// Copyright 2013 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package rules

import (
	"context"
	"errors"
	"log/slog"
	"math"
	"slices"
	"strings"
	"sync"
	"time"

	"go.uber.org/atomic"

	"github.com/prometheus/prometheus/promql/parser"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/common/model"
	"github.com/prometheus/common/promslog"
	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/codes"

	"github.com/prometheus/prometheus/model/labels"
	"github.com/prometheus/prometheus/model/timestamp"
	"github.com/prometheus/prometheus/model/value"
	"github.com/prometheus/prometheus/promql"
	"github.com/prometheus/prometheus/storage"
	"github.com/prometheus/prometheus/tsdb/chunkenc"
)

// Group is a set of rules that have a logical relation.
type Group struct {
	name                 string
	file                 string
	interval             time.Duration
	queryOffset          *time.Duration
	limit                int
	rules                []Rule
	seriesInPreviousEval []map[string]labels.Labels // One per Rule.
	staleSeries          []labels.Labels
	opts                 *ManagerOptions
	mtx                  sync.Mutex
	evaluationTime       time.Duration
	lastEvaluation       time.Time // Wall-clock time of most recent evaluation.
	lastEvalTimestamp    time.Time // Time slot used for most recent evaluation.

	shouldRestore bool

	markStale   bool
	done        chan struct{}
	terminated  chan struct{}
	managerDone chan struct{}

	logger *slog.Logger

	metrics *Metrics

	// Rule group evaluation iteration function,
	// defaults to DefaultEvalIterationFunc.
	evalIterationFunc GroupEvalIterationFunc

	// concurrencyController controls the rules evaluation concurrency.
	concurrencyController RuleConcurrencyController
	appOpts               *storage.AppendOptions
}

// GroupEvalIterationFunc is used to implement and extend rule group
// evaluation iteration logic. It is configured in Group.evalIterationFunc,
// and periodically invoked at each group evaluation interval to
// evaluate the rules in the group at that point in time.
// DefaultEvalIterationFunc is the default implementation.
type GroupEvalIterationFunc func(ctx context.Context, g *Group, evalTimestamp time.Time)

type GroupOptions struct {
	Name, File        string
	Interval          time.Duration
	Limit             int
	Rules             []Rule
	ShouldRestore     bool
	Opts              *ManagerOptions
	QueryOffset       *time.Duration
	done              chan struct{}
	EvalIterationFunc GroupEvalIterationFunc
}

// NewGroup makes a new Group with the given name, options, and rules.
func NewGroup(o GroupOptions) *Group {
	metrics := o.Opts.Metrics
	if metrics == nil {
		metrics = NewGroupMetrics(o.Opts.Registerer)
	}

	key := GroupKey(o.File, o.Name)
	metrics.IterationsMissed.WithLabelValues(key)
	metrics.IterationsScheduled.WithLabelValues(key)
	metrics.EvalTotal.WithLabelValues(key)
	metrics.EvalFailures.WithLabelValues(key)
	metrics.GroupLastEvalTime.WithLabelValues(key)
	metrics.GroupLastDuration.WithLabelValues(key)
	metrics.GroupRules.WithLabelValues(key).Set(float64(len(o.Rules)))
	metrics.GroupSamples.WithLabelValues(key)
	metrics.GroupInterval.WithLabelValues(key).Set(o.Interval.Seconds())

	evalIterationFunc := o.EvalIterationFunc
	if evalIterationFunc == nil {
		evalIterationFunc = DefaultEvalIterationFunc
	}

	concurrencyController := o.Opts.RuleConcurrencyController
	if concurrencyController == nil {
		concurrencyController = sequentialRuleEvalController{}
	}

	if o.Opts.Logger == nil {
		promslog.NewNopLogger()
	}

	return &Group{
		name:                  o.Name,
		file:                  o.File,
		interval:              o.Interval,
		queryOffset:           o.QueryOffset,
		limit:                 o.Limit,
		rules:                 o.Rules,
		shouldRestore:         o.ShouldRestore,
		opts:                  o.Opts,
		seriesInPreviousEval:  make([]map[string]labels.Labels, len(o.Rules)),
		done:                  make(chan struct{}),
		managerDone:           o.done,
		terminated:            make(chan struct{}),
		logger:                o.Opts.Logger.With("file", o.File, "group", o.Name),
		metrics:               metrics,
		evalIterationFunc:     evalIterationFunc,
		concurrencyController: concurrencyController,
		appOpts:               &storage.AppendOptions{DiscardOutOfOrder: true},
	}
}

// Name returns the group name.
func (g *Group) Name() string { return g.name }

// File returns the group's file.
func (g *Group) File() string { return g.file }

