// Copyright (c) 2026, Pim van Pelt <pim@ipng.ch>

package checker

import (
	"context"
	"log/slog"
	"net"
	"sort"
	"sync"
	"time"

	"git.ipng.ch/ipng/vpp-maglev/internal/config"
	"git.ipng.ch/ipng/vpp-maglev/internal/health"
	"git.ipng.ch/ipng/vpp-maglev/internal/prober"
)

// BackendSnapshot combines the live health state with the config entry for a backend.
type BackendSnapshot struct {
	Health *health.Backend
	Config config.Backend
}

// Event is emitted on every backend state transition, once per frontend that
// references the backend.
type Event struct {
	FrontendName string
	BackendName  string
	Backend      net.IP
	Transition   health.Transition
}

type worker struct {
	backend *health.Backend
	hc      config.HealthCheck
	entry   config.Backend
	cancel  context.CancelFunc
}

// Checker orchestrates health probing for all backends.
// Each backend is probed exactly once, regardless of how many frontends
// reference it.
type Checker struct {
	cfg     *config.Config
	mu      sync.RWMutex
	workers map[string]*worker // keyed by backend name

	subsMu  sync.Mutex
	nextID  int
	subs    map[int]chan Event
	eventCh chan Event
}

// New creates a Checker. Call Run to start probing.
func New(cfg *config.Config) *Checker {
	return &Checker{
		cfg:     cfg,
		workers: make(map[string]*worker),
		subs:    make(map[int]chan Event),
		eventCh: make(chan Event, 256),
	}
}

// Run starts all probe goroutines and blocks until ctx is cancelled.
func (c *Checker) Run(ctx context.Context) error {
	go c.fanOut(ctx)

	c.mu.Lock()
	names := activeBackendNames(c.cfg)
	for i, name := range names {
		b := c.cfg.Backends[name]
		hc := c.cfg.HealthChecks[b.HealthCheck]
		c.startWorker(ctx, name, b, hc, i, len(names))
	}
	c.mu.Unlock()

	<-ctx.Done()
	return nil
}

// Reload applies a new config without restarting the process.
// New backends are added, removed backends are stopped, changed backends are
// restarted. Backends whose healthcheck config is unchanged continue
// uninterrupted, even if the set of frontends referencing them changes.
func (c *Checker) Reload(ctx context.Context, cfg *config.Config) error {
	c.mu.Lock()
	defer c.mu.Unlock()

	desired := map[string]struct{}{}
	for _, name := range activeBackendNames(cfg) {
		desired[name] = struct{}{}
	}

	// Stop workers no longer needed.
	for name, w := range c.workers {
		if _, ok := desired[name]; !ok {
			slog.Info("backend-stop", "backend", name)
			w.cancel()
			delete(c.workers, name)
		}
	}

	// Add new or restart changed workers.
	names := activeBackendNames(cfg)
	for i, name := range names {
		b := cfg.Backends[name]
		hc := cfg.HealthChecks[b.HealthCheck]
		if w, ok := c.workers[name]; ok {
			if healthCheckEqual(w.hc, hc) {
				// Update entry metadata (weight, etc.) in place without restart.
				w.entry = b
				continue
			}
			slog.Info("backend-restart", "backend", name)
			w.cancel()
			c.startWorker(ctx, name, b, hc, i, len(names))
		} else {
			slog.Info("backend-start", "backend", name)
			c.startWorker(ctx, name, b, hc, i, len(names))
		}
	}

	c.cfg = cfg
	return nil
}

// Subscribe returns a channel that receives Events for every state transition.
// Call the returned cancel function to unsubscribe.
func (c *Checker) Subscribe() (<-chan Event, func()) {
	c.subsMu.Lock()
	defer c.subsMu.Unlock()
	id := c.nextID
	c.nextID++
	ch := make(chan Event, 64)
	c.subs[id] = ch
	return ch, func() {
		c.subsMu.Lock()
		defer c.subsMu.Unlock()
		delete(c.subs, id)
		close(ch)
	}
}

// ListFrontends returns the names of all configured frontends.
func (c *Checker) ListFrontends() []string {
	c.mu.RLock()
	defer c.mu.RUnlock()
	names := make([]string, 0, len(c.cfg.Frontends))
	for name := range c.cfg.Frontends {
		names = append(names, name)
	}
	return names
}

// GetFrontend returns the frontend config for the given name.
func (c *Checker) GetFrontend(name string) (config.Frontend, bool) {
	c.mu.RLock()
	defer c.mu.RUnlock()
	v, ok := c.cfg.Frontends[name]
	return v, ok
}

// ListHealthChecks returns the names of all configured health checks, sorted.
func (c *Checker) ListHealthChecks() []string {
	c.mu.RLock()
	defer c.mu.RUnlock()
	names := make([]string, 0, len(c.cfg.HealthChecks))
	for name := range c.cfg.HealthChecks {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

