Reapply "feat(loyalty): EnakPoint & EnakCoin" (#32)

This reverts commit 4e24f9bbb0.

internal/processor/payment_code_processor.go

@@ -0,0 +1,101 @@
+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
+}