apskel-pos-backend/internal/repository/payment_code_repository.go

package repository

import (
	"context"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/google/uuid"
	"github.com/redis/go-redis/v9"
)

var (
	// ErrPaymentCodeTaken means the code is already live for someone; draw another.
	ErrPaymentCodeTaken = errors.New("payment code already in use")
	// ErrPaymentCodeNotFound means the code does not exist: never issued, expired, or
	// already used.
	ErrPaymentCodeNotFound = errors.New("payment code not found")
	// ErrPaymentCodeWrongCustomer means the code belongs to another customer.
	ErrPaymentCodeWrongCustomer = errors.New("payment code belongs to another customer")
)

// PaymentCodeRepository keeps one-time EnakPoint payment codes in Redis
// (docs/prd-point-coin.md F9). A code expires by TTL and is removed when used.
type PaymentCodeRepository interface {
	// Save stores a code for a customer for ttl, and retires the customer's previous
	// code so only the newest one works. ErrPaymentCodeTaken if the code is live.
	Save(ctx context.Context, code string, customerID uuid.UUID, ttl time.Duration) error
	// Consume uses a code up if it belongs to the customer. A code of another customer
	// is left in place, so a cashier scanning it against the wrong order does not burn
	// it for its owner.
	Consume(ctx context.Context, code string, customerID uuid.UUID) error
}

type paymentCodeRepository struct {
	client *redis.Client
}

func NewPaymentCodeRepository(client *redis.Client) PaymentCodeRepository {
	return &paymentCodeRepository{client: client}
}

func paymentCodeKey(code string) string { return "wallet:paycode:" + code }

func paymentCodeCustomerKey(customerID uuid.UUID) string {
	return "wallet:paycode:customer:" + customerID.String()
}

func (r *paymentCodeRepository) Save(ctx context.Context, code string, customerID uuid.UUID, ttl time.Duration) error {
	ok, err := r.client.SetNX(ctx, paymentCodeKey(code), customerID.String(), ttl).Result()
	if err != nil {
		return fmt.Errorf("failed to store payment code: %w", err)
	}
	if !ok {
		return ErrPaymentCodeTaken
	}
	previous, err := r.client.GetSet(ctx, paymentCodeCustomerKey(customerID), code).Result()
	if err != nil && !errors.Is(err, redis.Nil) {
		return fmt.Errorf("failed to track payment code: %w", err)
	}
	r.client.Expire(ctx, paymentCodeCustomerKey(customerID), ttl)
	if previous != "" && previous != code {
		// Only if it is still that customer's: the number may have been reissued.
		if err := r.compareAndDelete(ctx, previous, customerID); err != nil && !errors.Is(err, ErrPaymentCodeNotFound) && !errors.Is(err, ErrPaymentCodeWrongCustomer) {
			return err
		}
	}
	return nil
}

// consumeScript deletes a code only if it belongs to the given customer, in one step.
// Returns 1 when used up, 0 when missing, -1 when it belongs to someone else.
var consumeScript = redis.NewScript(`
local owner = redis.call('GET', KEYS[1])
if not owner then return 0 end
if owner ~= ARGV[1] then return -1 end
redis.call('DEL', KEYS[1])
return 1
`)

func (r *paymentCodeRepository) Consume(ctx context.Context, code string, customerID uuid.UUID) error {
	code = strings.TrimSpace(code)
	if code == "" {
		return ErrPaymentCodeNotFound
	}
	return r.compareAndDelete(ctx, code, customerID)
}

func (r *paymentCodeRepository) compareAndDelete(ctx context.Context, code string, customerID uuid.UUID) error {
	result, err := consumeScript.Run(ctx, r.client, []string{paymentCodeKey(code)}, customerID.String()).Int()
	if err != nil {
		return fmt.Errorf("failed to use payment code: %w", err)
	}
	switch result {
	case 1:
		return nil
	case -1:
		return ErrPaymentCodeWrongCustomer
	default:
		return ErrPaymentCodeNotFound
	}
}