package main

import (
	"context"
	"database/sql"
	"fmt"
	"log"

	"github.com/cockroachdb/cockroach-go/crdb"
)

func transferFunds(tx *sql.Tx, from int, to int, amount int) error {
	// Read the balance.
	var fromBalance int
	if err := tx.QueryRow(
		"SELECT balance FROM accounts WHERE id = $1", from).Scan(&fromBalance); err != nil {
		return err
	}

	if fromBalance < amount {
		return fmt.Errorf("insufficient funds")
	}

	// Perform the transfer.
	if _, err := tx.Exec(
		"UPDATE accounts SET balance = balance - $1 WHERE id = $2", amount, from); err != nil {
		return err
	}
	if _, err := tx.Exec(
		"UPDATE accounts SET balance = balance + $1 WHERE id = $2", amount, to); err != nil {
		return err
	}
	return nil
}

func main() {
	db, err := sql.Open("postgres",
		"postgresql://maxroach@localhost:26257/bank?ssl=true&sslmode=require&sslrootcert=certs/ca.crt&sslkey=certs/client.maxroach.key&sslcert=certs/client.maxroach.crt")
	if err != nil {
		log.Fatal("error connecting to the database: ", err)
	}
	defer db.Close()

	// Run a transfer in a transaction.
	err = crdb.ExecuteTx(context.Background(), db, nil, func(tx *sql.Tx) error {
		return transferFunds(tx, 1 /* from acct# */, 2 /* to acct# */, 100 /* amount */)
	})
	if err == nil {
		fmt.Println("Success")
	} else {
		log.Fatal("error: ", err)
	}
}