package schedule

import (
	"context"
	"encoding/binary"
	"errors"
	"fmt"
	"hash/fnv"
	"math"
	"sort"
	"sync"
	"time"
	"unsafe"

	"github.com/grafana/grafana/pkg/services/ngalert/models"
	"github.com/grafana/grafana/pkg/util"
)

var errRuleDeleted = errors.New("rule deleted")

type alertRuleInfoRegistry struct {
	mu            sync.Mutex
	alertRuleInfo map[models.AlertRuleKey]*alertRuleInfo
}

// getOrCreateInfo gets rule routine information from registry by the key. If it does not exist, it creates a new one.
// Returns a pointer to the rule routine information and a flag that indicates whether it is a new struct or not.
func (r *alertRuleInfoRegistry) getOrCreateInfo(context context.Context, key models.AlertRuleKey) (*alertRuleInfo, bool) {
	r.mu.Lock()
	defer r.mu.Unlock()

	info, ok := r.alertRuleInfo[key]
	if !ok {
		info = newAlertRuleInfo(context)
		r.alertRuleInfo[key] = info
	}
	return info, !ok
}

func (r *alertRuleInfoRegistry) exists(key models.AlertRuleKey) bool {
	r.mu.Lock()
	defer r.mu.Unlock()

	_, ok := r.alertRuleInfo[key]
	return ok
}

// del removes pair that has specific key from alertRuleInfo.
// Returns 2-tuple where the first element is value of the removed pair
// and the second element indicates whether element with the specified key existed.
func (r *alertRuleInfoRegistry) del(key models.AlertRuleKey) (*alertRuleInfo, bool) {
	r.mu.Lock()
	defer r.mu.Unlock()
	info, ok := r.alertRuleInfo[key]
	if ok {
		delete(r.alertRuleInfo, key)
	}
	return info, ok
}

func (r *alertRuleInfoRegistry) keyMap() map[models.AlertRuleKey]struct{} {
	r.mu.Lock()
	defer r.mu.Unlock()
	definitionsIDs := make(map[models.AlertRuleKey]struct{}, len(r.alertRuleInfo))
	for k := range r.alertRuleInfo {
		definitionsIDs[k] = struct{}{}
	}
	return definitionsIDs
}

type ruleVersionAndPauseStatus struct {
	Fingerprint fingerprint
	IsPaused    bool
}

type alertRuleInfo struct {
	evalCh   chan *evaluation
	updateCh chan ruleVersionAndPauseStatus
	ctx      context.Context
	stop     func(reason error)
}

func newAlertRuleInfo(parent context.Context) *alertRuleInfo {
	ctx, stop := util.WithCancelCause(parent)
	return &alertRuleInfo{evalCh: make(chan *evaluation), updateCh: make(chan ruleVersionAndPauseStatus), ctx: ctx, stop: stop}
}

// eval signals the rule evaluation routine to perform the evaluation of the rule. Does nothing if the loop is stopped.
// Before sending a message into the channel, it does non-blocking read to make sure that there is no concurrent send operation.
// Returns a tuple where first element is
//   - true when message was sent
//   - false when the send operation is stopped
//
// the second element contains a dropped message that was sent by a concurrent sender.
func (a *alertRuleInfo) eval(eval *evaluation) (bool, *evaluation) {
	// read the channel in unblocking manner to make sure that there is no concurrent send operation.
	var droppedMsg *evaluation
	select {
	case droppedMsg = <-a.evalCh:
	default:
	}

	select {
	case a.evalCh <- eval:
		return true, droppedMsg
	case <-a.ctx.Done():
		return false, droppedMsg
	}
}

// update sends an instruction to the rule evaluation routine to update the scheduled rule to the specified version. The specified version must be later than the current version, otherwise no update will happen.
func (a *alertRuleInfo) update(lastVersion ruleVersionAndPauseStatus) bool {
	// check if the channel is not empty.
	select {
	case <-a.updateCh:
	case <-a.ctx.Done():
		return false
	default:
	}

	select {
	case a.updateCh <- lastVersion:
		return true
	case <-a.ctx.Done():
		return false
	}
}

type evaluation struct {
	scheduledAt time.Time
	rule        *models.AlertRule
	folderTitle string
}

type alertRulesRegistry struct {
	rules        map[models.AlertRuleKey]*models.AlertRule
	folderTitles map[models.FolderKey]string
	mu           sync.Mutex
}

// all returns all rules in the registry.
func (r *alertRulesRegistry) all() ([]*models.AlertRule, map[models.FolderKey]string) {
	r.mu.Lock()
	defer r.mu.Unlock()
	result := make([]*models.AlertRule, 0, len(r.rules))
	for _, rule := range r.rules {
		result = append(result, rule)
	}
	return result, r.folderTitles
}

func (r *alertRulesRegistry) get(k models.AlertRuleKey) *models.AlertRule {
	r.mu.Lock()
	defer r.mu.Unlock()
	return r.rules[k]
}

// set replaces all rules in the registry. Returns difference between previous and the new current version of the registry
func (r *alertRulesRegistry) set(rules []*models.AlertRule, folders map[models.FolderKey]string) diff {
	r.mu.Lock()
	defer r.mu.Unlock()
	rulesMap := make(map[models.AlertRuleKey]*models.AlertRule)
	for _, rule := range rules {
		rulesMap[rule.GetKey()] = rule
	}
	d := r.getDiff(rulesMap)
	r.rules = rulesMap
	// return the map as is without copying because it is not mutated
	r.folderTitles = folders
	return d
}

