package mysql

import (
	"context"
	"database/sql"
	"errors"
)

// SchemaVerification 是发布和恢复前的只读结构核验结果。
// 它不修改数据库，也不代表已经完成备份恢复或高可用验收。
type SchemaVerification struct {
	// SchemaVersion 是迁移账本中的最后版本。
	SchemaVersion uint64 `json:"schema_version"`
	// MigrationCount 是已记录且通过校验的迁移数量。
	MigrationCount int `json:"migration_count"`
	// ForeignKeyCount 是 Orbit 表上的数据库外键数量，必须为零。
	ForeignKeyCount int `json:"foreign_key_count"`
	// RequiredIndexCount 是有界观测所需固定索引数量，当前必须为六。
	RequiredIndexCount int `json:"required_index_count"`
	// Complete 表示迁移、外键和观测索引均满足当前二进制要求。
	Complete bool `json:"complete"`
}

var requiredOperationalIndexes = []struct {
	table string
	name  string
}{
	{"pulse_tasks", "ix_task_observe_state_available"},
	{"orbit_deliveries", "ix_delivery_observe_state_available"},
	{"weave_instances", "ix_workflow_observe_state_updated"},
	{"orbit_events", "ix_event_observe_type_occurred"},
	{"pulse_tasks", "ix_task_observe_terminal_completed"},
	{"weave_instances", "ix_workflow_observe_terminal_completed"},
}

// VerifyOperationalSchema performs the read-only preflight used before release
// or restore. It never runs migration or repairs a dirty database.
func VerifyOperationalSchema(ctx context.Context, db *sql.DB) (SchemaVerification, error) {
	if db == nil {
		return SchemaVerification{}, errors.New("database is required")
	}
	status, err := Status(ctx, db)
	if err != nil {
		return SchemaVerification{}, errors.New("schema status unavailable")
	}
	result := SchemaVerification{MigrationCount: len(status)}
	if len(status) > 0 {
		result.SchemaVersion = status[len(status)-1].Version
	}
	for _, item := range status {
		if item.Dirty {
			return result, errors.New("schema has a dirty migration")
		}
	}
	var foreignKeys int
	if err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM information_schema.REFERENTIAL_CONSTRAINTS WHERE CONSTRAINT_SCHEMA=DATABASE() AND LEFT(TABLE_NAME,6) IN ('orbit_','pulse_','weave_')`).Scan(&foreignKeys); err != nil {
		return result, errors.New("foreign key status unavailable")
	}
	result.ForeignKeyCount = foreignKeys
	for _, required := range requiredOperationalIndexes {
		var count int
		if err := db.QueryRowContext(ctx, `SELECT COUNT(*) FROM information_schema.STATISTICS WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME=? AND INDEX_NAME=?`, required.table, required.name).Scan(&count); err != nil {
			return result, errors.New("operational index status unavailable")
		}
		if count > 0 {
			result.RequiredIndexCount++
		}
	}
	wanted, err := Migrations()
	if err != nil || len(status) != len(wanted) {
		return result, errors.New("schema version mismatch")
	}
	for i, migration := range wanted {
		if status[i].Version != migration.Version || status[i].Name != migration.Name || status[i].Checksum != migration.Checksum {
			return result, errors.New("schema checksum mismatch")
		}
	}
	result.Complete = result.ForeignKeyCount == 0 && result.RequiredIndexCount == len(requiredOperationalIndexes)
	if !result.Complete {
		return result, errors.New("schema preflight requirements are not satisfied")
	}
	return result, nil
}