// Package alert owns Bell's small, deterministic rule and Alert domain.
package alert

import (
	"context"
	"crypto/sha256"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"regexp"
	"strings"
	"time"
	"unicode/utf8"
)

const (
	DefaultPageSize = 16
	MaxPageSize     = 100
	MaxRulesFile    = 256 << 10
)

var (
	ErrNotFound            = errors.New("alert not found")
	ErrIdempotencyConflict = errors.New("idempotency key was used for another command")
	ruleKeyPattern         = regexp.MustCompile(`^[a-z][a-z0-9_-]{2,63}$`)
	eventKindPattern       = regexp.MustCompile(`^[a-z][a-z0-9_]{2,63}$`)
	actorRefPattern        = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._:@/-]*$`)
	idempotencyKeyPattern  = regexp.MustCompile(`^[A-Za-z0-9][A-Za-z0-9._:-]*$`)
)

type RuleSpec struct {
	TenantID        int64     `json:"tenant_id"`
	SiteID          *int64    `json:"site_id,omitempty"`
	RuleKey         string    `json:"rule_key"`
	DisplayName     string    `json:"display_name"`
	EventKind       string    `json:"event_kind"`
	MinimumSeverity string    `json:"minimum_severity"`
	Enabled         bool      `json:"enabled"`
	EffectiveFrom   time.Time `json:"effective_from"`
}

func (r RuleSpec) Validate() error {
	if r.TenantID < 1 || (r.SiteID != nil && *r.SiteID < 1) {
		return errors.New("rule scope must use positive identifiers")
	}
	if !ruleKeyPattern.MatchString(r.RuleKey) || !eventKindPattern.MatchString(r.EventKind) {
		return errors.New("rule key or event kind is invalid")
	}
	if strings.TrimSpace(r.DisplayName) == "" || utf8.RuneCountInString(r.DisplayName) > 120 || r.EffectiveFrom.IsZero() {
		return errors.New("rule name or effective time is invalid")
	}
	if _, ok := SeverityRank(r.MinimumSeverity); !ok {
		return errors.New("rule minimum severity is invalid")
	}
	return nil
}

func (r RuleSpec) Digest() ([sha256.Size]byte, error) {
	value, err := json.Marshal(r)
	if err != nil {
		return [sha256.Size]byte{}, err
	}
	return sha256.Sum256(value), nil
}

type rulesDocument struct {
	Version int        `json:"version"`
	Rules   []RuleSpec `json:"rules"`
}

func LoadRules(path string) ([]RuleSpec, error) {
	file, err := os.Open(path)
	if err != nil {
		return nil, errors.New("open Bell alert rules file")
	}
	defer file.Close()
	info, err := file.Stat()
	if err != nil || info.Size() > MaxRulesFile {
		return nil, errors.New("Bell alert rules file is too large")
	}
	decoder := json.NewDecoder(file)
	decoder.DisallowUnknownFields()
	var document rulesDocument
	if err := decoder.Decode(&document); err != nil {
		return nil, errors.New("decode Bell alert rules file")
	}
	var trailing any
	if err := decoder.Decode(&trailing); !errors.Is(err, io.EOF) {
		return nil, errors.New("Bell alert rules file contains multiple JSON values")
	}
	if document.Version != 1 || len(document.Rules) < 1 || len(document.Rules) > 256 {
		return nil, errors.New("Bell alert rules file must contain 1 to 256 v1 rules")
	}
	seen := make(map[string]bool, len(document.Rules))
	for _, rule := range document.Rules {
		if err := rule.Validate(); err != nil {
			return nil, fmt.Errorf("invalid Bell alert rule %q: %w", rule.RuleKey, err)
		}
		key := fmt.Sprintf("%d:%s", rule.TenantID, rule.RuleKey)
		if seen[key] {
			return nil, fmt.Errorf("duplicate Bell alert rule %q", rule.RuleKey)
		}
		seen[key] = true
	}
	return document.Rules, nil
}

func SeverityRank(value string) (int, bool) {
	for rank, severity := range []string{"low", "medium", "high", "critical"} {
		if value == severity {
			return rank, true
		}
	}
	return 0, false
}

func ValidActorRef(value string) bool {
	return len(value) <= 80 && actorRefPattern.MatchString(value)
}

func ValidIdempotencyKey(value string) bool {
	return len(value) >= 8 && len(value) <= 128 && idempotencyKeyPattern.MatchString(value)
}

type Summary struct {
	ID          string    `json:"id"`
	Severity    string    `json:"severity"`
	Title       string    `json:"title"`
	State       string    `json:"state"`
	RuleKey     string    `json:"rule_key"`
	RuleVersion int       `json:"rule_version"`
	CreatedAt   time.Time `json:"created_at"`
}

type EventRef struct {
	ID         string    `json:"id"`
	DeviceID   int64     `json:"device_id"`
	Kind       string    `json:"kind"`
	Severity   string    `json:"severity"`
	OccurredAt time.Time `json:"occurred_at"`
}

type Transition struct {
	Sequence   int       `json:"sequence"`
	FromState  *string   `json:"from_state"`
	ToState    string    `json:"to_state"`
	ActorRef   string    `json:"actor_ref"`
	Note       *string   `json:"note"`
	OccurredAt time.Time `json:"occurred_at"`
}

type Detail struct {
	Summary
	Events         []EventRef   `json:"events"`
	Transitions    []Transition `json:"transitions"`
	EvidenceStatus string       `json:"evidence_status"`
	DeliveryStatus string       `json:"delivery_status"`
}

type Page struct {
	Items      []Summary `json:"items"`
	NextCursor *string   `json:"next_cursor"`
}

type CommandResponse struct {
	AlertID    string    `json:"alert_id"`
	State      string    `json:"state"`
	ActorRef   string    `json:"actor_ref"`
	OccurredAt time.Time `json:"occurred_at"`
	Code       string    `json:"code,omitempty"`
}

type Repository interface {
	AlertReady(context.Context) error
	PublishRule(context.Context, RuleSpec) (bool, error)
	EvaluateNext(context.Context) (bool, error)
	ListAlerts(context.Context, int64, int64, string, int, string) (Page, error)
	GetAlert(context.Context, int64, int64, string) (Detail, error)
	Command(context.Context, int64, int64, string, string, string, string, *string) (int, CommandResponse, error)
}

func Publish(ctx context.Context, repository Repository, rules []RuleSpec) error {
	for _, rule := range rules {
		if _, err := repository.PublishRule(ctx, rule); err != nil {
			return fmt.Errorf("publish Bell alert rule %q: %w", rule.RuleKey, err)
		}
	}
	return nil
}

func RunWorker(ctx context.Context, repository Repository, onError func(error)) {
	idle := time.NewTicker(250 * time.Millisecond)
	defer idle.Stop()
	for {
		worked, err := repository.EvaluateNext(ctx)
		if err != nil {
			if ctx.Err() != nil {
				return
			}
			if onError != nil {
				onError(err)
			}
			select {
			case <-ctx.Done():
				return
			case <-time.After(time.Second):
			}
			continue
		}
		if worked {
			continue
		}
		select {
		case <-ctx.Done():
			return
		case <-idle.C:
		}
	}
}