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Simple class to automatically detect text file encoding, with English-biased "best guess" heuristic based on byte patterns in the absence of BOM.

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TextFileEncodingDetector.cs
C#
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using System;
using System.Text;
using System.Text.RegularExpressions;
using System.IO;
 
namespace KlerksSoft
{
public static class TextFileEncodingDetector
{
/*
* Simple class to handle text file encoding woes (in a primarily English-speaking tech
* world).
*
* - This code is fully managed, no shady calls to MLang (the unmanaged codepage
* detection library originally developed for Internet Explorer).
*
* - This class does NOT try to detect arbitrary codepages/charsets, it really only
* aims to differentiate between some of the most common variants of Unicode
* encoding, and a "default" (western / ascii-based) encoding alternative provided
* by the caller.
*
* - As there is no "Reliable" way to distinguish between UTF-8 (without BOM) and
* Windows-1252 (in .Net, also incorrectly called "ASCII") encodings, we use a
* heuristic - so the more of the file we can sample the better the guess. If you
* are going to read the whole file into memory at some point, then best to pass
* in the whole byte byte array directly. Otherwise, decide how to trade off
* reliability against performance / memory usage.
*
* - The UTF-8 detection heuristic only works for western text, as it relies on
* the presence of UTF-8 encoded accented and other characters found in the upper
* ranges of the Latin-1 and (particularly) Windows-1252 codepages.
*
* - For more general detection routines, see existing projects / resources:
* - MLang - Microsoft library originally for IE6, available in Windows XP and later APIs now (I think?)
* - MLang .Net bindings: http://www.codeproject.com/KB/recipes/DetectEncoding.aspx
* - CharDet - Mozilla browser's detection routines
* - Ported to Java then .Net: http://www.conceptdevelopment.net/Localization/NCharDet/
* - Ported straight to .Net: http://code.google.com/p/chardetsharp/source/browse
*
* Copyright Tao Klerks, Jan 2010, tao@klerks.biz
*
* Permission is hereby granted to use, modify and distribute this code freely, provided
* the original copyright attribution remains.
*
*/
 
const long _defaultHeuristicSampleSize = 0x10000; //completely arbitrary - inappropriate for high numbers of files / speed
 
public static Encoding DetectTextFileEncoding(string InputFilename, Encoding DefaultEncoding)
{
using (FileStream textfileStream = File.OpenRead(InputFilename))
{
return DetectTextFileEncoding(textfileStream, DefaultEncoding, _defaultHeuristicSampleSize);
}
}
 
public static Encoding DetectTextFileEncoding(FileStream InputFileStream, Encoding DefaultEncoding, long HeuristicSampleSize)
{
if (InputFileStream == null)
throw new ArgumentNullException("Must provide a valid Filestream!", "InputFileStream");
 
if (!InputFileStream.CanRead)
throw new ArgumentException("Provided file stream is not readable!", "InputFileStream");
 
if (!InputFileStream.CanSeek)
throw new ArgumentException("Provided file stream cannot seek!", "InputFileStream");
 
Encoding encodingFound = null;
 
long originalPos = InputFileStream.Position;
 
InputFileStream.Position = 0;
 
 
//First read only what we need for BOM detection
 
byte[] bomBytes = new byte[InputFileStream.Length > 4 ? 4 : InputFileStream.Length];
InputFileStream.Read(bomBytes, 0, bomBytes.Length);
 
encodingFound = DetectBOMBytes(bomBytes);
 
if (encodingFound != null)
{
InputFileStream.Position = originalPos;
return encodingFound;
}
 
 
//BOM Detection failed, going for heuristics now.
// create sample byte array and populate it
byte[] sampleBytes = new byte[HeuristicSampleSize > InputFileStream.Length ? InputFileStream.Length : HeuristicSampleSize];
Array.Copy(bomBytes, sampleBytes, bomBytes.Length);
if (InputFileStream.Length > bomBytes.Length)
InputFileStream.Read(sampleBytes, bomBytes.Length, sampleBytes.Length - bomBytes.Length);
InputFileStream.Position = originalPos;
 
