YouTube-like GUID

Is it possible to generate short GUID like in YouTube (N7Et6c9nL9w)?

How can it be done? I want to use it in web app.

You could use Base64:

string base64Guid = Convert.ToBase64String(Guid.NewGuid().ToByteArray());

That generates a string like E1HKfn68Pkms5zsZsvKONw==. Since a GUID is always 128 bits, you can omit the == that you know will always be present at the end and that will give you a 22 character string. This isn't as short as YouTube though.

URL Friendly Solution

As mentioned in the accepted answer, base64 is a good solution but it can cause issues if you want to use the GUID in a URL. This is because + and / are valid base64 characters, but have special meaning in URLs.

Luckily, there are unused characters in base64 that are URL friendly. Here is a more complete answer:

public string ToShortString(Guid guid)
{
	var base64Guid = Convert.ToBase64String(guid.ToByteArray());

	// Replace URL unfriendly characters
	base64Guid = base64Guid.Replace('+', '-').Replace('/', '_');

	// Remove the trailing ==
	return base64Guid.Substring(0, base64Guid.Length - 2);
}

public Guid FromShortString(string str)
{
	str = str.Replace('_', '/').Replace('-', '+');
	var byteArray = Convert.FromBase64String(str + "==");
	return new Guid(byteArray);
}

Usage:

Guid guid = Guid.NewGuid();
string shortStr = ToShortString(guid);
// shortStr will look something like 2LP8GcHr-EC4D__QTizUWw
Guid guid2 = FromShortString(shortStr);
Assert.AreEqual(guid, guid2);

EDIT:

Can we do better? (Theoretical limit)

The above yields a 22 character, URL friendly GUID. This is because a GUID uses 128 bits, so representing it in base64 requires characters, which is 21.33, which rounds up to 22.

There are actually 66 URL friendly characters (we aren't using . and ~). So theoretically, we could use base66 to get characters, which is 21.17, which also rounds up to 22.

So this is optimal for a full, valid GUID.

However, GUID uses 6 bits to indicate the version and variant, which in our case are constant. So we technically only need 122 bits, which in both bases rounds to 21 ( = 20.33). So with more manipulation, we could remove another character. This requires wrangling the bits out however, so I leave this as an exercise to the reader.

How does youtube do it?

YouTube IDs use 11 characters. How do they do it?

A GUID uses 122 bits, which guarantees collisions are virtually impossible. This means you can generate a random GUID and be certain it is unique without checking. However, we don't need so many bits for just a regular ID.

We could use a smaller ID. If we use 66 bits or less, we have a higher risk of collision, but can represent this ID with 11 characters (even in base64). One could either accept the risk of collision, or test for a collision and regenerate.

With 122 bits (regular GUID), you would have to generate ~ GUIDs to have a 1% chance of collision.

With 66 bits, you would have to generate ~ or 1 billion IDs to have a 1% chance of collision. That is not that many IDs.

My guess is youtube uses 64 bits (which is more memory friendly than 66 bits), and checks for collisions to regenerate the ID if necessary.

If you want to abandon GUIDs in favor of smaller IDs, here is code for that:

class IdFactory
{
	private Random random = new Random();
	public int CharacterCount { get; }
	public IdFactory(int characterCount)
	{
		CharacterCount = characterCount;
	}

	public string Generate()
	{
		// bitCount = characterCount * log (targetBase) / log(2)
		var bitCount = 6 * CharacterCount;
		var byteCount = (int)Math.Ceiling(bitCount / 8f);
		byte[] buffer = new byte[byteCount];
		random.NextBytes(buffer);

		string guid = Convert.ToBase64String(buffer);
		// Replace URL unfriendly characters
		guid = guid.Replace('+', '-').Replace('/', '_');
		// Trim characters to fit the count
		return guid.Substring(0, CharacterCount);
	}
}

Usage:

var factory = new IdFactory(characterCount: 11);
string guid = factory.Generate();
// guid will look like Mh3darwiZhp

This uses 64 characters which is not optimal, but requires much less code (since we can reuse Convert.ToBase64String). You should be a lot more careful of collisions if you use this.

9 chars is not a GUID. Given that, you could use the hexadecimal representation of an int, which gives you a 8 char string.

