Improved IBAN validator

Switched to piecewise calculation as described here: https://en.wikipedia.org/wiki/International_Bank_Account_Number#Validating_the_IBAN

Author: codebude

QRCoder/PayloadGenerator.cs

@@ -2410,7 +2410,7 @@ public override string ToString()
                 _sb.AppendFormat("{0:000}", CalculateChecksum()).Append('\n');
                 return _sb.ToString();
             }
-        }
+        }      
 
         private static bool IsValidIban(string iban)
         {
@@ -2419,14 +2419,20 @@ private static bool IsValidIban(string iban)
 
             //Check for general structure
             var structurallyValid = Regex.IsMatch(ibanCleared, @"^[a-zA-Z]{2}[0-9]{2}([a-zA-Z0-9]?){16,30}$");
-                                         
+
             //Check IBAN checksum
+            var checksumValid = false;
             var sum = $"{ibanCleared.Substring(4)}{ibanCleared.Substring(0, 4)}".ToCharArray().Aggregate("", (current, c) => current + (char.IsLetter(c) ? (c - 55).ToString() : c.ToString()));
-            decimal sumDec;
-            if (!decimal.TryParse(sum, out sumDec))
-                return false;
-            var checksumValid = (sumDec % 97) == 1;
-            
+            int m = 0;
+            for (int i = 0; i < (int)Math.Ceiling((sum.Length - 2) / 7d); i++){
+                var offset = (i == 0 ? 0 : 2);
+                var start = i * 7 + offset;
+                var n = (i == 0 ? "" : m.ToString()) + sum.Substring(start, Math.Min(9 - offset, sum.Length - start));
+                if (!int.TryParse(n, NumberStyles.Any, CultureInfo.InvariantCulture, out m))
+                    break;
+                m = m % 97;
+            }
+            checksumValid = m == 1;
             return structurallyValid && checksumValid;
         }
 

QRCoderTests/PayloadGeneratorTests.cs

@@ -2104,6 +2104,20 @@ public void swissqrcode_generator_should_generate_iban()
                 .ShouldBe("CH2609000000857666015");
         }
 
+        [Fact]
+        [Category("PayloadGenerator/SwissQrCode.Iban")]
+        public void swissqrcode_generator_should_generate_iban_2()
+        {
+            var iban = "CH47048350000GABRIELS";
+            var ibanType = PayloadGenerator.SwissQrCode.Iban.IbanType.Iban;
+
+            var generator = new PayloadGenerator.SwissQrCode.Iban(iban, ibanType);
+
+            generator
+                .ToString()
+                .ShouldBe("CH47048350000GABRIELS");
+        }
+
         [Fact]
         [Category("PayloadGenerator/SwissQrCode.Iban")]
         public void swissqrcode_generator_should_generate_iban_qr()