package sync

import (
	"bufio"
	"crypto/sha256"
	"encoding/hex"
	"errors"
	"fmt"
	"log/slog"
	"strconv"
	"strings"
	"time"

	"git.mchus.pro/mchus/priceforge/internal/appmeta"
	"git.mchus.pro/mchus/priceforge/internal/localdb"
	"gorm.io/gorm"
)

const (
	ReadinessReady   = "ready"
	ReadinessBlocked = "blocked"
	ReadinessUnknown = "unknown"
)

var ErrSyncBlockedByReadiness = errors.New("sync blocked by readiness guard")

type SyncReadiness struct {
	Status                string     `json:"status"`
	Blocked               bool       `json:"blocked"`
	ReasonCode            string     `json:"reason_code,omitempty"`
	ReasonText            string     `json:"reason_text,omitempty"`
	RequiredMinAppVersion *string    `json:"required_min_app_version,omitempty"`
	LastCheckedAt         *time.Time `json:"last_checked_at,omitempty"`
}

type SyncBlockedError struct {
	Readiness SyncReadiness
}

func (e *SyncBlockedError) Error() string {
	if e == nil {
		return ErrSyncBlockedByReadiness.Error()
	}
	if strings.TrimSpace(e.Readiness.ReasonText) != "" {
		return e.Readiness.ReasonText
	}
	return ErrSyncBlockedByReadiness.Error()
}

func (s *Service) EnsureReadinessForSync() (*SyncReadiness, error) {
	readiness, err := s.GetReadiness()
	if err != nil {
		return nil, err
	}
	if readiness.Blocked {
		return readiness, &SyncBlockedError{Readiness: *readiness}
	}
	return readiness, nil
}

func (s *Service) GetReadiness() (*SyncReadiness, error) {
	now := time.Now().UTC()
	if !s.isOnline() {
		return s.blockedReadiness(
			now,
			"OFFLINE_UNVERIFIED_SCHEMA",
			"Синхронизация недоступна: нет соединения с сервером и нельзя проверить миграции локальной БД.",
			nil,
		)
	}

	mariaDB, err := s.getDB()
	if err != nil || mariaDB == nil {
		return s.blockedReadiness(
			now,
			"OFFLINE_UNVERIFIED_SCHEMA",
			"Синхронизация недоступна: нет соединения с сервером и нельзя проверить миграции локальной БД.",
			nil,
		)
	}

	migrations, err := listActiveClientMigrations(mariaDB)
	if err != nil {
		return s.blockedReadiness(
			now,
			"REMOTE_MIGRATION_REGISTRY_UNAVAILABLE",
			"Синхронизация заблокирована: не удалось проверить централизованные миграции локальной БД.",
			nil,
		)
	}

	for i := range migrations {
		m := migrations[i]
		if strings.TrimSpace(m.MinAppVersion) != "" {
			if compareVersions(appmeta.Version(), m.MinAppVersion) < 0 {
				min := m.MinAppVersion
				return s.blockedReadiness(
					now,
					"MIN_APP_VERSION_REQUIRED",
					fmt.Sprintf("Требуется обновление приложения до версии %s для безопасной синхронизации.", m.MinAppVersion),
					&min,
				)
			}
		}
	}

	if err := s.applyMissingRemoteMigrations(migrations); err != nil {
		if strings.Contains(strings.ToLower(err.Error()), "checksum") {
			return s.blockedReadiness(
				now,
				"REMOTE_MIGRATION_CHECKSUM_MISMATCH",
				"Синхронизация заблокирована: контрольная сумма миграции не совпадает.",
				nil,
			)
		}
		return s.blockedReadiness(
			now,
			"LOCAL_MIGRATION_APPLY_FAILED",
			"Синхронизация заблокирована: не удалось применить миграции локальной БД.",
			nil,
		)
	}

	if err := s.reportClientSchemaState(mariaDB, now); err != nil {
		slog.Warn("failed to report client schema state", "error", err)
	}

	ready := &SyncReadiness{Status: ReadinessReady, Blocked: false, LastCheckedAt: &now}
	if setErr := s.localDB.SetSyncGuardState(ReadinessReady, "", "", nil, &now); setErr != nil {
		slog.Warn("failed to persist sync guard state", "error", setErr)
	}
	return ready, nil
}