// Rules returns the group's rules.
func (g *Group) Rules(matcherSets ...[]*labels.Matcher) []Rule {
	if len(matcherSets) == 0 {
		return g.rules
	}
	var rules []Rule
	for _, rule := range g.rules {
		if matchesMatcherSets(matcherSets, rule.Labels()) {
			rules = append(rules, rule)
		}
	}
	return rules
}

func matches(lbls labels.Labels, matchers ...*labels.Matcher) bool {
	for _, m := range matchers {
		if v := lbls.Get(m.Name); !m.Matches(v) {
			return false
		}
	}
	return true
}

// matchesMatcherSets ensures all matches in each matcher set are ANDed and the set of those is ORed.
func matchesMatcherSets(matcherSets [][]*labels.Matcher, lbls labels.Labels) bool {
	if len(matcherSets) == 0 {
		return true
	}

	var ok bool
	for _, matchers := range matcherSets {
		if matches(lbls, matchers...) {
			ok = true
		}
	}
	return ok
}

// Queryable returns the group's queryable.
func (g *Group) Queryable() storage.Queryable { return g.opts.Queryable }

// Context returns the group's context.
func (g *Group) Context() context.Context { return g.opts.Context }

// Interval returns the group's interval.
func (g *Group) Interval() time.Duration { return g.interval }

// Limit returns the group's limit.
func (g *Group) Limit() int { return g.limit }

func (g *Group) Logger() *slog.Logger { return g.logger }

func (g *Group) run(ctx context.Context) {
	defer close(g.terminated)

	// Wait an initial amount to have consistently slotted intervals.
	evalTimestamp := g.EvalTimestamp(time.Now().UnixNano()).Add(g.interval)
	select {
	case <-time.After(time.Until(evalTimestamp)):
	case <-g.done:
		return
	}

	ctx = promql.NewOriginContext(ctx, map[string]interface{}{
		"ruleGroup": map[string]string{
			"file": g.File(),
			"name": g.Name(),
		},
	})

	// The assumption here is that since the ticker was started after having
	// waited for `evalTimestamp` to pass, the ticks will trigger soon
	// after each `evalTimestamp + N * g.interval` occurrence.
	tick := time.NewTicker(g.interval)
	defer tick.Stop()

	defer func() {
		if !g.markStale {
			return
		}
		go func(now time.Time) {
			for _, rule := range g.seriesInPreviousEval {
				for _, r := range rule {
					g.staleSeries = append(g.staleSeries, r)
				}
			}
			// That can be garbage collected at this point.
			g.seriesInPreviousEval = nil
			// Wait for 2 intervals to give the opportunity to renamed rules
			// to insert new series in the tsdb. At this point if there is a
			// renamed rule, it should already be started.
			select {
			case <-g.managerDone:
			case <-time.After(2 * g.interval):
				g.cleanupStaleSeries(ctx, now)
			}
		}(time.Now())
	}()

	g.evalIterationFunc(ctx, g, evalTimestamp)
	if g.shouldRestore {
		// If we have to restore, we wait for another Eval to finish.
		// The reason behind this is, during first eval (or before it)
		// we might not have enough data scraped, and recording rules would not
		// have updated the latest values, on which some alerts might depend.
		select {
		case <-g.done:
			return
		case <-tick.C:
			missed := (time.Since(evalTimestamp) / g.interval) - 1
			if missed > 0 {
				g.metrics.IterationsMissed.WithLabelValues(GroupKey(g.file, g.name)).Add(float64(missed))
				g.metrics.IterationsScheduled.WithLabelValues(GroupKey(g.file, g.name)).Add(float64(missed))
			}
			evalTimestamp = evalTimestamp.Add((missed + 1) * g.interval)
			g.evalIterationFunc(ctx, g, evalTimestamp)
		}

		restoreStartTime := time.Now()
		g.RestoreForState(restoreStartTime)
		totalRestoreTimeSeconds := time.Since(restoreStartTime).Seconds()
		g.metrics.GroupLastRestoreDuration.WithLabelValues(GroupKey(g.file, g.name)).Set(totalRestoreTimeSeconds)
		g.logger.Debug("'for' state restoration completed", "duration_seconds", totalRestoreTimeSeconds)
		g.shouldRestore = false
	}

	for {
		select {
		case <-g.done:
			return
		default:
			select {
			case <-g.done:
				return
			case <-tick.C:
				missed := (time.Since(evalTimestamp) / g.interval) - 1
				if missed > 0 {
					g.metrics.IterationsMissed.WithLabelValues(GroupKey(g.file, g.name)).Add(float64(missed))
					g.metrics.IterationsScheduled.WithLabelValues(GroupKey(g.file, g.name)).Add(float64(missed))
				}
				evalTimestamp = evalTimestamp.Add((missed + 1) * g.interval)

				g.evalIterationFunc(ctx, g, evalTimestamp)
			}
		}
	}
}

func (g *Group) stop() {
	close(g.done)
	<-g.terminated
}

func (g *Group) hash() uint64 {
	l := labels.New(
		labels.Label{Name: "name", Value: g.name},
		labels.Label{Name: "file", Value: g.file},
	)
	return l.Hash()
}

