leases[ip]; l != nil {
		c := *l
		return &c
	}
	return nil
}

// ErrNoLease is returned for an address without a lease.
var ErrNoLease = errors.New("no lease for that address")

// DeleteLease forgets a lease: the address is free at once (the client
// may still be using it until it next renews, and then gets a NAK).
func (s *Server) DeleteLease(ctx context.Context, ip netip.Addr) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	if s.leases[ip] == nil {
		return ErrNoLease
	}
	if err := s.store.DeleteLease(ctx, ip); err != nil {
		return err
	}
	delete(s.leases, ip)
	delete(s.offers, ip)
	return nil
}

// ImportLeases adds leases (from another server), replacing what is known
// for their addresses.
func (s *Server) ImportLeases(ctx context.Context, ls []*model.Lease) error {
	s.mu.Lock()
	defer s.mu.Unlock()
	for _, l := range ls {
		if err := s.store.PutLease(ctx, l); err != nil {
			return err
		}
		c := *l
		s.leases[l.IP] = &c
	}
	return nil
}

// Prune deletes leases that ended long ago, and forgets old offers.
func (s *Server) Prune(ctx context.Context) {
	now := s.now()
	s.mu.Lock()
	defer s.mu.Unlock()
	cut := now.Add(-keepEnded)
	n := 0
	for ip, l := range s.leases {
		if l.End.Before(cut) {
			delete(s.leases, ip)
			n++
		}
	}
	for ip, o := range s.offers {
		if now.After(o.expires) {
			delete(s.offers, ip)
		}
	}
	if n > 0 {
		if _, err := s.store.PruneLeases(ctx, cut); err != nil {
			s.log.Warn("pruning old leases failed", "err", err)
		}
	}
}

// putLease saves a lease, then keeps it (s.mu held).
func (s *Server) putLease(ctx context.Context, l *model.Lease) error {
	l.Updated = s.now()
	if err := s.store.PutLease(ctx, l); err != nil {
		return err
	}
	c := *l
	s.leases[l.IP] = &c
	return nil
}

// warnOnce logs a warning at most once per interval for a key.
func (s *Server) warnOnce(key string, every time.Duration, msg string, args ...any) {
	now := s.now()
	if v, ok := s.warned.Load(key); ok && now.Sub(v.(time.Time)) < every {
		return
	}
	s.warned.Store(key, now)
	s.log.Warn(msg, args...)
}

// ---- interfaces

// HostInterfaces reads this machine's interfaces (cached briefly, since
// every request asks).
type HostInterfaces struct {
	mu    sync.Mutex
	at    time.Time
	cache map[string][]netip.Prefix
	err   error
}

// Addrs implements Interfaces.
func (h *HostInterfaces) Addrs(name string) ([]netip.Prefix, error) {
	all, err := h.All()
	if err != nil {
		return nil, err
	}
	a, ok := all[name]
	if !ok {
		return nil, errors.New("no interface " + name)
	}
	return a, nil
}

// All returns every interface with its IPv4 prefixes.
func (h *HostInterfaces) All() (map[string][]netip.Prefix, error) {
	h.mu.Lock()
	defer h.mu.Unlock()
	if time.Since(h.at) < 5*time.Second && (h.cache != nil || h.err != nil) {
		return h.cache, h.err
	}
	h.at = time.Now()
	ifs, err := net.Interfaces()
	if err != nil {
		h.cache, h.err = nil, err
		return nil, err
	}
	m := map[string][]netip.Prefix{}
	for _, i := range ifs {
		var ps []netip.Prefix
		addrs, _ := i.Addrs()
		for _, a := range addrs {
			if n, ok := a.(*net.IPNet); ok {
				if ip4 := n.IP.To4(); ip4 != nil {
					bits, _ := n.Mask.Size()
					ps = append(ps, netip.PrefixFrom(netip.AddrFrom4([4]byte(ip4)), bits))
				}
			}
		}
		m[i.Name] = ps
	}
	h.cache, h.err = m, nil
	return m, nil
}

// serverAddr is the interface's address to answer a subnet's clients
// from: its address in the subnet, else its first IPv4 address.
func (s *Server) serverAddr(iface string, sub *subnet) (netip.Addr, error) {
	addrs, err := s.ifaces.Addrs(iface)
	if err != nil {
		return netip.Addr{}, err
	}
	for _, a := range addrs {
		if sub != nil && sub.Prefix.Contains(a.Addr()) {
			return a.Addr(), nil
		}
	}
	if len(addrs) > 0 {
		return addrs[0].Addr(), nil
	}
	return netip.Addr{}, errors.New("interface " + iface + " has no IPv4 address")
}

// ownAddrs is the set of the interface's addresses.
func (s *Server) ownAddrs(iface string) map[netip.Addr]bool {
	addrs, _ := s.ifaces.Addrs(iface)
	m := make(map[netip.Addr]bool, len(addrs))
	for _, p := range addrs {
		m[p.Addr()] = true
	}
	return m
}

// isOwnAddr reports whether a is one of the interface's addresses.
func (s *Server) isOwnAddr(iface string, a netip.Addr) bool {
	addrs, _ := s.ifaces.Addrs(iface)
	for _, p := range addrs {
		if p.Addr() == a {
			return true
		}
	}
	return false
}

// ---- events

// Event is one DHCP message in or out, for the activity log.
type Event struct {
	Time      time.Time `json:"time"`
	Interface string    `json:"interface"`
	Type      string    `json:"type"` // DHCPDISCOVER, DHCPOFFER, ...
	MAC       string    `json:"mac"`
	IP        string    `json:"ip,omitempty"`
	Hostname  string    `json:"hostname,omitempty"`
	Detail    string    `json:"detail,omitempty"`
}

type events struct {
	mu     sync.Mutex
	max    int
	list   []Event
	counts map[string]uint64
}

func (e *events) add(ev Event) {
	e.mu.Lock()
	defer e.mu.Unlock()
	e.list = append(e.list, ev)
	if len(e.list) > e.max {
		e.list = append([]Event(nil), e.list[len(e.list)-e.max:]...)
	}
	if e.counts == nil {
		e.counts = map[string]uint64{}
	}
	e.counts[ev.Type]++
}

// Events returns the latest events, newest first, optionally only those
// keep accepts.
func (s *Server) Events(limit int, keep func(Event) bool) []Event {
	s.events.mu.Lock()
	defer s.events.mu.Unlock()
	out := []Event{}
	for i := len(s.events.list) - 1; i >= 0 && (limit <= 0 || len(out) < limit); i-- {
		ev := s.events.list[i]
		if keep != nil && !keep(ev) {
			continue
		}
		out = append(out, ev)
	}
	return out
}

// Counts returns how many messages of each type were seen and sent since
// the start.
func (s *Server) Counts() map[string]uint64 {
	s.events.mu.Lock()
	defer s.events.mu.Unlock()
	out := map[string]uint64{}
	for k, v := range s.events.counts {
		out[k] = v
	}
	return out
}