// GetHealthCheck returns the config for a health check by name.
func (c *Checker) GetHealthCheck(name string) (config.HealthCheck, bool) {
	c.mu.RLock()
	defer c.mu.RUnlock()
	hc, ok := c.cfg.HealthChecks[name]
	return hc, ok
}

// ListBackends returns the names of all active backends.
func (c *Checker) ListBackends() []string {
	c.mu.RLock()
	defer c.mu.RUnlock()
	names := make([]string, 0, len(c.workers))
	for name := range c.workers {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

// ListFrontendBackends returns the backend health states for all backends of a frontend.
func (c *Checker) ListFrontendBackends(frontendName string) []*health.Backend {
	c.mu.RLock()
	defer c.mu.RUnlock()
	fe, ok := c.cfg.Frontends[frontendName]
	if !ok {
		return nil
	}
	var out []*health.Backend
	for _, name := range fe.Backends {
		if w, ok := c.workers[name]; ok {
			out = append(out, w.backend)
		}
	}
	return out
}

// GetBackend returns a snapshot of the health state and config for a backend by name.
func (c *Checker) GetBackend(name string) (BackendSnapshot, bool) {
	c.mu.RLock()
	defer c.mu.RUnlock()
	w, ok := c.workers[name]
	if !ok {
		return BackendSnapshot{}, false
	}
	return BackendSnapshot{Health: w.backend, Config: w.entry}, true
}

// PauseBackend pauses health checking for a backend by name.
func (c *Checker) PauseBackend(name string) (BackendSnapshot, bool) {
	c.mu.Lock()
	defer c.mu.Unlock()
	w, ok := c.workers[name]
	if !ok {
		return BackendSnapshot{}, false
	}
	maxHistory := c.cfg.HealthChecker.TransitionHistory
	if w.backend.Pause(maxHistory) {
		slog.Info("backend-pause", "backend", name)
		c.emitForBackend(name, w.backend.Address, w.backend.Transitions[0])
	}
	return BackendSnapshot{Health: w.backend, Config: w.entry}, true
}

// ResumeBackend resumes health checking for a backend by name.
func (c *Checker) ResumeBackend(name string) (BackendSnapshot, bool) {
	c.mu.Lock()
	defer c.mu.Unlock()
	w, ok := c.workers[name]
	if !ok {
		return BackendSnapshot{}, false
	}
	maxHistory := c.cfg.HealthChecker.TransitionHistory
	if w.backend.Resume(maxHistory) {
		slog.Info("backend-resume", "backend", name)
		c.emitForBackend(name, w.backend.Address, w.backend.Transitions[0])
	}
	return BackendSnapshot{Health: w.backend, Config: w.entry}, true
}

// ---- internal --------------------------------------------------------------

// startWorker creates a Backend and launches a probe goroutine.
// Must be called with c.mu held.
func (c *Checker) startWorker(ctx context.Context, name string, entry config.Backend, hc config.HealthCheck, pos, total int) {
	rise, fall := hc.Rise, hc.Fall
	if entry.HealthCheck == "" {
		// No healthcheck: one synthetic pass drives the backend to Up immediately.
		rise, fall = 1, 1
	}
	wCtx, cancel := context.WithCancel(ctx)
	w := &worker{
		backend: health.New(name, entry.Address, rise, fall),
		hc:      hc,
		entry:   entry,
		cancel:  cancel,
	}
	c.workers[name] = w
	go c.runProbe(wCtx, name, pos, total)
}

// runProbe is the per-backend probe loop.
func (c *Checker) runProbe(ctx context.Context, name string, pos, total int) {
	c.mu.RLock()
	w, ok := c.workers[name]
	if !ok {
		c.mu.RUnlock()
		return
	}
	initialDelay := staggerDelay(w.hc.Interval, pos, total)
	c.mu.RUnlock()

	if initialDelay > 0 {
		select {
		case <-ctx.Done():
			return
		case <-time.After(initialDelay):
		}
	}

	for {
		c.mu.RLock()
		w, ok := c.workers[name]
		if !ok {
			c.mu.RUnlock()
			return
		}
		hc := w.hc
		entry := w.entry
		maxHistory := c.cfg.HealthChecker.TransitionHistory
		netns := c.cfg.HealthChecker.Netns
		var sleepFor time.Duration
		if entry.HealthCheck == "" {
			sleepFor = 30 * time.Second
		} else {
			sleepFor = w.backend.NextInterval(hc.Interval, hc.FastInterval, hc.DownInterval)
		}
		c.mu.RUnlock()