// AlertingRules returns the list of the group's alerting rules.
func (g *Group) AlertingRules() []*AlertingRule {
	g.mtx.Lock()
	defer g.mtx.Unlock()

	var alerts []*AlertingRule
	for _, rule := range g.rules {
		if alertingRule, ok := rule.(*AlertingRule); ok {
			alerts = append(alerts, alertingRule)
		}
	}
	slices.SortFunc(alerts, func(a, b *AlertingRule) int {
		if a.State() == b.State() {
			return strings.Compare(a.Name(), b.Name())
		}
		return int(b.State() - a.State())
	})
	return alerts
}

// HasAlertingRules returns true if the group contains at least one AlertingRule.
func (g *Group) HasAlertingRules() bool {
	g.mtx.Lock()
	defer g.mtx.Unlock()

	for _, rule := range g.rules {
		if _, ok := rule.(*AlertingRule); ok {
			return true
		}
	}
	return false
}

// GetEvaluationTime returns the time in seconds it took to evaluate the rule group.
func (g *Group) GetEvaluationTime() time.Duration {
	g.mtx.Lock()
	defer g.mtx.Unlock()
	return g.evaluationTime
}

// setEvaluationTime sets the time in seconds the last evaluation took.
func (g *Group) setEvaluationTime(dur time.Duration) {
	g.metrics.GroupLastDuration.WithLabelValues(GroupKey(g.file, g.name)).Set(dur.Seconds())

	g.mtx.Lock()
	defer g.mtx.Unlock()
	g.evaluationTime = dur
}

// GetLastEvaluation returns the time the last evaluation of the rule group took place.
func (g *Group) GetLastEvaluation() time.Time {
	g.mtx.Lock()
	defer g.mtx.Unlock()
	return g.lastEvaluation
}

// setLastEvaluation updates evaluationTimestamp to the timestamp of when the rule group was last evaluated.
func (g *Group) setLastEvaluation(ts time.Time) {
	g.metrics.GroupLastEvalTime.WithLabelValues(GroupKey(g.file, g.name)).Set(float64(ts.UnixNano()) / 1e9)

	g.mtx.Lock()
	defer g.mtx.Unlock()
	g.lastEvaluation = ts
}

// GetLastEvalTimestamp returns the timestamp of the last evaluation.
func (g *Group) GetLastEvalTimestamp() time.Time {
	g.mtx.Lock()
	defer g.mtx.Unlock()
	return g.lastEvalTimestamp
}

// setLastEvalTimestamp updates lastEvalTimestamp to the timestamp of the last evaluation.
func (g *Group) setLastEvalTimestamp(ts time.Time) {
	g.mtx.Lock()
	defer g.mtx.Unlock()
	g.lastEvalTimestamp = ts
}

// EvalTimestamp returns the immediately preceding consistently slotted evaluation time.
func (g *Group) EvalTimestamp(startTime int64) time.Time {
	var (
		offset = int64(g.hash() % uint64(g.interval))

		// This group's evaluation times differ from the perfect time intervals by `offset` nanoseconds.
		// But we can only use `% interval` to align with the interval. And `% interval` will always
		// align with the perfect time intervals, instead of this group's. Because of this we add
		// `offset` _after_ aligning with the perfect time interval.
		//
		// There can be cases where adding `offset` to the perfect evaluation time can yield a
		// timestamp in the future, which is not what EvalTimestamp should do.
		// So we subtract one `offset` to make sure that `now - (now % interval) + offset` gives an
		// evaluation time in the past.
		adjNow = startTime - offset

		// Adjust to perfect evaluation intervals.
		base = adjNow - (adjNow % int64(g.interval))

		// Add one offset to randomize the evaluation times of this group.
		next = base + offset
	)

	return time.Unix(0, next).UTC()
}

func nameAndLabels(rule Rule) string {
	return rule.Name() + rule.Labels().String()
}

