// Package migrate allows you to update your database from your application
package migrate

import (
	"database/sql"
	"errors"
	"fmt"
	"strconv"
)

// Options contains all settings
type Options struct {
	TableName   string // Name used for version info table; defaults to DefaultTableName if not set
	Schema      string // Schema used for version info table; For PostgreSQL, ignored if not set
	AssetPrefix string
}

// DefaultTableName is the name used when no TableName is specified in Options
const DefaultTableName = `version`

// ErrUpdatesMissing indicates an update is missing, making it impossible to execute the migration
var ErrUpdatesMissing = errors.New(`Missing migration files`)

// ErrDatabaseNewer indicates that the database version is newer than the requested version. We throw an error because downgrades might cause dataloss
var ErrDatabaseNewer = errors.New(`Current version is newer than the requested version`)

const fileFormat = `%04d.sql`

// AssetFunc is a function that returns the data for the given name
type AssetFunc func(string) ([]byte, error)

// Migrate executes
func Migrate(db *sql.DB, version int, o Options, asset AssetFunc) error {
	if o.TableName == `` {
		o.TableName = DefaultTableName
	}

	table := o.TableName
	if o.Schema != `` {
		table = o.Schema + `.` + table
	}

	_, err := db.Exec(`CREATE TABLE IF NOT EXISTS ` + table + ` (Version integer NOT NULL PRIMARY KEY)`)
	if err != nil {
		return err
	}

	row := db.QueryRow(`SELECT Version FROM ` + table + ` ORDER BY Version DESC`)
	if err != nil {
		return err
	}

	var v int
	err = row.Scan(&v)
	if err != sql.ErrNoRows && err != nil {
		return err
	}

	if v > version {
		return ErrDatabaseNewer
	}

	tx, err := db.Begin()
	if err != nil {
		return err
	}
	defer tx.Rollback()

	for i := v + 1; i <= version; i++ {
		asset, err := asset(fmt.Sprintf(o.AssetPrefix+fileFormat, version))
		if err != nil {
			return ErrUpdatesMissing
		}

		_, err = tx.Exec(string(asset))
		if err != nil {
			return err
		}

		_, err = tx.Exec(`INSERT INTO ` + table + ` VALUES (` + strconv.Itoa(i) + `)`)
		if err != nil {
			return err
		}
	}

	err = tx.Commit()
	if err != nil {
		return err
	}

	return nil
}