		select {
		case <-ctx.Done():
			return
		case <-time.After(sleepFor):
		}

		var result health.ProbeResult
		if entry.HealthCheck == "" {
			// No healthcheck configured: synthesise a passing result so the
			// backend is assumed healthy without any network activity.
			result = health.ProbeResult{OK: true, Layer: health.LayerL7, Code: "L7OK"}
		} else {
			var probeSrc net.IP
			if entry.Address.To4() != nil {
				probeSrc = hc.ProbeIPv4Src
			} else {
				probeSrc = hc.ProbeIPv6Src
			}
			pcfg := prober.ProbeConfig{
				Target:           entry.Address,
				Port:             hc.Port,
				ProbeSrc:         probeSrc,
				HealthCheckNetns: netns,
				Timeout:          hc.Timeout,
				HTTP:             hc.HTTP,
				TCP:              hc.TCP,
			}
			probeCtx, cancel := context.WithTimeout(ctx, hc.Timeout)
			slog.Debug("probe-start", "backend", name, "type", hc.Type)
			start := time.Now()
			result = prober.ForType(hc.Type)(probeCtx, pcfg)
			cancel()
			slog.Debug("probe-done",
				"backend", name,
				"type", hc.Type,
				"ok", result.OK,
				"code", result.Code,
				"detail", result.Detail,
				"elapsed", time.Since(start).Round(time.Millisecond).String(),
			)
		}

		c.mu.Lock()
		w, exists := c.workers[name]
		if !exists {
			c.mu.Unlock()
			return
		}
		if w.backend.Record(result, maxHistory) {
			t := w.backend.Transitions[0]
			addr := w.backend.Address
			slog.Info("backend-transition",
				"backend", name,
				"from", t.From.String(),
				"to", t.To.String(),
				"code", result.Code,
				"detail", result.Detail,
			)
			c.emitForBackend(name, addr, t)
		}
		c.mu.Unlock()
	}
}

// emitForBackend emits one Event per frontend that references backendName.
// Must be called with c.mu held.
func (c *Checker) emitForBackend(backendName string, addr net.IP, t health.Transition) {
	for feName, fe := range c.cfg.Frontends {
		for _, name := range fe.Backends {
			if name == backendName {
				c.emit(Event{FrontendName: feName, BackendName: backendName, Backend: addr, Transition: t})
				break
			}
		}
	}
}

// emit sends an event to the internal fan-out channel (non-blocking).
// Must be called with c.mu held.
func (c *Checker) emit(e Event) {
	select {
	case c.eventCh <- e:
	default:
		slog.Warn("event-drop", "frontend", e.FrontendName, "backend", e.BackendName)
	}
}

// fanOut reads from eventCh and distributes to all subscribers.
func (c *Checker) fanOut(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		case e := <-c.eventCh:
			c.subsMu.Lock()
			for _, ch := range c.subs {
				select {
				case ch <- e:
				default:
					// Slow subscriber — drop rather than block.
				}
			}
			c.subsMu.Unlock()
		}
	}
}

// healthCheckEqual returns true if two HealthCheck configs are functionally identical.
func healthCheckEqual(a, b config.HealthCheck) bool {
	if a.Type != b.Type ||
		a.Interval != b.Interval ||
		a.FastInterval != b.FastInterval ||
		a.DownInterval != b.DownInterval ||
		a.Timeout != b.Timeout ||
		a.Rise != b.Rise ||
		a.Fall != b.Fall {
		return false
	}
	return httpParamsEqual(a.HTTP, b.HTTP) && tcpParamsEqual(a.TCP, b.TCP)
}

func httpParamsEqual(a, b *config.HTTPParams) bool {
	if a == nil && b == nil {
		return true
	}
	if a == nil || b == nil {
		return false
	}
	aRe, bRe := "", ""
	if a.ResponseRegexp != nil {
		aRe = a.ResponseRegexp.String()
	}
	if b.ResponseRegexp != nil {
		bRe = b.ResponseRegexp.String()
	}
	return a.Path == b.Path &&
		a.Host == b.Host &&
		a.ResponseCodeMin == b.ResponseCodeMin &&
		a.ResponseCodeMax == b.ResponseCodeMax &&
		aRe == bRe &&
		a.ServerName == b.ServerName &&
		a.InsecureSkipVerify == b.InsecureSkipVerify
}

func tcpParamsEqual(a, b *config.TCPParams) bool {
	if a == nil && b == nil {
		return true
	}
	if a == nil || b == nil {
		return false
	}
	return a.SSL == b.SSL &&
		a.ServerName == b.ServerName &&
		a.InsecureSkipVerify == b.InsecureSkipVerify
}

// activeBackendNames returns a sorted, deduplicated list of backend names that
// are referenced by at least one frontend and have Enabled: true.
func activeBackendNames(cfg *config.Config) []string {
	seen := map[string]struct{}{}
	for _, fe := range cfg.Frontends {
		for _, name := range fe.Backends {
			if b, ok := cfg.Backends[name]; ok && b.Enabled {
				seen[name] = struct{}{}
			}
		}
	}
	names := make([]string, 0, len(seen))
	for name := range seen {
		names = append(names, name)
	}
	sort.Strings(names)
	return names
}

// staggerDelay computes the initial probe delay for position pos out of total.
func staggerDelay(interval time.Duration, pos, total int) time.Duration {
	if total <= 1 {
		return 0
	}
	return time.Duration(int64(interval) * int64(pos) / int64(total))
}