package business

import (
	"errors"
	"fmt"
	"strings"

	"gorm.io/gorm"
)

// 保留旧 register_time 字符串及其 CAST 语义，用虚拟列索引避免逐行扫描主账号。
// MySQL 可将查询中相同的 CAST 表达式匹配到该索引，既有查询无需更改统计口径。
func migrateRegistrationIndex(db *gorm.DB) error {
	if db.Dialector.Name() != "mysql" || !db.Migrator().HasTable("tab_user") {
		return nil
	}
	const column = "sw_register_time_unix"
	const index = "idx_sw_user_registration"
	if !db.Migrator().HasColumn("tab_user", column) {
		if err := db.Exec("ALTER TABLE tab_user ADD COLUMN sw_register_time_unix BIGINT UNSIGNED GENERATED ALWAYS AS (CAST(register_time AS UNSIGNED)) VIRTUAL, ALGORITHM=INPLACE, LOCK=NONE").Error; err != nil {
			return err
		}
	} else {
		var expression string
		if err := db.Raw("SELECT generation_expression FROM information_schema.columns WHERE table_schema=DATABASE() AND table_name='tab_user' AND column_name=?", column).Scan(&expression).Error; err != nil {
			return err
		}
		normalized := strings.NewReplacer("`", "", " ", "", "\n", "", "\t", "").Replace(strings.ToLower(expression))
		if normalized != "cast(register_timeasunsigned)" {
			return fmt.Errorf("tab_user.%s 的现有定义与注册时间转换不符", column)
		}
	}
	if !db.Migrator().HasIndex("tab_user", index) {
		return addRegistrationIndex(db)
	}
	var columns []string
	if err := db.Raw("SELECT column_name FROM information_schema.statistics WHERE table_schema=DATABASE() AND table_name='tab_user' AND index_name=? ORDER BY seq_in_index", index).Scan(&columns).Error; err != nil {
		return err
	}
	if strings.Join(columns, ",") != "source_db,puid,sw_register_time_unix,id" {
		return fmt.Errorf("tab_user.%s 的现有索引列不符", index)
	}
	return nil
}

func addRegistrationIndex(db *gorm.DB) error {
	return withLegacyRegistrationCast(db, func(conn *gorm.DB) error {
		return conn.Exec("ALTER TABLE tab_user ADD INDEX idx_sw_user_registration(source_db,puid,sw_register_time_unix,id), ALGORITHM=INPLACE, LOCK=NONE").Error
	})
}

func withLegacyRegistrationCast(db *gorm.DB, migrate func(*gorm.DB) error) error {
	// 旧 SELECT CAST 会以警告方式接受尾部空格等历史值。建索引属于 DDL，
	// 严格模式会把同一警告升级为错误；仅在固定连接的索引构建期间保持旧转换行为。
	return db.Connection(func(conn *gorm.DB) (result error) {
		// Keep the bound connection's base handle free of statement errors so
		// restoration still executes after a failed ALTER TABLE.
		fresh := func() *gorm.DB { return conn.Session(&gorm.Session{NewDB: true}) }
		var previous string
		if err := fresh().Raw("SELECT @@session.sql_mode").Scan(&previous).Error; err != nil {
			return err
		}
		modes := []string{}
		for _, mode := range strings.Split(previous, ",") {
			if mode != "STRICT_TRANS_TABLES" && mode != "STRICT_ALL_TABLES" {
				modes = append(modes, mode)
			}
		}
		if err := fresh().Exec("SET SESSION sql_mode=?", strings.Join(modes, ",")).Error; err != nil {
			return err
		}
		defer func() {
			if err := fresh().Exec("SET SESSION sql_mode=?", previous).Error; err != nil {
				result = errors.Join(result, err)
			}
		}()
		return migrate(fresh())
	})
}