// update inserts or replaces a rule in the registry.
func (r *alertRulesRegistry) update(rule *models.AlertRule) {
	r.mu.Lock()
	defer r.mu.Unlock()
	r.rules[rule.GetKey()] = rule
}

// del removes pair that has specific key from alertRulesRegistry.
// Returns 2-tuple where the first element is value of the removed pair
// and the second element indicates whether element with the specified key existed.
func (r *alertRulesRegistry) del(k models.AlertRuleKey) (*models.AlertRule, bool) {
	r.mu.Lock()
	defer r.mu.Unlock()
	rule, ok := r.rules[k]
	if ok {
		delete(r.rules, k)
	}
	return rule, ok
}

func (r *alertRulesRegistry) isEmpty() bool {
	r.mu.Lock()
	defer r.mu.Unlock()
	return len(r.rules) == 0
}

func (r *alertRulesRegistry) needsUpdate(keys []models.AlertRuleKeyWithVersion) bool {
	if len(r.rules) != len(keys) {
		return true
	}
	for _, key := range keys {
		rule, ok := r.rules[key.AlertRuleKey]
		if !ok || rule.Version != key.Version {
			return true
		}
	}
	return false
}

type diff struct {
	updated map[models.AlertRuleKey]struct{}
}

func (d diff) IsEmpty() bool {
	return len(d.updated) == 0
}

// getDiff calculates difference between the list of rules fetched previously and provided keys. Returns diff where
// updated - a list of keys that exist in the registry but with different version,
func (r *alertRulesRegistry) getDiff(rules map[models.AlertRuleKey]*models.AlertRule) diff {
	result := diff{
		updated: map[models.AlertRuleKey]struct{}{},
	}
	for key, newRule := range rules {
		oldRule, ok := r.rules[key]
		if !ok || newRule.Version == oldRule.Version {
			// a new rule or not updated
			continue
		}
		result.updated[key] = struct{}{}
	}
	return result
}

type fingerprint uint64

func (f fingerprint) String() string {
	return fmt.Sprintf("%016x", uint64(f))
}

// fingerprintSeparator used during calculation of fingerprint to separate different fields. Contains a byte sequence that cannot happen in UTF-8 strings.
var fingerprintSeparator = []byte{255}

type ruleWithFolder struct {
	rule        *models.AlertRule
	folderTitle string
}

// fingerprint calculates a fingerprint that includes all fields except rule's Version and Update timestamp.
func (r ruleWithFolder) Fingerprint() fingerprint {
	rule := r.rule

	sum := fnv.New64()

	writeBytes := func(b []byte) {
		_, _ = sum.Write(b)
		_, _ = sum.Write(fingerprintSeparator)
	}
	writeString := func(s string) {
		if len(s) == 0 {
			writeBytes(nil)
			return
		}
		// avoid allocation when converting string to byte slice
		writeBytes(unsafe.Slice(unsafe.StringData(s), len(s)))
	}
	// this temp slice is used to convert ints to bytes.
	tmp := make([]byte, 8)
	writeInt := func(u int64) {
		binary.LittleEndian.PutUint64(tmp, uint64(u))
		writeBytes(tmp)
	}

	// allocate a slice that will be used for sorting keys, so we allocate it only once
	var keys []string
	maxLen := int(math.Max(math.Max(float64(len(rule.Annotations)), float64(len(rule.Labels))), float64(len(rule.Data))))
	if maxLen > 0 {
		keys = make([]string, maxLen)
	}

	writeLabels := func(lbls map[string]string) {
		// maps do not guarantee predictable sequence of keys.
		// Therefore, to make hash stable, we need to sort keys
		if len(lbls) == 0 {
			return
		}
		idx := 0
		for labelName := range lbls {
			keys[idx] = labelName
			idx++
		}
		sub := keys[:idx]
		sort.Strings(sub)
		for _, name := range sub {
			writeString(name)
			writeString(lbls[name])
		}
	}
	writeQuery := func() {
		// The order of queries is not important as they represent an expression tree.
		// Therefore, the order of elements should not change the hash. Sort by RefID because it is the unique key.
		for i, q := range rule.Data {
			keys[i] = q.RefID
		}
		sub := keys[:len(rule.Data)]
		sort.Strings(sub)
		for _, id := range sub {
			for _, q := range rule.Data {
				if q.RefID == id {
					writeString(q.RefID)
					writeString(q.DatasourceUID)
					writeString(q.QueryType)
					writeInt(int64(q.RelativeTimeRange.From))
					writeInt(int64(q.RelativeTimeRange.To))
					writeBytes(q.Model)
					break
				}
			}
		}
	}

	// fields that determine the rule state
	writeString(rule.UID)
	writeString(rule.Title)
	writeString(rule.NamespaceUID)
	writeString(r.folderTitle)
	writeLabels(rule.Labels)
	writeString(rule.Condition)
	writeQuery()

	if rule.IsPaused {
		writeInt(1)
	} else {
		writeInt(0)
	}

	// fields that do not affect the state.
	// TODO consider removing fields below from the fingerprint
	writeInt(rule.ID)
	writeInt(rule.OrgID)
	writeInt(rule.IntervalSeconds)
	writeInt(int64(rule.For))
	writeLabels(rule.Annotations)
	if rule.DashboardUID != nil {
		writeString(*rule.DashboardUID)
	}
	if rule.PanelID != nil {
		writeInt(*rule.PanelID)
	}
	writeString(rule.RuleGroup)
	writeInt(int64(rule.RuleGroupIndex))
	writeString(string(rule.NoDataState))
	writeString(string(rule.ExecErrState))
	return fingerprint(sum.Sum64())
}