package stats

import (
	"context"
	"errors"
	"fmt"
	"strings"
	"time"

	"go.mongodb.org/mongo-driver/v2/bson"
	"go.mongodb.org/mongo-driver/v2/mongo"
	"go.mongodb.org/mongo-driver/v2/mongo/options"

	"github.com/tiennm99/tiennm99bot/internal/storage"
	"github.com/tiennm99/tiennm99bot/internal/systemstate"
)

const (
	statsCommandUsersIndexName   = "stats_cmd_n_user"
	statsUserCommandsIndexName   = "stats_uid_n_cmd"
	statsUsernameLookupIndexName = "stats_user_uid"
)

// commandRename moves one command's usage history to its new name. Each
// rename has its own completion marker in the system collection.
type commandRename struct {
	oldCmd, newCmd, markerKey string
}

// commandRenames run in order, each at most once. The noitu renames swap two
// names, so /noitu (the solo game, now /noitubot) must move out before
// /noitupvp (the group game, now /noitu) moves in; the reverse order would
// merge both histories into noitubot.
var commandRenames = []commandRename{
	{oldCmd: "noitu", newCmd: "noitubot", markerKey: "stats:command-rename:noitu-to-noitubot"},
	{oldCmd: "noitupvp", newCmd: "noitu", markerKey: "stats:command-rename:noitupvp-to-noitu"},
}

// InitStore performs stats collection startup maintenance. It is safe to call
// every boot: MongoDB indexes are created idempotently, and one-time command
// rename migrations are guarded by markers in the shared system collection.
func InitStore(ctx context.Context, statsColl, systemColl storage.Collection) error {
	if mongoColl, ok := storage.MongoCollection(statsColl); ok {
		if err := ensureUsageIndexes(ctx, mongoColl); err != nil {
			return err
		}
	}
	for _, r := range commandRenames {
		if err := migrateCommandRename(ctx, statsColl, systemColl, r); err != nil {
			return err
		}
	}
	return nil
}

// migrateCommandRename merges every usage row of r.oldCmd, the anonymous
// total and the per-user rows, into the matching r.newCmd row, deletes the
// old row, and records completion. A completed marker makes it a no-op, so a
// later rename that reuses r.oldCmd as its new name is never moved again.
func migrateCommandRename(ctx context.Context, statsColl, systemColl storage.Collection, r commandRename) error {
	state := systemstate.New(systemColl)
	if rec, ok, err := state.Get(ctx, r.markerKey); err != nil {
		return fmt.Errorf("stats command rename marker %s: %w", r.markerKey, err)
	} else if ok && rec.Status == "complete" {
		return nil
	}

	docs := storage.Typed[usageEntry](statsColl)
	keys, err := docs.List(ctx, r.oldCmd)
	if err != nil {
		return fmt.Errorf("stats command rename list %s: %w", r.oldCmd, err)
	}

	var moved int64
	for _, key := range keys {
		// The key prefix also matches longer command names.
		if key != r.oldCmd && !strings.HasPrefix(key, r.oldCmd+":") {
			continue
		}
		entry, _, err := docs.Get(ctx, key)
		if err != nil {
			return fmt.Errorf("stats command rename get %s: %w", key, err)
		}
		if entry.Cmd != r.oldCmd {
			continue
		}

		targetKey := usageKey(r.newCmd, entry.UserID)
		target, _, err := docs.Get(ctx, targetKey)
		missingTarget := errors.Is(err, storage.ErrNotFound)
		if err != nil && !missingTarget {
			return fmt.Errorf("stats command rename get target %s: %w", targetKey, err)
		}
		if missingTarget {
			target = usageEntry{Deleted: entry.Deleted}
		}
		target.Cmd = r.newCmd
		target.UserID = entry.UserID
		if entry.UserID == 0 {
			target.Username = ""
		} else if entry.Username != "" {
			target.Username = entry.Username
		}
		target.N += entry.N
		if !entry.Deleted {
			target.Deleted = false
		}
		if err := docs.Put(ctx, targetKey, target); err != nil {
			return fmt.Errorf("stats command rename put target %s: %w", targetKey, err)
		}
		if err := docs.Delete(ctx, key); err != nil {
			return fmt.Errorf("stats command rename delete old %s: %w", key, err)
		}
		moved += entry.N
	}

	now := time.Now().UTC().UnixMilli()
	if err := state.Put(ctx, r.markerKey, systemstate.Record{
		Kind:        "migration",
		Name:        r.markerKey,
		Status:      "complete",
		Count:       moved,
		CompletedAt: now,
		UpdatedAt:   now,
	}); err != nil {
		return fmt.Errorf("stats command rename marker put %s: %w", r.markerKey, err)
	}
	return nil
}

func ensureUsageIndexes(ctx context.Context, coll *mongo.Collection) error {
	models := []mongo.IndexModel{
		{
			Keys:    bson.D{bsonField("cmd", 1), bsonField("n", -1), bsonField("user", 1)},
			Options: options.Index().SetName(statsCommandUsersIndexName),
		},
		{
			Keys:    bson.D{bsonField("uid", 1), bsonField("n", -1), bsonField("cmd", 1)},
			Options: options.Index().SetName(statsUserCommandsIndexName),
		},
		{
			Keys:    bson.D{bsonField("user", 1), bsonField("uid", 1)},
			Options: options.Index().SetName(statsUsernameLookupIndexName),
		},
	}
	if _, err := coll.Indexes().CreateMany(ctx, models); err != nil {
		return fmt.Errorf("stats indexes: %w", err)
	}
	return nil
}