package internal

import (
	"context"
	"sync"
	"time"

	log "github.com/sirupsen/logrus"

	mgmProto "github.com/netbirdio/netbird/shared/management/proto"
)

// mapStateManager is the single read/write point between the management stream
// (writes) and the convergence loop (reads/applies).
//
// The stream calls SetTarget with the latest full SyncResponse — the complete
// desired state. A single background goroutine (run) applies it to the engine in
// bounded passes via apply() until converged, releasing syncMsgMux between passes
// so other subsystems interleave. If a newer update arrives mid-flight, the loop
// coalesces: it keeps converging toward the latest target and the intermediate one
// is SKIPPED — never applied on its own (logged, no onConverged).
//
// Convergence is a single comparison: appliedGen == targetGen. targetGen
// increments on every SetTarget (an internal generation counter, so it also covers
// config-only updates that carry no network-map serial).
//
// onConverged fires once for each — and only each — map that is actually processed
// (i.e. converged as the target). Skipped/superseded maps and dropped-on-error maps
// do NOT fire it. So "sync finished in X" / RecordSyncDuration always corresponds
// to a real, completed alignment.
type mapStateManager struct {
	// apply performs one bounded apply pass and reports whether more passes are needed.
	// firstPass is true on the first pass of a given target, so the caller can run
	// wholesale (firewall/routes/DNS/forward-rules) once per target and skip it on the
	// re-runs that only drain the bounded peer batches. The manager owns this signal
	// because it owns the convergence boundary; the engine need not track serials for it.
	apply func(update *mgmProto.SyncResponse, firstPass bool) (bool, error)
	// onConverged is called once per processed map, with the elapsed time since that
	// map was received (for the sync-duration metric / "sync finished" log).
	onConverged func(time.Duration)
	// persist snapshots an update to disk for restore-on-restart. Called once per
	// update received from management (in SetTarget), including ones later coalesced
	// or skipped from apply, so the on-disk state mirrors what management last sent.
	// The impl skips config-only updates (nil NetworkMap). May be nil.
	persist func(*mgmProto.SyncResponse)

	mu          sync.Mutex
	target      *mgmProto.SyncResponse
	targetGen   uint64
	appliedGen  uint64
	targetSetAt time.Time

	wake chan struct{}
}

func newMapStateManager(apply func(update *mgmProto.SyncResponse, firstPass bool) (bool, error), persist func(*mgmProto.SyncResponse), onConverged func(time.Duration)) *mapStateManager {
	return &mapStateManager{
		apply:       apply,
		persist:     persist,
		onConverged: onConverged,
		wake:        make(chan struct{}, 1),
	}
}

// SetTarget records the latest update as the desired state and wakes the loop.
// It returns immediately; convergence happens in the background. Serial-based
// staleness of the network map is still enforced inside apply (updateNetworkMap).
func (m *mapStateManager) SetTarget(update *mgmProto.SyncResponse) error {
	m.mu.Lock()
	// A target that has not settled yet (targetGen > appliedGen) is being superseded
	// before it converged: we coalesce to the latest map and never apply this one on
	// its own. It is SKIPPED — logged here, and it will not fire onConverged.
	if m.target != nil && m.targetGen > m.appliedGen {
		log.Debugf("sync map (gen %d) superseded before convergence, skipping", m.targetGen)
	}
	m.target = m.mergeTarget(m.target, update)
	// Bump an internal generation counter, NOT the map serial: config-only updates
	// (relay token rotation, STUN/TURN) arrive with NetworkMap == nil and carry no
	// serial, yet must still be applied. Every SetTarget is therefore a distinct
	// target regardless of payload. Map-serial staleness is enforced separately
	// inside apply (updateNetworkMap).
	m.targetGen++
	m.targetSetAt = time.Now()
	m.mu.Unlock()

	select {
	case m.wake <- struct{}{}:
	default:
	}

	// Persist every update received from management — once per update (not per apply
	// pass), and including ones that get coalesced/skipped from apply, so the on-disk
	// state always reflects the latest map management sent. Done after waking the loop
	// so convergence can start in parallel with the disk write. The persist impl skips
	// config-only updates (nil NetworkMap).
	if m.persist != nil {
		m.persist(update)
	}
	return nil
}

