// Package metricalert implements a background goroutine that
// periodically evaluates operator-configured metric-threshold rules
// against recent container stats samples. On breach (subject to a
// per-rule-per-workload cooldown) it emits an event into the existing
// event_log + event-bus pipeline — the same fan-out used by the
// log-scanner — instead of building any new notification plumbing.
package metricalert

import (
	"encoding/json"
	"fmt"
	"log/slog"
	"sync"
	"time"

	"github.com/alexei/tinyforge/internal/events"
	"github.com/alexei/tinyforge/internal/store"
)

// EvalInterval is how often the evaluator tick fires.
const EvalInterval = 30 * time.Second

// lookbackSeconds bounds how far back we pull samples each tick. Stats
// are collected at most every few seconds (see internal/stats), so a
// 120s window comfortably captures the latest reading per container
// even if collection briefly stalls.
const lookbackSeconds = 120

// RuleSource is the read-side seam for fetching the current rule rows.
// Real callers pass *store.Store; tests pass a fake.
type RuleSource interface {
	ListMetricAlertRules() ([]store.MetricAlertRule, error)
}

// SampleSource fetches the recent container stats samples to evaluate.
type SampleSource interface {
	ListAllRecentContainerStatsSamples(sinceTS int64) ([]store.ContainerStatsSample, error)
}

// EventSink writes a breach into event_log.
type EventSink interface {
	InsertEvent(store.EventLog) (store.EventLog, error)
}

// Publisher fans the breach out on the event bus. Matches *events.Bus.
type Publisher interface {
	Publish(events.Event)
}

// Source identifies metric-alert events in event_log + the bus.
const eventSource = "metric_alert"

// Manager owns the evaluation loop lifecycle. It mirrors
// stats.Collector: a once-guarded Start/Stop pair with stop/done
// channels and a single-goroutine run loop.
type Manager struct {
	rules   RuleSource
	samples SampleSource
	sink    EventSink
	pub     Publisher

	// now is swappable in tests so cooldown windows can be exercised
	// deterministically. Defaults to time.Now.
	now func() time.Time

	// mu guards lastFired. The run loop is single-goroutine today, but
	// Start/Stop and a future ReloadRules may touch shared state; the
	// mutex is cheap insurance.
	mu        sync.Mutex
	lastFired map[string]time.Time // "ruleID:ownerID" -> last emit time

	startOnce sync.Once
	stopOnce  sync.Once
	started   bool
	stop      chan struct{}
	done      chan struct{}
}

// New wires a manager with the supplied dependencies. Call Start to
// begin evaluating.
func New(rules RuleSource, samples SampleSource, sink EventSink, pub Publisher) *Manager {
	return &Manager{
		rules:     rules,
		samples:   samples,
		sink:      sink,
		pub:       pub,
		now:       time.Now,
		lastFired: map[string]time.Time{},
		stop:      make(chan struct{}),
		done:      make(chan struct{}),
	}
}

// Start launches the background loop. Returns immediately. The loop
// exits when Stop is called. Safe to call multiple times — only the
// first call has an effect.
func (m *Manager) Start() {
	m.startOnce.Do(func() {
		m.started = true
		go m.run()
	})
}

// Stop signals the loop to exit and blocks until it has finished the
// in-flight tick. If Start was never called, Stop returns immediately.
func (m *Manager) Stop() {
	m.stopOnce.Do(func() {
		close(m.stop)
		if !m.started {
			close(m.done)
		}
	})
	<-m.done
}

// run is the main loop. It evaluates once shortly after start, then on
// every EvalInterval tick, until Stop is called.
func (m *Manager) run() {
	defer close(m.done)

	// Settle delay so the app + first stats samples exist before the
	// first evaluation.
	select {
	case <-time.After(3 * time.Second):
	case <-m.stop:
		return
	}

	ticker := time.NewTicker(EvalInterval)
	defer ticker.Stop()
	m.evaluate(m.now())
	for {
		select {
		case <-m.stop:
			return
		case <-ticker.C:
			m.evaluate(m.now())
		}
	}
}

// evaluate runs one pass: load rules + recent samples, reduce to the
// freshest sample per (owner, container), and emit on breach subject to
// cooldown. Best-effort throughout — a bad rule or sample never crashes
// the loop.
func (m *Manager) evaluate(now time.Time) {
	rules, err := m.rules.ListMetricAlertRules()
	if err != nil {
		slog.Warn("metricalert: list rules", "error", err)
		return
	}
	if len(rules) == 0 {
		return
	}

	since := now.Unix() - lookbackSeconds
	samples, err := m.samples.ListAllRecentContainerStatsSamples(since)
	if err != nil {
		slog.Warn("metricalert: list samples", "error", err)
		return
	}
	latest := latestPerContainer(samples)
	if len(latest) == 0 {
		return
	}

	for _, rule := range rules {
		if !rule.Enabled {
			continue
		}
		for _, sample := range latest {
			// Per-workload rules only match their workload; "" matches all.
			if rule.WorkloadID != "" && rule.WorkloadID != sample.OwnerID {
				continue
			}
			value, ok := metricValue(rule.Metric, sample)
			if !ok {
				continue // e.g. memory_percent with a zero limit
			}
			if !breached(rule.Comparator, value, rule.Threshold) {
				continue
			}
			if m.coolingDown(rule, sample.OwnerID, now) {
				continue
			}
			m.emit(rule, sample, value)
			m.recordFire(rule, sample.OwnerID, now)
		}
	}
}

