apskel-pos-backend/internal/processor/payment_code_processor.go

package processor

import (
	"context"
	"crypto/rand"
	"errors"
	"fmt"
	"math/big"
	"strings"
	"time"

	"github.com/google/uuid"

	"apskel-pos-be/internal/models"
	"apskel-pos-be/internal/repository"
)

const (
	paymentCodeDigits   = 6
	paymentCodeTTL      = 2 * time.Minute
	paymentCodeAttempts = 5
	// PaymentCodeQRPrefix marks a scanned QR as an EnakPoint payment code.
	PaymentCodeQRPrefix = "enakpoint:"
)

// ErrPaymentCodeInvalid means the code was never issued, has expired, has been used,
// or belongs to another customer.
var ErrPaymentCodeInvalid = errors.New("payment code is invalid or expired")

type pinVerifier interface {
	VerifyPin(ctx context.Context, customerID uuid.UUID, pin string, action PinAction, info models.CustomerPinRequestInfo) error
}

// PaymentCodeProcessor issues and redeems the one-time codes that let a cashier take a
// customer's EnakPoint (docs/prd-point-coin.md F9, K8). The customer approves with
// their PIN on their own phone and shows the code; the PIN is never typed on the
// cashier's device.
type PaymentCodeProcessor struct {
	codes repository.PaymentCodeRepository
	pins  pinVerifier
	now   func() time.Time
}

func NewPaymentCodeProcessor(codes repository.PaymentCodeRepository, pins pinVerifier) *PaymentCodeProcessor {
	return &PaymentCodeProcessor{codes: codes, pins: pins, now: time.Now}
}

// Issue checks the customer's PIN and returns a fresh 6-digit code, valid for two
// minutes and bound to the customer. A new code retires the previous one.
func (p *PaymentCodeProcessor) Issue(ctx context.Context, customerID uuid.UUID, pin string, info models.CustomerPinRequestInfo) (*models.PaymentCode, error) {
	if err := p.pins.VerifyPin(ctx, customerID, pin, PinActionPay, info); err != nil {
		return nil, err
	}
	for attempt := 0; attempt < paymentCodeAttempts; attempt++ {
		code, err := randomDigits(paymentCodeDigits)
		if err != nil {
			return nil, err
		}
		err = p.codes.Save(ctx, code, customerID, paymentCodeTTL)
		if errors.Is(err, repository.ErrPaymentCodeTaken) {
			continue
		}
		if err != nil {
			return nil, err
		}
		return &models.PaymentCode{
			Code:      code,
			QRPayload: PaymentCodeQRPrefix + code,
			ExpiresAt: p.now().Add(paymentCodeTTL),
		}, nil
	}
	return nil, fmt.Errorf("could not draw a free payment code after %d attempts", paymentCodeAttempts)
}

// Redeem uses a code up for a payment by the given customer. It accepts the code as
// typed or as scanned from the QR. Every failure is ErrPaymentCodeInvalid.
func (p *PaymentCodeProcessor) Redeem(ctx context.Context, code string, customerID uuid.UUID) error {
	code = strings.TrimPrefix(strings.TrimSpace(code), PaymentCodeQRPrefix)
	if len(code) != paymentCodeDigits {
		return ErrPaymentCodeInvalid
	}
	err := p.codes.Consume(ctx, code, customerID)
	if errors.Is(err, repository.ErrPaymentCodeNotFound) || errors.Is(err, repository.ErrPaymentCodeWrongCustomer) {
		return ErrPaymentCodeInvalid
	}
	return err
}

// randomDigits draws n decimal digits from a cryptographic source, so codes cannot be
// predicted.
func randomDigits(n int) (string, error) {
	var b strings.Builder
	for i := 0; i < n; i++ {
		d, err := rand.Int(rand.Reader, big.NewInt(10))
		if err != nil {
			return "", fmt.Errorf("failed to draw a payment code: %w", err)
		}
		b.WriteByte(byte('0' + d.Int64()))
	}
	return b.String(), nil
}