CsvFieldIndexer: Fix calculation of offsets with unicode characters.

Fixes #4

CsvLib/BufferedTextReader.cs

@@ -1,4 +1,3 @@
-using System;
 using System.IO;
 using System.Text;
 
@@ -10,15 +9,29 @@ namespace CsvLib
         private int _position;
         private readonly StringBuilder _sbBuffer = new StringBuilder();
 
+        private readonly Encoding _currentEncoding = Encoding.Default;
+
         public BufferedTextReader(TextReader baseReader)
         {
             _baseReader = baseReader;
+            if (baseReader is StreamReader streamReader)
+            {
+                _currentEncoding = streamReader.CurrentEncoding;
+            }
         }
 
         public override int Read()
         {
-            _position++;
             int read = _baseReader.Read();
+            if (read > 127)
+            {
+                int count = _currentEncoding.GetByteCount(((char)read).ToString());
+                _position += count;
+            }
+            else
+            {
+                _position++;
+            }
             if (read != -1)
             {
                 _sbBuffer.Append((char)read);
@@ -26,11 +39,6 @@ namespace CsvLib
             return read;
         }
 
-        public override int Read(char[] buffer, int index, int count)
-        {
-            throw new NotImplementedException("Read buffered method on BufferedTextReader");
-        }
-
         public override int Peek()
         {
             return _baseReader.Peek();

CsvLib/CsvFieldIndexer.cs

@@ -9,6 +9,8 @@ namespace CsvLib
     {
         private bool _insideString;
 
+        private Encoding _currentEncoding = Encoding.Default;
+        
         private readonly char _separator;
         private readonly char _quoteChar;
         private readonly char _escapeChar;
@@ -59,6 +61,7 @@ namespace CsvLib
             List<long> fieldPositions = new List<long>();
             long? fieldStartPosition = null;
             long? fieldEndPosition = null;
+            int unicodeDelta = 0;
             for (int i = 0; i < line.Length; i++)
             {
                 char c = line[i];
@@ -90,7 +93,12 @@ namespace CsvLib
                 }
                 else
                 {
-                    long absolutePosition = lineOffset + i;
+                    if (c > 127)
+                    {
+                        unicodeDelta += _currentEncoding.GetByteCount(c.ToString()) - 1;
+                    }
+                    
+                    long absolutePosition = lineOffset + i + unicodeDelta;
                     if (fieldStartPosition == null) { fieldStartPosition = absolutePosition; }
                     fieldEndPosition = absolutePosition;
                 }
@@ -121,6 +129,10 @@ namespace CsvLib
             _index.Clear();
             _index.Add(0);
             int idxRow = 0;
+            if (textReader is StreamReader streamReader)
+            {
+                _currentEncoding = streamReader.CurrentEncoding;
+            }
             using (BufferedTextReader reader = new BufferedTextReader(textReader))
             {
                 string currentLine;
@@ -180,7 +192,7 @@ namespace CsvLib
         public void LoadIndexOfFile(string file)
         {
             DateTime dtFile = File.GetCreationTime(file);
-            string indexFile = $"{file}.idx";
+            string indexFile = $"{file}.idx2";
             if (File.Exists(indexFile) && File.GetCreationTime(indexFile) > dtFile)
             {
                 _index = Index_LoadFile(indexFile);

CsvLib/TrackingTextReader.cs

@@ -1,5 +1,5 @@
-using System;
 using System.IO;
+using System.Text;
 
 namespace CsvLib
 {
@@ -7,21 +7,30 @@ namespace CsvLib
     {
         private readonly TextReader _baseReader;
         private int _position;
+        private readonly Encoding _currentEncoding = Encoding.Default;
 
         public TrackingTextReader(TextReader baseReader)
         {
             _baseReader = baseReader;
+            if (baseReader is StreamReader streamReader)
+            {
+                _currentEncoding = streamReader.CurrentEncoding;
+            }
         }
 
         public override int Read()
         {
-            _position++;
-            return _baseReader.Read();
-        }
-
-        public override int Read(char[] buffer, int index, int count)
-        {
-            throw new NotImplementedException("Read buffered method on TrackingTextReader");
+            int read = _baseReader.Read();
+            if (read > 127)
+            {
+                int count = _currentEncoding.GetByteCount(((char)read).ToString());
+                _position += count;
+            }
+            else
+            {
+                _position++;
+            }
+            return read;
         }
 
         public override int Peek()