// Package runtimecfg owns the in-memory view of everything the DNS data path
// needs: settings, the compiled zone index, the compiled policy index and the
// recursion ACL.
//
// The whole view is an immutable Snapshot behind an atomic pointer. Query
// handling reads the pointer once and then works entirely from immutable data,
// so it never blocks on a lock and never touches SQLite. Configuration changes
// build a brand new snapshot and swap it in; readers already in flight finish
// against the old one.
package runtimecfg

import (
	"context"
	"log/slog"
	"sync"
	"sync/atomic"
	"time"

	"github.com/owen/vibedns/internal/authoritative"
	"github.com/owen/vibedns/internal/blacklist"
	"github.com/owen/vibedns/internal/config"
	"github.com/owen/vibedns/internal/database"
	"github.com/owen/vibedns/internal/policy"
	"github.com/owen/vibedns/internal/resolver"
)

// Snapshot is an immutable view of the runtime configuration.
type Snapshot struct {
	Settings config.Settings
	Zones    *authoritative.Index
	Policy   *policy.Index
	ACL      *resolver.ACL

	BuiltAt  time.Time
	BuildMS  int64
	Problems []authoritative.BuildError

	// Counts are computed at build time for the dashboard, so the UI never
	// has to walk the indexes.
	ZoneCount        int
	RecordCount      int
	BlacklistDomains int
	AllowlistDomains int
	NetworkCount     int
}

// Manager builds and publishes snapshots.
type Manager struct {
	db  *database.DB
	log *slog.Logger

	cur atomic.Pointer[Snapshot]

	buildMu sync.Mutex // serialises snapshot construction

	subMu sync.RWMutex
	subs  []func(*Snapshot)

	trigger chan struct{}
	wg      sync.WaitGroup
	once    sync.Once

	reloads  atomic.Int64
	failures atomic.Int64
}

// New creates a manager and builds the first snapshot.
func New(ctx context.Context, db *database.DB, log *slog.Logger) (*Manager, error) {
	m := &Manager{
		db:      db,
		log:     log,
		trigger: make(chan struct{}, 1),
	}
	if err := m.Reload(ctx); err != nil {
		return nil, err
	}
	return m, nil
}

// Current returns the active snapshot. It is never nil after New succeeds.
func (m *Manager) Current() *Snapshot { return m.cur.Load() }

// Settings is a shorthand for the active settings.
func (m *Manager) Settings() config.Settings { return m.cur.Load().Settings }

// OnReload registers a callback invoked after every successful reload. It is
// how the cache, resolver, rate limiter and query logger pick up new settings.
func (m *Manager) OnReload(fn func(*Snapshot)) {
	m.subMu.Lock()
	m.subs = append(m.subs, fn)
	m.subMu.Unlock()
}

// Reload rebuilds the snapshot from the database and publishes it.
func (m *Manager) Reload(ctx context.Context) error {
	m.buildMu.Lock()
	defer m.buildMu.Unlock()

	start := time.Now()
	snap, err := m.build(ctx)
	if err != nil {
		m.failures.Add(1)
		return err
	}
	snap.BuildMS = time.Since(start).Milliseconds()
	m.cur.Store(snap)
	m.reloads.Add(1)

	m.subMu.RLock()
	subs := make([]func(*Snapshot), len(m.subs))
	copy(subs, m.subs)
	m.subMu.RUnlock()
	for _, fn := range subs {
		fn(snap)
	}

	m.log.Debug("configuration reloaded",
		"zones", snap.ZoneCount, "records", snap.RecordCount,
		"blacklist_domains", snap.BlacklistDomains, "networks", snap.NetworkCount,
		"duration_ms", snap.BuildMS)
	for _, p := range snap.Problems {
		m.log.Warn("record skipped while building the zone index",
			"zone", p.ZoneName, "name", p.Name, "type", p.Type, "error", p.Err)
	}
	return nil
}

// build assembles a snapshot. It performs a handful of bulk queries rather
// than per-object lookups, so a reload is cheap even with large lists.
func (m *Manager) build(ctx context.Context) (*Snapshot, error) {
	stored, err := m.db.Settings(ctx)
	if err != nil {
		return nil, err
	}
	settings := config.LoadSettings(stored)

	zones, records, err := m.db.SnapshotZones(ctx)
	if err != nil {
		return nil, err
	}
	zoneIdx, problems := authoritative.Build(zones, records)

	lists, err := m.buildDomainSets(ctx)
	if err != nil {
		return nil, err
	}

	networks, assignments, err := m.db.SnapshotNetworks(ctx)
	if err != nil {
		return nil, err
	}
	policies, err := m.db.Policies(ctx)
	if err != nil {
		return nil, err
	}
	policyIdx := policy.Build(networks, assignments, policies, lists)