// CopyState copies the alerting rule and staleness related state from the given group.
//
// Rules are matched based on their name and labels. If there are duplicates, the
// first is matched with the first, second with the second etc.
func (g *Group) CopyState(from *Group) {
	g.evaluationTime = from.evaluationTime
	g.lastEvaluation = from.lastEvaluation

	ruleMap := make(map[string][]int, len(from.rules))

	for fi, fromRule := range from.rules {
		nameAndLabels := nameAndLabels(fromRule)
		l := ruleMap[nameAndLabels]
		ruleMap[nameAndLabels] = append(l, fi)
	}

	for i, rule := range g.rules {
		nameAndLabels := nameAndLabels(rule)
		indexes := ruleMap[nameAndLabels]
		if len(indexes) == 0 {
			continue
		}
		fi := indexes[0]
		g.seriesInPreviousEval[i] = from.seriesInPreviousEval[fi]
		ruleMap[nameAndLabels] = indexes[1:]

		ar, ok := rule.(*AlertingRule)
		if !ok {
			continue
		}
		far, ok := from.rules[fi].(*AlertingRule)
		if !ok {
			continue
		}

		for fp, a := range far.active {
			ar.active[fp] = a
		}
	}

	// Handle deleted and unmatched duplicate rules.
	g.staleSeries = from.staleSeries
	for fi, fromRule := range from.rules {
		nameAndLabels := nameAndLabels(fromRule)
		l := ruleMap[nameAndLabels]
		if len(l) != 0 {
			for _, series := range from.seriesInPreviousEval[fi] {
				g.staleSeries = append(g.staleSeries, series)
			}
		}
	}
}

// Eval runs a single evaluation cycle in which all rules are evaluated sequentially.
// Rules can be evaluated concurrently if the `concurrent-rule-eval` feature flag is enabled.
func (g *Group) Eval(ctx context.Context, ts time.Time) {
	var (
		samplesTotal atomic.Float64
		wg           sync.WaitGroup
	)

	ruleQueryOffset := g.QueryOffset()

	for i, rule := range g.rules {
		select {
		case <-g.done:
			return
		default:
		}

		eval := func(i int, rule Rule, cleanup func()) {
			if cleanup != nil {
				defer cleanup()
			}

			logger := g.logger.With("name", rule.Name(), "index", i)
			ctx, sp := otel.Tracer("").Start(ctx, "rule")
			sp.SetAttributes(attribute.String("name", rule.Name()))
			defer func(t time.Time) {
				sp.End()

				since := time.Since(t)
				g.metrics.EvalDuration.Observe(since.Seconds())
				rule.SetEvaluationDuration(since)
				rule.SetEvaluationTimestamp(t)
			}(time.Now())

			if sp.SpanContext().IsSampled() && sp.SpanContext().HasTraceID() {
				logger = logger.With("trace_id", sp.SpanContext().TraceID())
			}

			g.metrics.EvalTotal.WithLabelValues(GroupKey(g.File(), g.Name())).Inc()

			vector, err := rule.Eval(ctx, ruleQueryOffset, ts, g.opts.QueryFunc, g.opts.ExternalURL, g.Limit())
			if err != nil {
				rule.SetHealth(HealthBad)
				rule.SetLastError(err)
				sp.SetStatus(codes.Error, err.Error())
				g.metrics.EvalFailures.WithLabelValues(GroupKey(g.File(), g.Name())).Inc()

				// Canceled queries are intentional termination of queries. This normally
				// happens on shutdown and thus we skip logging of any errors here.
				var eqc promql.ErrQueryCanceled
				if !errors.As(err, &eqc) {
					logger.Warn("Evaluating rule failed", "rule", rule, "err", err)
				}
				return
			}
			rule.SetHealth(HealthGood)
			rule.SetLastError(nil)
			samplesTotal.Add(float64(len(vector)))

			if ar, ok := rule.(*AlertingRule); ok {
				ar.sendAlerts(ctx, ts, g.opts.ResendDelay, g.interval, g.opts.NotifyFunc)
			}
			var (
				numOutOfOrder = 0
				numTooOld     = 0
				numDuplicates = 0
			)

			app := g.opts.Appendable.Appender(ctx)
			seriesReturned := make(map[string]labels.Labels, len(g.seriesInPreviousEval[i]))
			defer func() {
				if err := app.Commit(); err != nil {
					rule.SetHealth(HealthBad)
					rule.SetLastError(err)
					sp.SetStatus(codes.Error, err.Error())
					g.metrics.EvalFailures.WithLabelValues(GroupKey(g.File(), g.Name())).Inc()

					logger.Warn("Rule sample appending failed", "err", err)
					return
				}
				g.seriesInPreviousEval[i] = seriesReturned
			}()

			for _, s := range vector {
				if s.H != nil {
					_, err = app.AppendHistogram(0, s.Metric, s.T, nil, s.H)
				} else {
					app.SetOptions(g.appOpts)
					_, err = app.Append(0, s.Metric, s.T, s.F)
				}