//test byte array content
encodingFound = DetectUnicodeInByteSampleByHeuristics(sampleBytes);
 
if (encodingFound != null)
return encodingFound;
else
return DefaultEncoding;
}
 
public static Encoding DetectTextByteArrayEncoding(byte[] TextData, Encoding DefaultEncoding)
{
if (TextData == null)
throw new ArgumentNullException("Must provide a valid text data byte array!", "TextData");
 
Encoding encodingFound = null;
 
encodingFound = DetectBOMBytes(TextData);
 
if (encodingFound != null)
{
return encodingFound;
}
else
{
//test byte array content
encodingFound = DetectUnicodeInByteSampleByHeuristics(TextData);
 
if (encodingFound != null)
return encodingFound;
else
return DefaultEncoding;
}
 
 
}
 
public static Encoding DetectBOMBytes(byte[] BOMBytes)
{
if (BOMBytes == null)
throw new ArgumentNullException("Must provide a valid BOM byte array!", "BOMBytes");
 
if (BOMBytes.Length < 2)
return null;
 
if (BOMBytes[0] == 0xff && BOMBytes[1] == 0xfe && (BOMBytes.Length < 4 || BOMBytes[2] != 0 || BOMBytes[3] != 0))
return Encoding.Unicode;
 
if (BOMBytes[0] == 0xfe && BOMBytes[1] == 0xff)
return Encoding.BigEndianUnicode;
 
if (BOMBytes.Length < 3)
return null;
 
if (BOMBytes[0] == 0xef && BOMBytes[1] == 0xbb && BOMBytes[2] == 0xbf)
return Encoding.UTF8;
 
if (BOMBytes[0] == 0x2b && BOMBytes[1] == 0x2f && BOMBytes[2] == 0x76)
return Encoding.UTF7;
 
if (BOMBytes.Length < 4)
return null;
 
if (BOMBytes[0] == 0xff && BOMBytes[1] == 0xfe && BOMBytes[2] == 0 && BOMBytes[3] == 0)
return Encoding.UTF32;
 
if (BOMBytes[0] == 0 && BOMBytes[1] == 0 && BOMBytes[2] == 0xfe && BOMBytes[3] == 0xff)
return Encoding.GetEncoding(12001);
 
return null;
}
 
public static Encoding DetectUnicodeInByteSampleByHeuristics(byte[] SampleBytes)
{
long oddBinaryNullsInSample = 0;
long evenBinaryNullsInSample = 0;
long suspiciousUTF8SequenceCount = 0;
long suspiciousUTF8BytesTotal = 0;
long likelyUSASCIIBytesInSample = 0;
 
//Cycle through, keeping count of binary null positions, possible UTF-8
// sequences from upper ranges of Windows-1252, and probable US-ASCII
// character counts.
 
long currentPos = 0;
int skipUTF8Bytes = 0;
 
while (currentPos < SampleBytes.Length)
{
//binary null distribution
if (SampleBytes[currentPos] == 0)
{
if (currentPos % 2 == 0)
evenBinaryNullsInSample++;
else
oddBinaryNullsInSample++;
}
 
//likely US-ASCII characters
if (IsCommonUSASCIIByte(SampleBytes[currentPos]))
likelyUSASCIIBytesInSample++;
//suspicious sequences (look like UTF-8)
if (skipUTF8Bytes == 0)
{
int lengthFound = DetectSuspiciousUTF8SequenceLength(SampleBytes, currentPos);
 
if (lengthFound > 0)
{
suspiciousUTF8SequenceCount++;
suspiciousUTF8BytesTotal += lengthFound;
skipUTF8Bytes = lengthFound - 1;
}
}
else
{
skipUTF8Bytes--;
}
 
currentPos++;
}
 
//1: UTF-16 LE - in english / european environments, this is usually characterized by a
// high proportion of odd binary nulls (starting at 0), with (as this is text) a low
// proportion of even binary nulls.
// The thresholds here used (less than 20% nulls where you expect non-nulls, and more than
// 60% nulls where you do expect nulls) are completely arbitrary.
 
