Fix structured append parity

Author: manuelbl

QrCodeGenerator/StructuredAppend.cs

@@ -296,9 +296,16 @@ private static Tuple<DataSegment, DataSegment> SplitSegment(DataSegment segment,
             );
         }
 
+        /// <summary>
+        /// Calculates the parity of the data, i.e. the XOR of all its bytes.
+        /// <para>
+        /// Every QR code of the sequence carries it, so that a scanner can tell whether the codes
+        /// it has collected belong together (see ISO/IEC 18004, section 8.4.1).
+        /// </para>
+        /// </summary>
         private static byte CalculateParity(byte[] data)
         {
-            return data.Aggregate<byte, byte>(0, (current, value) => (byte)(current ^ (byte)(value >> 8)));
+            return data.Aggregate<byte, byte>(0, (current, value) => (byte)(current ^ value));
         }
 
     }

QrCodeGeneratorTest/StructuredAppendTest.cs

@@ -49,6 +49,19 @@ public void EncodeVeryLongStringInMultipleCodes()
             }
         }
 
+        [Fact]
+        public void StructuredAppendHeadersCarryDataParity()
+        {
+            var text = RandomData.MakeAlphanumericString(3000, seed: 1001);
+            var data = Encoding.GetEncoding("ISO-8859-1").GetBytes(text);
+            var expectedParity = data.Aggregate<byte, byte>(0, (current, value) => (byte)(current ^ value));
+
+            var qrCodes = StructuredAppend.BuildSegments(data, 29, QrCode.Ecc.Medium, ECI.Latin1, false);
+
+            Assert.NotEqual(0, expectedParity);
+            Assert.All(qrCodes, segments => Assert.Equal(expectedParity, segments[0].StructuredAppendParity));
+        }
+
         [Fact]
         public void RejectTooLongString()
         {