				if err != nil {
					rule.SetHealth(HealthBad)
					rule.SetLastError(err)
					sp.SetStatus(codes.Error, err.Error())
					unwrappedErr := errors.Unwrap(err)
					if unwrappedErr == nil {
						unwrappedErr = err
					}
					switch {
					case errors.Is(unwrappedErr, storage.ErrOutOfOrderSample):
						numOutOfOrder++
						logger.Debug("Rule evaluation result discarded", "err", err, "sample", s)
					case errors.Is(unwrappedErr, storage.ErrTooOldSample):
						numTooOld++
						logger.Debug("Rule evaluation result discarded", "err", err, "sample", s)
					case errors.Is(unwrappedErr, storage.ErrDuplicateSampleForTimestamp):
						numDuplicates++
						logger.Debug("Rule evaluation result discarded", "err", err, "sample", s)
					default:
						logger.Warn("Rule evaluation result discarded", "err", err, "sample", s)
					}
				} else {
					buf := [1024]byte{}
					seriesReturned[string(s.Metric.Bytes(buf[:]))] = s.Metric
				}
			}
			if numOutOfOrder > 0 {
				logger.Warn("Error on ingesting out-of-order result from rule evaluation", "num_dropped", numOutOfOrder)
			}
			if numTooOld > 0 {
				logger.Warn("Error on ingesting too old result from rule evaluation", "num_dropped", numTooOld)
			}
			if numDuplicates > 0 {
				logger.Warn("Error on ingesting results from rule evaluation with different value but same timestamp", "num_dropped", numDuplicates)
			}

			for metric, lset := range g.seriesInPreviousEval[i] {
				if _, ok := seriesReturned[metric]; !ok {
					// Series no longer exposed, mark it stale.
					_, err = app.Append(0, lset, timestamp.FromTime(ts.Add(-ruleQueryOffset)), math.Float64frombits(value.StaleNaN))
					unwrappedErr := errors.Unwrap(err)
					if unwrappedErr == nil {
						unwrappedErr = err
					}
					switch {
					case unwrappedErr == nil:
					case errors.Is(unwrappedErr, storage.ErrOutOfOrderSample),
						errors.Is(unwrappedErr, storage.ErrTooOldSample),
						errors.Is(unwrappedErr, storage.ErrDuplicateSampleForTimestamp):
						// Do not count these in logging, as this is expected if series
						// is exposed from a different rule.
					default:
						logger.Warn("Adding stale sample failed", "sample", lset.String(), "err", err)
					}
				}
			}
		}

		if ctrl := g.concurrencyController; ctrl.Allow(ctx, g, rule) {
			wg.Add(1)

			go eval(i, rule, func() {
				wg.Done()
				ctrl.Done(ctx)
			})
		} else {
			eval(i, rule, nil)
		}
	}

	wg.Wait()

	g.metrics.GroupSamples.WithLabelValues(GroupKey(g.File(), g.Name())).Set(samplesTotal.Load())
	g.cleanupStaleSeries(ctx, ts)
}

func (g *Group) QueryOffset() time.Duration {
	if g.queryOffset != nil {
		return *g.queryOffset
	}

	if g.opts.DefaultRuleQueryOffset != nil {
		return g.opts.DefaultRuleQueryOffset()
	}

	return time.Duration(0)
}

func (g *Group) cleanupStaleSeries(ctx context.Context, ts time.Time) {
	if len(g.staleSeries) == 0 {
		return
	}
	app := g.opts.Appendable.Appender(ctx)
	app.SetOptions(g.appOpts)
	queryOffset := g.QueryOffset()
	for _, s := range g.staleSeries {
		// Rule that produced series no longer configured, mark it stale.
		_, err := app.Append(0, s, timestamp.FromTime(ts.Add(-queryOffset)), math.Float64frombits(value.StaleNaN))
		unwrappedErr := errors.Unwrap(err)
		if unwrappedErr == nil {
			unwrappedErr = err
		}
		switch {
		case unwrappedErr == nil:
		case errors.Is(unwrappedErr, storage.ErrOutOfOrderSample),
			errors.Is(unwrappedErr, storage.ErrTooOldSample),
			errors.Is(unwrappedErr, storage.ErrDuplicateSampleForTimestamp):
			// Do not count these in logging, as this is expected if series
			// is exposed from a different rule.
		default:
			g.logger.Warn("Adding stale sample for previous configuration failed", "sample", s, "err", err)
		}
	}
	if err := app.Commit(); err != nil {
		g.logger.Warn("Stale sample appending for previous configuration failed", "err", err)
	} else {
		g.staleSeries = nil
	}
}

