What is the most efficient way to encode an arbitrary GUID into readable ASCII (33-127)?
This is an old question, but I had to solve it in order for a system I was working on to be backward compatible.
The exact requirement was for a client-generated identifier that would be written to the database and stored in a 20-character unique column. It never got shown to the user and was not indexed in any way.
Since I couldn't eliminate the requirement, I really wanted to use a Guid (which is statistically unique) and if I could encode it losslessly into 20 characters, then it would be a good solution given the constraints.
Ascii-85 allows you to encode 4 bytes of binary data into 5 bytes of Ascii data. So a 16 byte guid will just fit into 20 Ascii characters using this encoding scheme. A Guid can have 3.1962657931507848761677563491821e+38 discrete values whereas 20 characters of Ascii-85 can have 3.8759531084514355873123178482056e+38 discrete values.
When writing to the database I had some concerns about truncation so no whitespace characters are included in the encoding. I also had issues with collation, which I addressed by excluding lowercase characters from the encoding. Also, it would only ever be passed through a paramaterized command, so any special SQL characters would be escaped automatically.
I've included the C# code to perform Ascii-85 encoding and decoding in case it helps anyone out there. Obviously, depending on your usage you might need to choose a different character set as my constraints made me choose some unusual characters like 'ß' and 'Ø' - but that's the easy part:
/// <summary>
/// This code implements an encoding scheme that uses 85 printable ascii characters
/// to encode the same volume of information as contained in a Guid.
///
/// Ascii-85 can represent 4 binary bytes as 5 Ascii bytes. So a 16 byte Guid can be
/// represented in 20 Ascii bytes. A Guid can have
/// 3.1962657931507848761677563491821e+38 discrete values whereas 20 characters of
/// Ascii-85 can have 3.8759531084514355873123178482056e+38 discrete values.
///
/// Lower-case characters are not included in this encoding to avoid collation
/// issues.
/// This is a departure from standard Ascii-85 which does include lower case
/// characters.
/// In addition, no whitespace characters are included as these may be truncated in
/// the database depending on the storage mechanism - ie VARCHAR vs CHAR.
/// </summary>
internal static class Ascii85
{
/// <summary>
/// 85 printable ascii characters with no lower case ones, so database
/// collation can't bite us. No ' ' character either so database can't
/// truncate it!
/// Unfortunately, these limitation mean resorting to some strange
/// characters like 'Æ' but we won't ever have to type these, so it's ok.
/// </summary>
private static readonly char[] kEncodeMap = new[]
{
'0','1','2','3','4','5','6','7','8','9', // 10
'A','B','C','D','E','F','G','H','I','J', // 20
'K','L','M','N','O','P','Q','R','S','T', // 30
'U','V','W','X','Y','Z','|','}','~','{', // 40
'!','"','#','$','%','&','\'','(',')','`', // 50
'*','+',',','-','.','/','[','\\',']','^', // 60
':',';','<','=','>','?','@','_','¼','½', // 70
'¾','ß','Ç','Ð','€','«','»','¿','•','Ø', // 80
'£','†','‡','§','¥' // 85
};
/// <summary>
/// A reverse mapping of the <see cref="kEncodeMap"/> array for decoding
/// purposes.
/// </summary>
private static readonly IDictionary<char, byte> kDecodeMap;
/// <summary>
/// Initialises the <see cref="kDecodeMap"/>.
/// </summary>
static Ascii85()
{
kDecodeMap = new Dictionary<char, byte>();
for (byte i = 0; i < kEncodeMap.Length; i++)
{
kDecodeMap.Add(kEncodeMap[i], i);
}
}
/// <summary>
/// Decodes an Ascii-85 encoded Guid.
/// </summary>
/// <param name="ascii85Encoding">The Guid encoded using Ascii-85.</param>
/// <returns>A Guid decoded from the parameter.</returns>
public static Guid Decode(string ascii85Encoding)
{
// Ascii-85 can encode 4 bytes of binary data into 5 bytes of Ascii.