You can use an id you might already have. Also you can use .GetHashCode against different simple types and there you have a different int. You can also xor different fields. And if you are into it, you might even use a Random number - hey, you have well above 2.000.000.000+ possible values if you stick to the positives ;)

> It's not a GUID

Let me jump in with the following

It uses the TotalMilliseconds from EPOCH and a valid set of characters. > This will not be globally unique but unique to the instance where it's defines

public string YoutubeLikeId()
{
    Thread.Sleep(1);//make everything unique while looping
	long ticks = (long)(DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1,0,0,0,0))).TotalMilliseconds;//EPOCH
	char[] baseChars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz".ToCharArray();
    
    int i = 32;
    char[] buffer = new char[i];
    int targetBase= baseChars.Length;

    do
    {
        buffer[--i] = baseChars[ticks % targetBase];
        ticks = ticks / targetBase;
    }
    while (ticks > 0);

    char[] result = new char[32 - i];
    Array.Copy(buffer, i, result, 0, 32 - i);

    return new string(result);
}

The output will come something like

XOTgBsu
XOTgBtB
XOTgBtR
XOTgBtg
XOTgBtw
XOTgBuE

Update: The same can be achieved from Guid as

var guid = Guid.NewGuid(); 
guid.ToString("N");
guid.ToString("N").Substring(0,8);
guid.ToString("N").Substring(8,4);
guid.ToString("N").Substring(12,4);
guid.ToString("N").Substring(16,4);
guid.ToString("N").Substring(20,12);

For a Guid ecd65132-ab5a-4587-87b8-b875e2fe0f35 it will break it down in chunks as ecd65132 ,ab5a , 4587,87b8,b875e2fe0f35 > but i can't guarantee it to be unique always.

Update 2: There is also a project called ShortGuid to get a url friendly GUID it can be converted fron/to a regular Guid

It works by encoding the Guid to Base64 as the code below

public static string Encode(Guid guid)
{
    string encoded = Convert.ToBase64String(guid.ToByteArray());

    encoded = encoded
        .Replace("/", "_")
        .Replace("+", "-");
    return encoded.Substring(0, 22);
}

The good thing about it it can be decoded again to get the Guid back with

public static Guid Decode(string value)
{
    // avoid parsing larger strings/blobs
    if (value.Length != 22)
    {
        throw new ArgumentException("A ShortGuid must be exactly 22 characters long. Receive a character string.");
    }

    string base64 = value
        .Replace("_", "/")
        .Replace("-", "+") + "==";

    byte[] blob = Convert.FromBase64String(base64);
    var guid = new Guid(blob);

    var sanityCheck = Encode(guid);
    if (sanityCheck != value)
    {
        throw new FormatException(
            @"Invalid strict ShortGuid encoded string. The string '{value}' is valid URL-safe Base64, " +
            @"but failed a round-trip test expecting '{sanityCheck}'."
        );
    }

    return guid;
}

So a Guid 4039124b-6153-4721-84dc-f56f5b057ac2 will be encoded as SxI5QFNhIUeE3PVvWwV6wg and the Output will look something like.

ANf-MxRHHky2TptaXBxcwA
zpjp-stmVE6ZCbOjbeyzew
jk7P-XYFokmqgGguk_530A
81t6YZtkikGfLglibYkDhQ
qiM2GmqCK0e8wQvOSn-zLA

As others have mentioned, YouTube's VideoId is not technically a GUID since it's not inherently unique.

As per Wikipedia:

> The total number of unique keys is 2¹²⁸ or 3.4×10³⁸. This number is so > large that the probability of the same number being generated randomly > twice is negligible.

The uniqueness YouTube's VideoId is maintained by their generator algorithm.

You can either write your own algorithm, or you can use some sort of random string generator and utilize the UNIQUE CONSTRAINT constraint in SQL to enforce its uniqueness.

First, create a UNIQUE CONSTRAINT in your database:

ALTER TABLE MyTable
ADD CONSTRAINT UniqueUrlId
UNIQUE (UrlId);

Then, for example, generate a random string (from philipproplesch's answer):

string shortUrl = System.Web.Security.Membership.GeneratePassword(11, 0);

If the generated UrlId is sufficiently random and sufficiently long you should rarely encounter the exception that is thrown when SQL encounters a duplicate UrlId. In such an event, you can easily handle the exception in your web app.

Technically it's not a Guid. Youtube has a simple randomized string generator that you can probably whip up in a few minutes using an array of allowed characters and a random number generator.

It might be not the best solution, but you can do something like that:

string shortUrl = System.Web.Security.Membership.GeneratePassword(11, 0);

This id is probably not globally unique. GUID's should be globally unique as they include elements which should not occur elsewhere (the MAC address of the machine generating the ID, the time the ID was generated, etc.)

If what you need is an ID that is unique within your application, use a number fountain - perhaps encoding the value as a hexadecimal number. Every time you need an id, grab it from the number fountain.

If you have multiple servers allocating id's, you could grab a range of numbers (a few tens or thousands depending on how quickly you're allocating ids) and that should do the job. an 8 digit hex number will give you 4 billion ids - but your first id's will be a lot shorter.