// RestoreForState restores the 'for' state of the alerts
// by looking up last ActiveAt from storage.
func (g *Group) RestoreForState(ts time.Time) {
	maxtMS := int64(model.TimeFromUnixNano(ts.UnixNano()))
	// We allow restoration only if alerts were active before after certain time.
	mint := ts.Add(-g.opts.OutageTolerance)
	mintMS := int64(model.TimeFromUnixNano(mint.UnixNano()))
	q, err := g.opts.Queryable.Querier(mintMS, maxtMS)
	if err != nil {
		g.logger.Error("Failed to get Querier", "err", err)
		return
	}
	defer func() {
		if err := q.Close(); err != nil {
			g.logger.Error("Failed to close Querier", "err", err)
		}
	}()

	for _, rule := range g.Rules() {
		alertRule, ok := rule.(*AlertingRule)
		if !ok {
			continue
		}

		alertHoldDuration := alertRule.HoldDuration()
		if alertHoldDuration < g.opts.ForGracePeriod {
			// If alertHoldDuration is already less than grace period, we would not
			// like to make it wait for `g.opts.ForGracePeriod` time before firing.
			// Hence we skip restoration, which will make it wait for alertHoldDuration.
			alertRule.SetRestored(true)
			continue
		}

		sset, err := alertRule.QueryForStateSeries(g.opts.Context, q)
		if err != nil {
			g.logger.Error(
				"Failed to restore 'for' state",
				labels.AlertName, alertRule.Name(),
				"stage", "Select",
				"err", err,
			)
			// Even if we failed to query the `ALERT_FOR_STATE` series, we currently have no way to retry the restore process.
			// So the best we can do is mark the rule as restored and let it eventually fire.
			alertRule.SetRestored(true)
			continue
		}

		// While not technically the same number of series we expect, it's as good of an approximation as any.
		seriesByLabels := make(map[string]storage.Series, alertRule.ActiveAlertsCount())
		for sset.Next() {
			seriesByLabels[sset.At().Labels().DropMetricName().String()] = sset.At()
		}

		// No results for this alert rule.
		if len(seriesByLabels) == 0 {
			g.logger.Debug("No series found to restore the 'for' state of the alert rule", labels.AlertName, alertRule.Name())
			alertRule.SetRestored(true)
			continue
		}

		alertRule.ForEachActiveAlert(func(a *Alert) {
			var s storage.Series

			s, ok := seriesByLabels[a.Labels.String()]
			if !ok {
				return
			}
			// Series found for the 'for' state.
			var t int64
			var v float64
			it := s.Iterator(nil)
			for it.Next() == chunkenc.ValFloat {
				t, v = it.At()
			}
			if it.Err() != nil {
				g.logger.Error("Failed to restore 'for' state",
					labels.AlertName, alertRule.Name(), "stage", "Iterator", "err", it.Err())
				return
			}
			if value.IsStaleNaN(v) { // Alert was not active.
				return
			}

			downAt := time.Unix(t/1000, 0).UTC()
			restoredActiveAt := time.Unix(int64(v), 0).UTC()
			timeSpentPending := downAt.Sub(restoredActiveAt)
			timeRemainingPending := alertHoldDuration - timeSpentPending

			switch {
			case timeRemainingPending <= 0:
				// It means that alert was firing when prometheus went down.
				// In the next Eval, the state of this alert will be set back to
				// firing again if it's still firing in that Eval.
				// Nothing to be done in this case.
			case timeRemainingPending < g.opts.ForGracePeriod:
				// (new) restoredActiveAt = (ts + m.opts.ForGracePeriod) - alertHoldDuration
				//                            /* new firing time */      /* moving back by hold duration */
				//
				// Proof of correctness:
				// firingTime = restoredActiveAt.Add(alertHoldDuration)
				//            = ts + m.opts.ForGracePeriod - alertHoldDuration + alertHoldDuration
				//            = ts + m.opts.ForGracePeriod
				//
				// Time remaining to fire = firingTime.Sub(ts)
				//                        = (ts + m.opts.ForGracePeriod) - ts
				//                        = m.opts.ForGracePeriod
				restoredActiveAt = ts.Add(g.opts.ForGracePeriod).Add(-alertHoldDuration)
			default:
				// By shifting ActiveAt to the future (ActiveAt + some_duration),
				// the total pending time from the original ActiveAt
				// would be `alertHoldDuration + some_duration`.
				// Here, some_duration = downDuration.
				downDuration := ts.Sub(downAt)
				restoredActiveAt = restoredActiveAt.Add(downDuration)
			}

