// Package mysql owns SQL connections, explicit migrations and transaction boundaries.
package mysql

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

	driver "github.com/go-sql-driver/mysql"
)

// Open configures UTC, bounded I/O and an explicit connection pool. It never
// applies migrations and never logs the DSN. TLS is chosen in the secret DSN.
func Open(ctx context.Context, dsn string, maxOpen, maxIdle int) (*sql.DB, error) {
	if maxOpen <= 0 || maxIdle < 0 || maxIdle > maxOpen {
		return nil, errors.New("invalid database pool configuration")
	}
	cfg, err := driver.ParseDSN(dsn)
	if err != nil {
		return nil, errors.New("invalid MySQL DSN")
	}
	if cfg.DBName == "" {
		return nil, errors.New("MySQL database name required")
	}
	cfg.ParseTime = true
	cfg.Loc = time.UTC
	cfg.MultiStatements = false
	cfg.InterpolateParams = false
	cfg.Timeout = 5 * time.Second
	cfg.ReadTimeout = 10 * time.Second
	cfg.WriteTimeout = 10 * time.Second
	if cfg.Params == nil {
		cfg.Params = map[string]string{}
	}
	cfg.Params["time_zone"] = "'+00:00'"
	cfg.Params["sql_mode"] = "'STRICT_ALL_TABLES,NO_ZERO_DATE,NO_ZERO_IN_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION'"
	connector, err := driver.NewConnector(cfg)
	if err != nil {
		return nil, errors.New("invalid MySQL connection configuration")
	}
	db := sql.OpenDB(connector)
	db.SetMaxOpenConns(maxOpen)
	db.SetMaxIdleConns(maxIdle)
	db.SetConnMaxLifetime(5 * time.Minute)
	db.SetConnMaxIdleTime(time.Minute)
	if err := db.PingContext(ctx); err != nil {
		db.Close()
		return nil, errors.New("MySQL connection failed; check endpoint, credentials and TLS configuration")
	}
	return db, nil
}

const transactionRetryLimit = 2

// WithTx retries only a callback failure caused by a MySQL deadlock or lock wait
// timeout before COMMIT. It never retries Begin/Commit errors because a commit
// response may be lost after the database has already applied the transaction;
// callers must reconcile such an ambiguous write by stable operation identity.
func WithTx(ctx context.Context, db *sql.DB, fn func(*sql.Tx) error) error {
	for attempt := 0; attempt <= transactionRetryLimit; attempt++ {
		err, retryable := withTxAttempt(ctx, db, fn)
		if err == nil || !retryable || attempt == transactionRetryLimit {
			return err
		}
		if err := waitTransactionRetry(ctx, attempt); err != nil {
			return err
		}
	}
	return errors.New("transaction retry loop exhausted")
}

func withTxAttempt(ctx context.Context, db *sql.DB, fn func(*sql.Tx) error) (error, bool) {
	tx, err := db.BeginTx(ctx, &sql.TxOptions{Isolation: sql.LevelReadCommitted})
	if err != nil {
		return err, false
	}
	defer tx.Rollback()
	if err := fn(tx); err != nil {
		return err, retryableTransactionError(err)
	}
	if err := tx.Commit(); err != nil {
		return err, false
	}
	return nil, false
}

func retryableTransactionError(err error) bool {
	var mysqlErr *driver.MySQLError
	if !errors.As(err, &mysqlErr) {
		return false
	}
	return mysqlErr.Number == 1205 || mysqlErr.Number == 1213
}

func waitTransactionRetry(ctx context.Context, attempt int) error {
	delay := time.Duration(attempt+1) * 25 * time.Millisecond
	timer := time.NewTimer(delay)
	defer timer.Stop()
	select {
	case <-ctx.Done():
		return ctx.Err()
	case <-timer.C:
		return nil
	}
}