feat(validation): add card number validation

backend/internal/card/validation.go

@@ -0,0 +1,27 @@
+package card
+
+import "errors"
+
+var (
+	ErrCardRequired = errors.New("card number is required")
+	ErrCardLength   = errors.New("card number must contain exactly 16 digits")
+	ErrCardNumeric  = errors.New("card number must contain only digits")
+)
+
+func Validate(number string) error {
+	if number == "" {
+		return ErrCardRequired
+	}
+
+	if len(number) != 16 {
+		return ErrCardLength
+	}
+
+	for _, char := range number {
+		if char < '0' || char > '9' {
+			return ErrCardNumeric
+		}
+	}
+
+	return nil
+}

backend/internal/card/validation_test.go

@@ -0,0 +1,55 @@
+package card
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestValidate(t *testing.T) {
+	tests := []struct {
+		name    string
+		number  string
+		wantErr error
+	}{
+		{
+			name:    "empty card number",
+			number:  "",
+			wantErr: ErrCardRequired,
+		},
+		{
+			name:    "card number is too short",
+			number:  "123456789",
+			wantErr: ErrCardLength,
+		},
+		{
+			name:    "card number is too long",
+			number:  "12345678901234567",
+			wantErr: ErrCardLength,
+		},
+		{
+			name:    "card number contains letters",
+			number:  "603799ABC4567890",
+			wantErr: ErrCardNumeric,
+		},
+		{
+			name:    "card number contains spaces",
+			number:  "6037 912 3456 78",
+			wantErr: ErrCardNumeric,
+		},
+		{
+			name:    "valid card number structure",
+			number:  "6037991234567890",
+			wantErr: nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := Validate(tt.number)
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Errorf("Validate() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}