// mergeTarget combines the currently pending target with a freshly received update
// and returns the new desired state. It is called under m.mu from SetTarget and is
// the single seam where the replace-vs-squash decision lives.
//
// Today management always sends a FULL map (the complete desired state), so the
// update simply replaces whatever was pending — prev is ignored. When management
// starts sending incremental/delta updates, squash `update` onto `prev` here; the
// rest of the manager (generation tracking, convergence, signaling) is unaffected
// because it already treats target as "the complete desired state, whatever it is".
func (m *mapStateManager) mergeTarget(prev, update *mgmProto.SyncResponse) *mgmProto.SyncResponse {
	// Nothing pending to preserve (no prev, or prev already fully applied): plain replace.
	if prev == nil || update == nil || m.targetGen == m.appliedGen {
		return update
	}

	// prev still has unapplied state (targetGen > appliedGen). In the sync protocol a
	// nil component means "no change", so if `update` omits a component that prev
	// carried, carry prev's forward — otherwise coalescing an update that superseded a
	// not-yet-applied one would silently drop the map or config it uniquely brought.
	// A present component in `update` is newer and wins. Management may send map-only
	// updates (nil config) and config-only updates (nil map); both are handled here.
	// A nil component in `update` means "no change", so fill it in from prev — otherwise
	// coalescing an update that superseded a not-yet-applied one would drop the map or
	// config it uniquely carried. A present component in `update` is newer and wins.
	// We mutate `update` in place: it is a fresh per-message allocation from the sync
	// stream (see receiveUpdatesEvents — not reused), and persisting this squashed target
	// is correct, since it is the current full (superset) desired state.
	if update.GetNetworkMap() == nil && prev.GetNetworkMap() != nil {
		update.NetworkMap = prev.GetNetworkMap()
		update.Checks = prev.Checks // checks travel with the map
	}
	if update.GetNetbirdConfig() == nil && prev.GetNetbirdConfig() != nil {
		update.NetbirdConfig = prev.GetNetbirdConfig()
	}
	return update
}

// run drives convergence until ctx is done. It is meant to run in its own goroutine.
func (m *mapStateManager) run(ctx context.Context) {
	// passGen is the generation of the most recent apply() call (0 = none). A pass is
	// the first for its target when its generation differs from the previous one —
	// true on a fresh target and on a coalesced switch to a newer target mid-flight.
	var passGen uint64
	for {
		m.mu.Lock()
		target, tg, ag := m.target, m.targetGen, m.appliedGen
		m.mu.Unlock()

		// Fully converged (or nothing yet): block until a new target arrives.
		if target == nil || ag == tg {
			select {
			case <-ctx.Done():
				return
			case <-m.wake:
				continue
			}
		}

		firstPass := tg != passGen
		passGen = tg
		more, err := m.apply(target, firstPass)
		if err != nil {
			if ctx.Err() != nil {
				return
			}
			// Log and DROP this target — do not retry it. A deterministic failure
			// (e.g. a malformed peer in the map) would otherwise spin every pass
			// making no progress. Management is the source of truth and re-delivers
			// the full map on the next sync, so dropping is safe; peers already
			// applied this convergence stay (idempotent diffs) and the remainder is
			// reconciled by the next target. Mirrors the legacy handleSync path,
			// where the apply error was logged by the gRPC client and the update
			// dropped. No onConverged: this target did not converge.
			log.Errorf("apply sync pass, dropping update: %v", err)
			m.settle(tg, false)
			continue
		}

		if more {
			// keep converging the current target; syncMsgMux was released by apply
			// between passes so other subsystems interleave.
			continue
		}

		// This pass converged. Mark applied and signal this one map.
		m.settle(tg, true)
		// if a newer target arrived mid-pass, settle is a no-op (targetGen != tg) and
		// ag<tg next iteration -> apply it; this generation was skipped (logged in
		// SetTarget) and is not signaled.
	}
}

// settle marks generation tg as processed so the loop goes idle instead of
// re-applying the same target. It is a no-op when a newer target arrived during the
// pass (targetGen != tg), leaving appliedGen behind so that target re-applies — the
// just-finished generation was already counted as skipped.
//
// When signal is true (the pass converged) it fires onConverged once for this map;
// when false (the target was dropped on error) it does not — the map did not converge.
func (m *mapStateManager) settle(tg uint64, signal bool) {
	m.mu.Lock()
	if m.targetGen != tg {
		m.mu.Unlock()
		return
	}
	m.appliedGen = tg
	setAt := m.targetSetAt
	m.mu.Unlock()

	if signal && m.onConverged != nil {
		m.onConverged(time.Since(setAt))
	}
}