			a.ActiveAt = restoredActiveAt
			g.logger.Debug("'for' state restored",
				labels.AlertName, alertRule.Name(), "restored_time", a.ActiveAt.Format(time.RFC850),
				"labels", a.Labels.String())
		})

		alertRule.SetRestored(true)
	}
}

// Equals return if two groups are the same.
func (g *Group) Equals(ng *Group) bool {
	if g.name != ng.name {
		return false
	}

	if g.file != ng.file {
		return false
	}

	if g.interval != ng.interval {
		return false
	}

	if g.limit != ng.limit {
		return false
	}

	if ((g.queryOffset == nil) != (ng.queryOffset == nil)) || (g.queryOffset != nil && ng.queryOffset != nil && *g.queryOffset != *ng.queryOffset) {
		return false
	}

	if len(g.rules) != len(ng.rules) {
		return false
	}

	for i, gr := range g.rules {
		if gr.String() != ng.rules[i].String() {
			return false
		}
	}

	return true
}

// GroupKey group names need not be unique across filenames.
func GroupKey(file, name string) string {
	return file + ";" + name
}

// Constants for instrumentation.
const namespace = "prometheus"

// Metrics for rule evaluation.
type Metrics struct {
	EvalDuration             prometheus.Summary
	IterationDuration        prometheus.Summary
	IterationsMissed         *prometheus.CounterVec
	IterationsScheduled      *prometheus.CounterVec
	EvalTotal                *prometheus.CounterVec
	EvalFailures             *prometheus.CounterVec
	GroupInterval            *prometheus.GaugeVec
	GroupLastEvalTime        *prometheus.GaugeVec
	GroupLastDuration        *prometheus.GaugeVec
	GroupLastRestoreDuration *prometheus.GaugeVec
	GroupRules               *prometheus.GaugeVec
	GroupSamples             *prometheus.GaugeVec
}

// NewGroupMetrics creates a new instance of Metrics and registers it with the provided registerer,
// if not nil.
func NewGroupMetrics(reg prometheus.Registerer) *Metrics {
	m := &Metrics{
		EvalDuration: prometheus.NewSummary(
			prometheus.SummaryOpts{
				Namespace:  namespace,
				Name:       "rule_evaluation_duration_seconds",
				Help:       "The duration for a rule to execute.",
				Objectives: map[float64]float64{0.5: 0.05, 0.9: 0.01, 0.99: 0.001},
			}),
		IterationDuration: prometheus.NewSummary(prometheus.SummaryOpts{
			Namespace:  namespace,
			Name:       "rule_group_duration_seconds",
			Help:       "The duration of rule group evaluations.",
			Objectives: map[float64]float64{0.01: 0.001, 0.05: 0.005, 0.5: 0.05, 0.90: 0.01, 0.99: 0.001},
		}),
		IterationsMissed: prometheus.NewCounterVec(
			prometheus.CounterOpts{
				Namespace: namespace,
				Name:      "rule_group_iterations_missed_total",
				Help:      "The total number of rule group evaluations missed due to slow rule group evaluation.",
			},
			[]string{"rule_group"},
		),
		IterationsScheduled: prometheus.NewCounterVec(
			prometheus.CounterOpts{
				Namespace: namespace,
				Name:      "rule_group_iterations_total",
				Help:      "The total number of scheduled rule group evaluations, whether executed or missed.",
			},
			[]string{"rule_group"},
		),
		EvalTotal: prometheus.NewCounterVec(
			prometheus.CounterOpts{
				Namespace: namespace,
				Name:      "rule_evaluations_total",
				Help:      "The total number of rule evaluations.",
			},
			[]string{"rule_group"},
		),
		EvalFailures: prometheus.NewCounterVec(
			prometheus.CounterOpts{
				Namespace: namespace,
				Name:      "rule_evaluation_failures_total",
				Help:      "The total number of rule evaluation failures.",
			},
			[]string{"rule_group"},
		),
		GroupInterval: prometheus.NewGaugeVec(
			prometheus.GaugeOpts{
				Namespace: namespace,
				Name:      "rule_group_interval_seconds",
				Help:      "The interval of a rule group.",
			},
			[]string{"rule_group"},
		),
		GroupLastEvalTime: prometheus.NewGaugeVec(
			prometheus.GaugeOpts{
				Namespace: namespace,
				Name:      "rule_group_last_evaluation_timestamp_seconds",
				Help:      "The timestamp of the last rule group evaluation in seconds.",
			},
			[]string{"rule_group"},
		),
		GroupLastDuration: prometheus.NewGaugeVec(
			prometheus.GaugeOpts{
				Namespace: namespace,
				Name:      "rule_group_last_duration_seconds",
				Help:      "The duration of the last rule group evaluation.",
			},
			[]string{"rule_group"},
		),
		GroupLastRestoreDuration: prometheus.NewGaugeVec(
			prometheus.GaugeOpts{
				Namespace: namespace,
				Name:      "rule_group_last_restore_duration_seconds",
				Help:      "The duration of the last alert rules alerts restoration using the `ALERTS_FOR_STATE` series.",
			},
			[]string{"rule_group"},
		),
		GroupRules: prometheus.NewGaugeVec(
			prometheus.GaugeOpts{
				Namespace: namespace,
				Name:      "rule_group_rules",
				Help:      "The number of rules.",
			},
			[]string{"rule_group"},
		),
		GroupSamples: prometheus.NewGaugeVec(
			prometheus.GaugeOpts{
				Namespace: namespace,
				Name:      "rule_group_last_evaluation_samples",
				Help:      "The number of samples returned during the last rule group evaluation.",
			},
			[]string{"rule_group"},
		),
	}

