Add CharSetUtil and PreParser for parsing MSH-18 from

src/NHapi.Base/Util/CharsetUtil.cs

@@ -0,0 +1,85 @@
+namespace NHapi.Base.Util
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Linq;
+    using System.Text;
+
+    public class CharsetUtil
+    {
+        private static readonly List<Bom> Boms = new()
+        {
+            new Bom(new byte[] { }, Encoding.ASCII),
+            new Bom(new byte[] { 0xEF, 0xBB, 0xBF }, Encoding.UTF8),
+            new Bom(new byte[] { 0xFF, 0xFE }, Encoding.Unicode), // UTF-16LE
+            new Bom(new byte[] { 0xFE, 0xFF, 0xBF }, Encoding.BigEndianUnicode), // UTF-16BE
+            new Bom(new byte[] { 0xFF, 0xFE, 0x00, 0x00 }, Encoding.UTF32), // UTF-32LE
+            new Bom(new byte[] { 0x00, 0x00, 0xFE, 0xFF }, new UTF32Encoding(true, true)), // UTF-32BE
+        };
+
+        public static Encoding CheckCharset(byte[] messageBytes, Encoding fallbackEncoding)
+        {
+            var messageString = SkipBom(messageBytes);
+            return CheckCharset(messageString, fallbackEncoding);
+        }
+
+        public static Encoding CheckCharset(string messageString, Encoding fallbackEncoding)
+        {
+            throw NotImplementedException();
+        }
+
+        public static byte[] WithoutBom(byte[] messageBytes)
+        {
+            var bom = GetBom(messageBytes);
+            return messageBytes.Skip(bom.BomBytes.Length).ToArray();
+        }
+
+        private static string SkipBom(byte[] messageBytes)
+        {
+            var bom = GetBom(messageBytes);
+            var encoding = bom.Encoding;
+            var messageBytesWithoutBom = messageBytes.Skip(bom.BomBytes.Length).ToArray();
+            return encoding.GetString(messageBytesWithoutBom);
+        }
+
+        private static Bom GetBom(byte[] messageBytes)
+        {
+            if (messageBytes == null)
+            {
+                return Boms[0];
+            }
+
+            foreach (var bom in Boms)
+            {
+                var messageBomBytes = messageBytes.Take(bom.BomBytes.Length);
+                if (bom.BomBytes.SequenceEqual(messageBomBytes))
+                {
+                    return bom;
+                }
+            }
+
+            return Boms[0];
+        }
+
+        private class Bom
+        {
+            public Bom(byte[] bomBytes, Encoding encoding)
+            {
+                BomBytes = bomBytes;
+                Encoding = encoding;
+            }
+
+            public byte[] BomBytes { get; }
+
+            public Encoding Encoding { get; }
+        }
+    }
+
+    public static class PreParser
+    {
+        public static string GetFields(string message, params string[] fields)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}