func (s *Service) blockedReadiness(now time.Time, code, text string, minVersion *string) (*SyncReadiness, error) {
	readiness := &SyncReadiness{
		Status:                ReadinessBlocked,
		Blocked:               true,
		ReasonCode:            code,
		ReasonText:            text,
		RequiredMinAppVersion: minVersion,
		LastCheckedAt:         &now,
	}
	if err := s.localDB.SetSyncGuardState(ReadinessBlocked, code, text, minVersion, &now); err != nil {
		slog.Warn("failed to persist blocked sync guard state", "error", err)
	}
	return readiness, nil
}

func (s *Service) isOnline() bool {
	if s.directDB != nil {
		return true
	}
	if s.connMgr == nil {
		return false
	}
	return s.connMgr.IsOnline()
}

type clientLocalMigration struct {
	ID            string    `gorm:"column:id"`
	Name          string    `gorm:"column:name"`
	SQLText       string    `gorm:"column:sql_text"`
	Checksum      string    `gorm:"column:checksum"`
	MinAppVersion string    `gorm:"column:min_app_version"`
	OrderNo       int       `gorm:"column:order_no"`
	CreatedAt     time.Time `gorm:"column:created_at"`
}

func listActiveClientMigrations(db *gorm.DB) ([]clientLocalMigration, error) {
	if strings.EqualFold(db.Dialector.Name(), "sqlite") {
		return []clientLocalMigration{}, nil
	}
	if err := ensureClientMigrationRegistryTable(db); err != nil {
		return nil, err
	}

	rows := make([]clientLocalMigration, 0)
	if err := db.Raw(`
		SELECT id, name, sql_text, checksum, COALESCE(min_app_version, '') AS min_app_version, order_no, created_at
		FROM qt_client_local_migrations
		WHERE is_active = 1
		ORDER BY order_no ASC, created_at ASC, id ASC
	`).Scan(&rows).Error; err != nil {
		return nil, fmt.Errorf("load client local migrations: %w", err)
	}

	return rows, nil
}

func ensureClientMigrationRegistryTable(db *gorm.DB) error {
	if err := db.Exec(`
		CREATE TABLE IF NOT EXISTS qt_client_local_migrations (
			id VARCHAR(128) NOT NULL,
			name VARCHAR(255) NOT NULL,
			sql_text LONGTEXT NOT NULL,
			checksum VARCHAR(128) NOT NULL,
			min_app_version VARCHAR(64) NULL,
			order_no INT NOT NULL DEFAULT 0,
			is_active TINYINT(1) NOT NULL DEFAULT 1,
			created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
			PRIMARY KEY (id),
			INDEX idx_qt_client_local_migrations_active_order (is_active, order_no, created_at)
		)
	`).Error; err != nil {
		return err
	}
	return db.Exec(`
		CREATE TABLE IF NOT EXISTS qt_client_schema_state (
			username VARCHAR(100) NOT NULL,
			last_applied_migration_id VARCHAR(128) NULL,
			app_version VARCHAR(64) NULL,
			last_checked_at DATETIME NOT NULL,
			updated_at DATETIME NOT NULL,
			PRIMARY KEY (username),
			INDEX idx_qt_client_schema_state_checked (last_checked_at)
		)
	`).Error
}

func (s *Service) applyMissingRemoteMigrations(migrations []clientLocalMigration) error {
	for i := range migrations {
		m := migrations[i]
		computedChecksum := digestSQL(m.SQLText)
		checksum := strings.TrimSpace(m.Checksum)
		if checksum == "" {
			checksum = computedChecksum
		} else if !strings.EqualFold(checksum, computedChecksum) {
			return fmt.Errorf("checksum mismatch for migration %s", m.ID)
		}

		applied, err := s.localDB.GetRemoteMigrationApplied(m.ID)
		if err == nil {
			if strings.TrimSpace(applied.Checksum) != checksum {
				return fmt.Errorf("checksum mismatch for migration %s", m.ID)
			}
			continue
		}
		if !errors.Is(err, gorm.ErrRecordNotFound) {
			return fmt.Errorf("check local applied migration %s: %w", m.ID, err)
		}

		if strings.TrimSpace(m.SQLText) == "" {
			if err := s.localDB.UpsertRemoteMigrationApplied(m.ID, checksum, appmeta.Version(), time.Now().UTC()); err != nil {
				return fmt.Errorf("mark empty migration %s as applied: %w", m.ID, err)
			}
			continue
		}