// Since a Guid is 16 bytes long, the Ascii-85 encoding should be 20
// characters long.
if(ascii85Encoding.Length != 20)
{
throw new ArgumentException(
"An encoded Guid should be 20 characters long.",
"ascii85Encoding");
}
// We only support upper case characters.
ascii85Encoding = ascii85Encoding.ToUpper();
// Split the string in half and decode each substring separately.
var higher = ascii85Encoding.Substring(0, 10).AsciiDecode();
var lower = ascii85Encoding.Substring(10, 10).AsciiDecode();
// Convert the decoded substrings into an array of 16-bytes.
var byteArray = new[]
{
(byte)((higher & 0xFF00000000000000) >> 56),
(byte)((higher & 0x00FF000000000000) >> 48),
(byte)((higher & 0x0000FF0000000000) >> 40),
(byte)((higher & 0x000000FF00000000) >> 32),
(byte)((higher & 0x00000000FF000000) >> 24),
(byte)((higher & 0x0000000000FF0000) >> 16),
(byte)((higher & 0x000000000000FF00) >> 8),
(byte)((higher & 0x00000000000000FF)),
(byte)((lower & 0xFF00000000000000) >> 56),
(byte)((lower & 0x00FF000000000000) >> 48),
(byte)((lower & 0x0000FF0000000000) >> 40),
(byte)((lower & 0x000000FF00000000) >> 32),
(byte)((lower & 0x00000000FF000000) >> 24),
(byte)((lower & 0x0000000000FF0000) >> 16),
(byte)((lower & 0x000000000000FF00) >> 8),
(byte)((lower & 0x00000000000000FF)),
};
return new Guid(byteArray);
}
/// <summary>
/// Encodes binary data into a plaintext Ascii-85 format string.
/// </summary>
/// <param name="guid">The Guid to encode.</param>
/// <returns>Ascii-85 encoded string</returns>
public static string Encode(Guid guid)
{
// Convert the 128-bit Guid into two 64-bit parts.
var byteArray = guid.ToByteArray();
var higher =
((UInt64)byteArray[0] << 56) | ((UInt64)byteArray[1] << 48) |
((UInt64)byteArray[2] << 40) | ((UInt64)byteArray[3] << 32) |
((UInt64)byteArray[4] << 24) | ((UInt64)byteArray[5] << 16) |
((UInt64)byteArray[6] << 8) | byteArray[7];
var lower =
((UInt64)byteArray[ 8] << 56) | ((UInt64)byteArray[ 9] << 48) |
((UInt64)byteArray[10] << 40) | ((UInt64)byteArray[11] << 32) |
((UInt64)byteArray[12] << 24) | ((UInt64)byteArray[13] << 16) |
((UInt64)byteArray[14] << 8) | byteArray[15];
var encodedStringBuilder = new StringBuilder();
// Encode each part into an ascii-85 encoded string.
encodedStringBuilder.AsciiEncode(higher);
encodedStringBuilder.AsciiEncode(lower);
return encodedStringBuilder.ToString();
}
/// <summary>
/// Encodes the given integer using Ascii-85.
/// </summary>
/// <param name="encodedStringBuilder">The <see cref="StringBuilder"/> to
/// append the results to.</param>
/// <param name="part">The integer to encode.</param>
private static void AsciiEncode(
this StringBuilder encodedStringBuilder, UInt64 part)
{
// Nb, the most significant digits in our encoded character will
// be the right-most characters.
var charCount = (UInt32)kEncodeMap.Length;
// Ascii-85 can encode 4 bytes of binary data into 5 bytes of Ascii.
// Since a UInt64 is 8 bytes long, the Ascii-85 encoding should be
// 10 characters long.
for (var i = 0; i < 10; i++)
{
// Get the remainder when dividing by the base.
var remainder = part % charCount;
// Divide by the base.
part /= charCount;
// Add the appropriate character for the current value (0-84).
encodedStringBuilder.Append(kEncodeMap[remainder]);
}
}
/// <summary>
/// Decodes the given string from Ascii-85 to an integer.
/// </summary>
/// <param name="ascii85EncodedString">Decodes a 10 character Ascii-85
/// encoded string.</param>
/// <returns>The integer representation of the parameter.</returns>
private static UInt64 AsciiDecode(this string ascii85EncodedString)
{
if (ascii85EncodedString.Length != 10)
{
throw new ArgumentException(
"An Ascii-85 encoded Uint64 should be 10 characters long.",
"ascii85EncodedString");
}
// Nb, the most significant digits in our encoded character
// will be the right-most characters.
var charCount = (UInt32)kEncodeMap.Length;
UInt64 result = 0;
// Starting with the right-most (most-significant) character,
// iterate through the encoded string and decode.
for (var i = ascii85EncodedString.Length - 1; i >= 0; i--)
{
// Multiply the current decoded value by the base.
result *= charCount;
// Add the integer value for that encoded character.
result += kDecodeMap[ascii85EncodedString[i]];
}
return result;
}
}
Also, here are the unit tests. They aren't as thorough as I'd like, and I don't like the non-determinism of where Guid.NewGuid() is used, but they should get you started:
/// <summary>
/// Tests to verify that the Ascii-85 encoding is functioning as expected.