// latestPerContainer keeps only the most recent sample per
// (OwnerID, ContainerID), so each container is judged on its freshest
// reading rather than every historical row in the window.
func latestPerContainer(samples []store.ContainerStatsSample) []store.ContainerStatsSample {
	newest := map[string]store.ContainerStatsSample{}
	for _, s := range samples {
		key := s.OwnerID + "\x00" + s.ContainerID
		if prev, ok := newest[key]; !ok || s.TS > prev.TS {
			newest[key] = s
		}
	}
	out := make([]store.ContainerStatsSample, 0, len(newest))
	for _, s := range newest {
		out = append(out, s)
	}
	return out
}

// metricValue resolves a rule's metric against a sample. The bool is
// false when the sample can't be judged for that metric (memory_percent
// with a zero/unknown limit) so the caller skips it instead of dividing
// by zero.
func metricValue(metric string, s store.ContainerStatsSample) (float64, bool) {
	switch metric {
	case store.MetricCPUPercent:
		return s.CPUPercent, true
	case store.MetricMemoryPercent:
		if s.MemoryLimit <= 0 {
			return 0, false
		}
		return float64(s.MemoryUsage) / float64(s.MemoryLimit) * 100, true
	case store.MetricMemoryBytes:
		return float64(s.MemoryUsage), true
	default:
		return 0, false
	}
}

// breached returns whether value crosses threshold per the comparator.
func breached(comparator string, value, threshold float64) bool {
	switch comparator {
	case store.MetricComparatorGT:
		return value > threshold
	case store.MetricComparatorLT:
		return value < threshold
	default:
		return false
	}
}

// cooldownKey is the per-rule-per-workload cooldown key.
func cooldownKey(ruleID int64, ownerID string) string {
	return fmt.Sprintf("%d:%s", ruleID, ownerID)
}

func (m *Manager) coolingDown(rule store.MetricAlertRule, ownerID string, now time.Time) bool {
	if rule.CooldownSeconds <= 0 {
		return false
	}
	m.mu.Lock()
	defer m.mu.Unlock()
	last, ok := m.lastFired[cooldownKey(rule.ID, ownerID)]
	if !ok {
		return false
	}
	return now.Sub(last) < time.Duration(rule.CooldownSeconds)*time.Second
}

func (m *Manager) recordFire(rule store.MetricAlertRule, ownerID string, now time.Time) {
	m.mu.Lock()
	m.lastFired[cooldownKey(rule.ID, ownerID)] = now
	m.mu.Unlock()
}

// emit persists the breach as an event_log row and publishes it on the
// bus. WorkloadID routes the alert to that app's activity timeline.
// Metadata is JSON-marshalled (never string-concatenated). Any
// marshal/insert failure is logged and skipped — emitting must never
// crash the loop.
func (m *Manager) emit(rule store.MetricAlertRule, sample store.ContainerStatsSample, value float64) {
	message := formatMessage(rule, value)
	meta := map[string]any{
		"workload_id": sample.OwnerID,
		"rule":        rule.Name,
		"metric":      rule.Metric,
		"value":       value,
		"threshold":   rule.Threshold,
		"comparator":  rule.Comparator,
	}
	metaJSON, err := json.Marshal(meta)
	if err != nil {
		slog.Error("metricalert: marshal metadata", "rule", rule.Name, "error", err)
		return
	}
	severity := rule.Severity
	if severity == "" {
		severity = store.LogScanSeverityWarn
	}
	evt, err := m.sink.InsertEvent(store.EventLog{
		Source:     eventSource,
		Severity:   severity,
		Message:    message,
		WorkloadID: sample.OwnerID,
		Metadata:   string(metaJSON),
	})
	if err != nil {
		slog.Error("metricalert: persist event", "rule", rule.Name, "error", err)
		return
	}
	if m.pub != nil {
		m.pub.Publish(events.Event{
			Type: events.EventLog,
			Payload: events.EventLogPayload{
				ID:         evt.ID,
				Source:     eventSource,
				WorkloadID: sample.OwnerID,
				Severity:   severity,
				Message:    message,
				Metadata:   string(metaJSON),
				CreatedAt:  evt.CreatedAt,
			},
		})
	}
}

// formatMessage builds a concise, human, secret-free breach line. The
// only operator-supplied text is rule.Name; the rest are numbers and
// fixed labels.
func formatMessage(rule store.MetricAlertRule, value float64) string {
	label, unit := metricLabelUnit(rule.Metric)
	word := comparatorWord(rule.Comparator)
	return fmt.Sprintf("%s: %s is %.0f%s (threshold %s %.0f%s)",
		rule.Name, label, value, unit, word, rule.Threshold, unit)
}

func metricLabelUnit(metric string) (label, unit string) {
	switch metric {
	case store.MetricCPUPercent:
		return "CPU", "%"
	case store.MetricMemoryPercent:
		return "Memory", "%"
	case store.MetricMemoryBytes:
		return "Memory", " bytes"
	default:
		return metric, ""
	}
}

func comparatorWord(comparator string) string {
	switch comparator {
	case store.MetricComparatorGT:
		return ">"
	case store.MetricComparatorLT:
		return "<"
	default:
		return comparator
	}
}