Universally Unique Lexicographically Sortable Identifier

Kotlin implementation of ULID with multiplatform support.

UUID can be suboptimal for many use-cases because:

• It isn't the most character efficient way of encoding 128 bits of randomness
• UUID v1/v2 is impractical in many environments, as it requires access to a unique, stable MAC address
• UUID v3/v5 requires a unique seed and produces randomly distributed IDs, which can cause fragmentation in many data structures
• UUID v4 provides no other information than randomness which can cause fragmentation in many data structures

Instead, herein is proposed ULID:

• 128-bit compatibility with UUID
• 1.21e+24 unique ULIDs per millisecond
• Lexicographically sortable!
• Canonically encoded as a 26 character string, as opposed to the 36 character UUID
• Uses Crockford's base32 for better efficiency and readability (5 bits per character)
• Case-insensitive
• No special characters (URL safe)
• Monotonic sort order (correctly detects and handles the same millisecond)

Add the Maven Central repository if it is not already there:

repositories {
    mavenCentral()
}

Add a dependency to the dependencies block:

dependencies {
    implementation "com.aallam.ulid:ulid-kotlin:$version"
}

In multiplatform projects, add a dependency to the commonMain source set dependencies.

• Generating ULID String:

ULID.randomULID()

• Generating ULID instance:

val ulid = ULID.nextULID()
val ulidString = ulid.toString()

• Generating ULID using ULID.Factory

val factory = ULID.Factory()
val ulid = factory.nextULID()
val ulidString = ulid.toString()

The default constructor is using default kotlin.random.Random but you can also use the ULID.Factory(Random) builder function to use a different Random instance.

// generate a ULID String
val ulidString: String = ULID.randomULID()

// generate a ULID instance
val ulid: ULID = ULID.nextULID()

// generate the ByteArray for a ULID instance
val data: ByteArray = ulid.toBytes()

// generate a ULID from given ByteArray using 'fromBytes' function
val ulidFromBytes: ByteArray = ULID.fromBytes(data)

// generate a ULID from given String using 'parseULID' function
val ulidFromString: ULID = ULID.parseULID(ulidString)

// generate a ULID String from ULID instance
val ulidStringFromULID: ULID = ulid.toString()

Specification

// generate ULID instance using a monotonic factory
val ulid: ULID = ULID.Monotonic.nextULID(previousULID)

// using a monotonic factory, generate a ULID instance or null in case of overflow
val ulidStrict: ULID? = ULID.Monotonic.nextULIDStrict(previousULID)

A thread-safe, stateful monotonic generator that internally tracks the previously generated ULID:

val generator = ULID.StatefulMonotonic()

// generate the next monotonic ULID
val ulid: ULID = generator.nextULID()

// generate the next strict monotonic ULID, or null on overflow
val ulidStrict: ULID? = generator.nextULIDStrict()

Below is the current specification of ULID as implemented in this repository.

Timestamp

• 48 bits
• UNIX-time in milliseconds
• Won't run out of space till the year 10889 AD

Entropy

• 80 bits
• User defined entropy source.

Crockford's Base32 is used as shown. This alphabet excludes the letters I, L, O, and U to avoid confusion and abuse.

0123456789ABCDEFGHJKMNPQRSTVWXYZ

The components are encoded as 16 octets. Each component is encoded with the Most Significant Byte first (network byte order).

0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      32_bit_uint_time_high                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|     16_bit_uint_time_low      |       16_bit_uint_random      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       32_bit_uint_random                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       32_bit_uint_random                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

 01AN4Z07BY      79KA1307SR9X4MV3
|----------|    |----------------|
 Timestamp           Entropy
  10 chars           16 chars
   48bits             80bits
   base32             base32

• huxi/sulky

ULID for Kotlin is an open-sourced software licensed under the MIT license.