if (((evenBinaryNullsInSample * 2.0) / SampleBytes.Length) < 0.2 && ((oddBinaryNullsInSample * 2.0) / SampleBytes.Length) > 0.6)
return Encoding.Unicode;
 
 
//2: UTF-16 BE - in english / european environments, this is usually characterized by a
// high proportion of even binary nulls (starting at 0), with (as this is text) a low
// proportion of odd binary nulls.
// The thresholds here used (less than 20% nulls where you expect non-nulls, and more than
// 60% nulls where you do expect nulls) are completely arbitrary.
 
if (((oddBinaryNullsInSample * 2.0) / SampleBytes.Length) < 0.2 && ((evenBinaryNullsInSample * 2.0) / SampleBytes.Length) > 0.6)
return Encoding.BigEndianUnicode;
 
 
//3: UTF-8 - Martin Dürst outlines a method for detecting whether something CAN be UTF-8 content
// using regexp, in his w3c.org unicode FAQ entry:
// http://www.w3.org/International/questions/qa-forms-utf-8
// adapted here for C#.
string potentiallyMangledString = Encoding.ASCII.GetString(SampleBytes);
Regex UTF8Validator = new Regex(@"\A([\x09\x0A\x0D\x20-\x7E]|[\xC2-\xDF][\x80-\xBF]|\xE0[\xA0-\xBF][\x80-\xBF]|[\xE1-\xEC\xEE\xEF][\x80-\xBF]{2}|\xED[\x80-\x9F][\x80-\xBF]|\xF0[\x90-\xBF][\x80-\xBF]{2}|[\xF1-\xF3][\x80-\xBF]{3}|\xF4[\x80-\x8F][\x80-\xBF]{2})*\z");
if (UTF8Validator.IsMatch(potentiallyMangledString))
{
//Unfortunately, just the fact that it CAN be UTF-8 doesn't tell you much about probabilities.
//If all the characters are in the 0-127 range, no harm done, most western charsets are same as UTF-8 in these ranges.
//If some of the characters were in the upper range (western accented characters), however, they would likely be mangled to 2-byte by the UTF-8 encoding process.
// So, we need to play stats.
 
// The "Random" likelihood of any pair of randomly generated characters being one
// of these "suspicious" character sequences is:
// 128 / (256 * 256) = 0.2%.
//
// In western text data, that is SIGNIFICANTLY reduced - most text data stays in the <127
// character range, so we assume that more than 1 in 500,000 of these character
// sequences indicates UTF-8. The number 500,000 is completely arbitrary - so sue me.
//
// We can only assume these character sequences will be rare if we ALSO assume that this
// IS in fact western text - in which case the bulk of the UTF-8 encoded data (that is
// not already suspicious sequences) should be plain US-ASCII bytes. This, I
// arbitrarily decided, should be 80% (a random distribution, eg binary data, would yield
// approx 40%, so the chances of hitting this threshold by accident in random data are
// VERY low).
 
if ((suspiciousUTF8SequenceCount * 500000.0 / SampleBytes.Length >= 1) //suspicious sequences
&& (
SampleBytes.Length - suspiciousUTF8BytesTotal == 0 //all suspicious, so cannot evaluate proportion of US-Ascii
||
likelyUSASCIIBytesInSample * 1.0 / (SampleBytes.Length - suspiciousUTF8BytesTotal) >= 0.8
)
)
return Encoding.UTF8;
}
 
return null;
}
 
private static bool IsCommonUSASCIIByte(byte testByte)
{
if (testByte == 0x0A //lf
|| testByte == 0x0D //cr
|| testByte == 0x09 //tab
|| (testByte >= 0x20 && testByte <= 0x2F) //common punctuation
|| (testByte >= 0x30 && testByte <= 0x39) //digits
|| (testByte >= 0x3A && testByte <= 0x40) //common punctuation
|| (testByte >= 0x41 && testByte <= 0x5A) //capital letters
|| (testByte >= 0x5B && testByte <= 0x60) //common punctuation
|| (testByte >= 0x61 && testByte <= 0x7A) //lowercase letters
|| (testByte >= 0x7B && testByte <= 0x7E) //common punctuation
)
return true;
else
return false;
}
 
