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Shorter ULID

A fork from oklog/ulid, but shorter.

Differences

The only difference is that the random field of SULID is shorter.

I cut the last 32 bits of the random field, which will save 6 bytes of memory space. (The length of the ULID is 26 and the length of the SULID is 20.)
I think that a collision probability of 1/(2^48) per millisecond can still cover most scenarios.

Binary Layout and Byte Order of ULID

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

Binary Layout and Byte Order of SULID

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

Install

This package requires Go modules.

go get -u github.com/cyberxnomad/sulid

Specification

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

Components

Timestamp

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

Entropy

  • 48 bits
  • User defined entropy source.

Encoding

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

0123456789ABCDEFGHJKMNPQRSTVWXYZ

String Representation

 0000XSNJG0      55WMAVS5Z8
|----------|    |----------|
 Timestamp        Entropy
  10 chars        10 chars
   48bits          48bits
   base32          base32

Test

go test ./...