/// </summary>
[TestClass]
[UsedImplicitly]
public class Ascii85Tests
{
[TestMethod]
[Description("Ensure that the Ascii-85 encoding is correct.")]
[UsedImplicitly]
public void CanEncodeAndDecodeAGuidUsingAscii85()
{
var guidStrings = new[]
{
"00000000-0000-0000-0000-000000000000",
"00000000-0000-0000-0000-0000000000FF",
"00000000-0000-0000-0000-00000000FF00",
"00000000-0000-0000-0000-000000FF0000",
"00000000-0000-0000-0000-0000FF000000",
"00000000-0000-0000-0000-00FF00000000",
"00000000-0000-0000-0000-FF0000000000",
"00000000-0000-0000-00FF-000000000000",
"00000000-0000-0000-FF00-000000000000",
"00000000-0000-00FF-0000-000000000000",
"00000000-0000-FF00-0000-000000000000",
"00000000-00FF-0000-0000-000000000000",
"00000000-FF00-0000-0000-000000000000",
"000000FF-0000-0000-0000-000000000000",
"0000FF00-0000-0000-0000-000000000000",
"00FF0000-0000-0000-0000-000000000000",
"FF000000-0000-0000-0000-000000000000",
"FF000000-0000-0000-0000-00000000FFFF",
"00000000-0000-0000-0000-0000FFFF0000",
"00000000-0000-0000-0000-FFFF00000000",
"00000000-0000-0000-FFFF-000000000000",
"00000000-0000-FFFF-0000-000000000000",
"00000000-FFFF-0000-0000-000000000000",
"0000FFFF-0000-0000-0000-000000000000",
"FFFF0000-0000-0000-0000-000000000000",
"00000000-0000-0000-0000-0000FFFFFFFF",
"00000000-0000-0000-FFFF-FFFF00000000",
"00000000-FFFF-FFFF-0000-000000000000",
"FFFFFFFF-0000-0000-0000-000000000000",
"00000000-0000-0000-FFFF-FFFFFFFFFFFF",
"FFFFFFFF-FFFF-FFFF-0000-000000000000",
"FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF",
"1000000F-100F-100F-100F-10000000000F"
};
foreach (var guidString in guidStrings)
{
var guid = new Guid(guidString);
var encoded = Ascii85.Encode(guid);
Assert.AreEqual(
20,
encoded.Length,
"A guid encoding should not exceed 20 characters.");
var decoded = Ascii85.Decode(encoded);
Assert.AreEqual(
guid,
decoded,
"The guids are different after being encoded and decoded.");
}
}
[TestMethod]
[Description(
"The Ascii-85 encoding is not susceptible to changes in character case.")]
[UsedImplicitly]
public void Ascii85IsCaseInsensitive()
{
const int kCount = 50;
for (var i = 0; i < kCount; i++)
{
var guid = Guid.NewGuid();
// The encoding should be all upper case. A reliance
// on mixed case will make the generated string
// vulnerable to sql collation.
var encoded = Ascii85.Encode(guid);
Assert.AreEqual(
encoded,
encoded.ToUpper(),
"The Ascii-85 encoding should produce only uppercase characters.");
}
}
}
I hope this saves somebody some trouble.
Also, if you find any bugs then let me know ;-)
Use Base 85. See section 4.1. Why 85? of A Compact Representation of IPv6 Addresses
An IPv6 address, like a GUID is made up of eight 16-bit pieces.
You have 95 characters available -- so, more than 6 bits, but not quite as many as 7 (about 6.57 actually). You could use 128/log2(95) = about 19.48 characters, to encode into 20 characters. If saving 2 characters in the encoded form is worth the loss of readability to you, something like (pseudocode):
char encoded[21];
long long guid; // 128 bits number
for(int i=0; i<20; ++i) {
encoded[i] = chr(guid % 95 + 33);
guid /= 95;
}
encoded[20] = chr(0);
which is basically the generic "encode a number in some base" code, except that there's no need to reverse the "digits" since the order's arbitrary anyway (and little-endian is more direct and natural). To get back the guid from the encoded string is, in a very similar way, the polynomial computation in base 95 (after subtracting 33 from each digit of course):
guid = 0;
for(int i=0; i<20; ++i) {
guid *= 95;
guid += ord(encoded[i]) - 33;
}
essentially using Horner's approach to polynomial evaluation.