	acl, err := resolver.NewACL(settings.Resolver.AllowNetworks, settings.Resolver.DenyNetworks)
	if err != nil {
		// A bad ACL must not silently become permissive; fall back to an
		// empty allow list, which denies recursion to everyone.
		m.log.Error("recursion ACL is invalid, denying recursion to all clients", "error", err)
		acl, _ = resolver.NewACL(nil, nil)
	}

	snap := &Snapshot{
		Settings: settings,
		Zones:    zoneIdx,
		Policy:   policyIdx,
		ACL:      acl,
		BuiltAt:  time.Now(),
		Problems: problems,
	}
	snap.ZoneCount = zoneIdx.Len()
	for _, z := range zoneIdx.Zones() {
		snap.RecordCount += z.RecordCount()
	}
	pstats := policyIdx.Stats()
	snap.BlacklistDomains = int(pstats.BlockedDomains)
	snap.AllowlistDomains = int(pstats.AllowedDomains)
	snap.NetworkCount = pstats.Networks
	return snap, nil
}

// buildDomainSets compiles every enabled domain list into a matcher. Sets are
// shared by pointer across policies, so a list used by several policies costs
// memory exactly once.
func (m *Manager) buildDomainSets(ctx context.Context) (map[int64]*blacklist.Set, error) {
	meta, err := m.db.DomainLists(ctx, "", "")
	if err != nil {
		return nil, err
	}
	builders := make(map[int64]*blacklist.Builder, len(meta))
	for _, l := range meta {
		if !l.Enabled {
			continue
		}
		builders[l.ID] = blacklist.NewBuilder(l.ID, l.Name, l.Kind, l.DomainCount)
	}

	err = m.db.SnapshotDomains(ctx, func(e database.SnapshotDomainEntry) {
		if b, ok := builders[e.ListID]; ok {
			b.Add(e.Domain, e.MatchSubdomains)
		}
	})
	if err != nil {
		return nil, err
	}

	out := make(map[int64]*blacklist.Set, len(builders))
	for id, b := range builders {
		out[id] = b.Build()
	}
	return out, nil
}

// RequestReload schedules a reload without blocking the caller.
//
// Requests arriving while one is pending are coalesced, so importing a hundred
// thousand domains one API call at a time still results in a bounded number of
// index rebuilds.
func (m *Manager) RequestReload() {
	select {
	case m.trigger <- struct{}{}:
	default:
	}
}

// Start launches the debounced reload worker.
func (m *Manager) Start(ctx context.Context) {
	m.once.Do(func() {
		m.wg.Add(1)
		go m.reloadLoop(ctx)
	})
}

// Stop waits for the reload worker to exit.
func (m *Manager) Stop() { m.wg.Wait() }

const reloadDebounce = 250 * time.Millisecond

func (m *Manager) reloadLoop(ctx context.Context) {
	defer m.wg.Done()
	for {
		select {
		case <-ctx.Done():
			return
		case <-m.trigger:
			// Coalesce a burst of changes into one rebuild.
			timer := time.NewTimer(reloadDebounce)
		drain:
			for {
				select {
				case <-m.trigger:
					if !timer.Stop() {
						<-timer.C
					}
					timer.Reset(reloadDebounce)
				case <-timer.C:
					break drain
				case <-ctx.Done():
					timer.Stop()
					return
				}
			}
			if err := m.Reload(ctx); err != nil {
				m.log.Error("could not reload configuration", "error", err)
			}
		}
	}
}

// Stats reports reload activity for the dashboard.
type Stats struct {
	Reloads     int64     `json:"reloads"`
	Failures    int64     `json:"failures"`
	LastBuiltAt time.Time `json:"last_built_at"`
	LastBuildMS int64     `json:"last_build_ms"`
	Problems    int       `json:"problems"`
}

// Stats returns reload counters.
func (m *Manager) Stats() Stats {
	snap := m.Current()
	s := Stats{Reloads: m.reloads.Load(), Failures: m.failures.Load()}
	if snap != nil {
		s.LastBuiltAt = snap.BuiltAt
		s.LastBuildMS = snap.BuildMS
		s.Problems = len(snap.Problems)
	}
	return s
}

// ZoneProblems returns the records that could not be compiled, for display on
// the zone pages.
func (m *Manager) ZoneProblems(zoneID int64) []authoritative.BuildError {
	snap := m.Current()
	if snap == nil {
		return nil
	}
	var out []authoritative.BuildError
	for _, p := range snap.Problems {
		if zoneID == 0 || p.ZoneID == zoneID {
			out = append(out, p)
		}
	}
	return out
}