		statements := splitSQLStatementsLite(m.SQLText)
		if err := s.localDB.DB().Transaction(func(tx *gorm.DB) error {
			for _, stmt := range statements {
				if err := tx.Exec(stmt).Error; err != nil {
					return fmt.Errorf("apply migration %s statement %q: %w", m.ID, stmt, err)
				}
			}
			return nil
		}); err != nil {
			return err
		}

		if err := s.localDB.UpsertRemoteMigrationApplied(m.ID, checksum, appmeta.Version(), time.Now().UTC()); err != nil {
			return fmt.Errorf("record applied migration %s: %w", m.ID, err)
		}
	}
	return nil
}

func splitSQLStatementsLite(script string) []string {
	scanner := bufio.NewScanner(strings.NewReader(script))
	scanner.Buffer(make([]byte, 1024), 1024*1024)

	lines := make([]string, 0, 64)
	for scanner.Scan() {
		line := strings.TrimSpace(scanner.Text())
		if line == "" || strings.HasPrefix(line, "--") {
			continue
		}
		lines = append(lines, scanner.Text())
	}
	combined := strings.Join(lines, "\n")
	raw := strings.Split(combined, ";")
	stmts := make([]string, 0, len(raw))
	for _, stmt := range raw {
		trimmed := strings.TrimSpace(stmt)
		if trimmed == "" {
			continue
		}
		stmts = append(stmts, trimmed)
	}
	return stmts
}

func digestSQL(sqlText string) string {
	hash := sha256.Sum256([]byte(sqlText))
	return hex.EncodeToString(hash[:])
}

func compareVersions(left, right string) int {
	leftParts := normalizeVersionParts(left)
	rightParts := normalizeVersionParts(right)
	maxLen := len(leftParts)
	if len(rightParts) > maxLen {
		maxLen = len(rightParts)
	}
	for i := 0; i < maxLen; i++ {
		lv := 0
		rv := 0
		if i < len(leftParts) {
			lv = leftParts[i]
		}
		if i < len(rightParts) {
			rv = rightParts[i]
		}
		if lv < rv {
			return -1
		}
		if lv > rv {
			return 1
		}
	}
	return 0
}

func (s *Service) reportClientSchemaState(mariaDB *gorm.DB, checkedAt time.Time) error {
	if strings.EqualFold(mariaDB.Dialector.Name(), "sqlite") {
		return nil
	}
	username := strings.TrimSpace(s.localDB.GetDBUser())
	if username == "" {
		return nil
	}
	lastMigrationID := ""
	if id, err := s.localDB.GetLatestAppliedRemoteMigrationID(); err == nil {
		lastMigrationID = id
	}
	return mariaDB.Exec(`
		INSERT INTO qt_client_schema_state (username, last_applied_migration_id, app_version, last_checked_at, updated_at)
		VALUES (?, ?, ?, ?, ?)
		ON DUPLICATE KEY UPDATE
			last_applied_migration_id = VALUES(last_applied_migration_id),
			app_version = VALUES(app_version),
			last_checked_at = VALUES(last_checked_at),
			updated_at = VALUES(updated_at)
	`, username, lastMigrationID, appmeta.Version(), checkedAt, checkedAt).Error
}

func normalizeVersionParts(v string) []int {
	trimmed := strings.TrimSpace(v)
	trimmed = strings.TrimPrefix(trimmed, "v")
	chunks := strings.Split(trimmed, ".")
	parts := make([]int, 0, len(chunks))
	for _, chunk := range chunks {
		clean := strings.TrimSpace(chunk)
		if clean == "" {
			parts = append(parts, 0)
			continue
		}
		n := 0
		for i := 0; i < len(clean); i++ {
			if clean[i] < '0' || clean[i] > '9' {
				clean = clean[:i]
				break
			}
		}
		if clean != "" {
			if parsed, err := strconv.Atoi(clean); err == nil {
				n = parsed
			}
		}
		parts = append(parts, n)
	}
	return parts
}

func toReadinessFromState(state *localdb.LocalSyncGuardState) *SyncReadiness {
	if state == nil {
		return nil
	}
	blocked := state.Status == ReadinessBlocked
	return &SyncReadiness{
		Status:                state.Status,
		Blocked:               blocked,
		ReasonCode:            state.ReasonCode,
		ReasonText:            state.ReasonText,
		RequiredMinAppVersion: state.RequiredMinAppVersion,
		LastCheckedAt:         state.LastCheckedAt,
	}
}