	if reg != nil {
		reg.MustRegister(
			m.EvalDuration,
			m.IterationDuration,
			m.IterationsMissed,
			m.IterationsScheduled,
			m.EvalTotal,
			m.EvalFailures,
			m.GroupInterval,
			m.GroupLastEvalTime,
			m.GroupLastDuration,
			m.GroupLastRestoreDuration,
			m.GroupRules,
			m.GroupSamples,
		)
	}

	return m
}

// dependencyMap is a data-structure which contains the relationships between rules within a group.
// It is used to describe the dependency associations between rules in a group whereby one rule uses the
// output metric produced by another rule in its expression (i.e. as its "input").
type dependencyMap map[Rule][]Rule

// dependents returns the count of rules which use the output of the given rule as one of their inputs.
func (m dependencyMap) dependents(r Rule) int {
	return len(m[r])
}

// dependencies returns the count of rules on which the given rule is dependent for input.
func (m dependencyMap) dependencies(r Rule) int {
	if len(m) == 0 {
		return 0
	}

	var count int
	for _, children := range m {
		for _, child := range children {
			if child == r {
				count++
			}
		}
	}

	return count
}

// isIndependent determines whether the given rule is not dependent on another rule for its input, nor is any other rule
// dependent on its output.
func (m dependencyMap) isIndependent(r Rule) bool {
	if m == nil {
		return false
	}

	return m.dependents(r)+m.dependencies(r) == 0
}

// buildDependencyMap builds a data-structure which contains the relationships between rules within a group.
//
// Alert rules, by definition, cannot have any dependents - but they can have dependencies. Any recording rule on whose
// output an Alert rule depends will not be able to run concurrently.
//
// There is a class of rule expressions which are considered "indeterminate", because either relationships cannot be
// inferred, or concurrent evaluation of rules depending on these series would produce undefined/unexpected behaviour:
//   - wildcard queriers like {cluster="prod1"} which would match every series with that label selector
//   - any "meta" series (series produced by Prometheus itself) like ALERTS, ALERTS_FOR_STATE
//
// Rules which are independent can run concurrently with no side-effects.
func buildDependencyMap(rules []Rule) dependencyMap {
	dependencies := make(dependencyMap)

	if len(rules) <= 1 {
		// No relationships if group has 1 or fewer rules.
		return dependencies
	}

	inputs := make(map[string][]Rule, len(rules))
	outputs := make(map[string][]Rule, len(rules))

	var indeterminate bool

	for _, rule := range rules {
		if indeterminate {
			break
		}

		name := rule.Name()
		outputs[name] = append(outputs[name], rule)

		parser.Inspect(rule.Query(), func(node parser.Node, path []parser.Node) error {
			if n, ok := node.(*parser.VectorSelector); ok {
				// A wildcard metric expression means we cannot reliably determine if this rule depends on any other,
				// which means we cannot safely run any rules concurrently.
				if n.Name == "" && len(n.LabelMatchers) > 0 {
					indeterminate = true
					return nil
				}

				// Rules which depend on "meta-metrics" like ALERTS and ALERTS_FOR_STATE will have undefined behaviour
				// if they run concurrently.
				if n.Name == alertMetricName || n.Name == alertForStateMetricName {
					indeterminate = true
					return nil
				}

				inputs[n.Name] = append(inputs[n.Name], rule)
			}
			return nil
		})
	}

	if indeterminate {
		return nil
	}

	for output, outRules := range outputs {
		for _, outRule := range outRules {
			if inRules, found := inputs[output]; found && len(inRules) > 0 {
				dependencies[outRule] = append(dependencies[outRule], inRules...)
			}
		}
	}

	return dependencies
}