private static int DetectSuspiciousUTF8SequenceLength(byte[] SampleBytes, long currentPos)
{
int lengthFound = 0;
 
if (SampleBytes.Length >= currentPos + 1 && SampleBytes[currentPos] == 0xC2)
{
if (SampleBytes[currentPos + 1] == 0x81 || SampleBytes[currentPos + 1] == 0x8D || SampleBytes[currentPos + 1] == 0x8F)
lengthFound = 2;
else if (SampleBytes[currentPos + 1] == 0x90 || SampleBytes[currentPos + 1] == 0x9D)
lengthFound = 2;
else if (SampleBytes[currentPos + 1] >= 0xA0 && SampleBytes[currentPos + 1] <= 0xBF)
lengthFound = 2;
}
else if (SampleBytes.Length >= currentPos + 1 && SampleBytes[currentPos] == 0xC3)
{
if (SampleBytes[currentPos + 1] >= 0x80 && SampleBytes[currentPos + 1] <= 0xBF)
lengthFound = 2;
}
else if (SampleBytes.Length >= currentPos + 1 && SampleBytes[currentPos] == 0xC5)
{
if (SampleBytes[currentPos + 1] == 0x92 || SampleBytes[currentPos + 1] == 0x93)
lengthFound = 2;
else if (SampleBytes[currentPos + 1] == 0xA0 || SampleBytes[currentPos + 1] == 0xA1)
lengthFound = 2;
else if (SampleBytes[currentPos + 1] == 0xB8 || SampleBytes[currentPos + 1] == 0xBD || SampleBytes[currentPos + 1] == 0xBE)
lengthFound = 2;
}
else if (SampleBytes.Length >= currentPos + 1 && SampleBytes[currentPos] == 0xC6)
{
if (SampleBytes[currentPos + 1] == 0x92)
lengthFound = 2;
}
else if (SampleBytes.Length >= currentPos + 1 && SampleBytes[currentPos] == 0xCB)
{
if (SampleBytes[currentPos + 1] == 0x86 || SampleBytes[currentPos + 1] == 0x9C)
lengthFound = 2;
}
else if (SampleBytes.Length >= currentPos + 2 && SampleBytes[currentPos] == 0xE2)
{
if (SampleBytes[currentPos + 1] == 0x80)
{
if (SampleBytes[currentPos + 2] == 0x93 || SampleBytes[currentPos + 2] == 0x94)
lengthFound = 3;
if (SampleBytes[currentPos + 2] == 0x98 || SampleBytes[currentPos + 2] == 0x99 || SampleBytes[currentPos + 2] == 0x9A)
lengthFound = 3;
if (SampleBytes[currentPos + 2] == 0x9C || SampleBytes[currentPos + 2] == 0x9D || SampleBytes[currentPos + 2] == 0x9E)
lengthFound = 3;
if (SampleBytes[currentPos + 2] == 0xA0 || SampleBytes[currentPos + 2] == 0xA1 || SampleBytes[currentPos + 2] == 0xA2)
lengthFound = 3;
if (SampleBytes[currentPos + 2] == 0xA6)
lengthFound = 3;
if (SampleBytes[currentPos + 2] == 0xB0)
lengthFound = 3;
if (SampleBytes[currentPos + 2] == 0xB9 || SampleBytes[currentPos + 2] == 0xBA)
lengthFound = 3;
}
else if (SampleBytes[currentPos + 1] == 0x82 && SampleBytes[currentPos + 2] == 0xAC)
lengthFound = 3;
else if (SampleBytes[currentPos + 1] == 0x84 && SampleBytes[currentPos + 2] == 0xA2)
lengthFound = 3;
}
 
return lengthFound;
}
 
}
}

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