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+# Globally Unique ID Generator
+
+[![godoc](http://img.shields.io/badge/godoc-reference-blue.svg?style=flat)](https://godoc.org/github.com/rs/xid) [![license](http://img.shields.io/badge/license-MIT-red.svg?style=flat)](https://raw.githubusercontent.com/rs/xid/master/LICENSE) [![Build Status](https://travis-ci.org/rs/xid.svg?branch=master)](https://travis-ci.org/rs/xid) [![Coverage](http://gocover.io/_badge/github.com/rs/xid)](http://gocover.io/github.com/rs/xid)
+
+Package xid is a globally unique id generator library, ready to safely be used directly in your server code.
+
+Xid uses the Mongo Object ID algorithm to generate globally unique ids with a different serialization (base64) to make it shorter when transported as a string:
+https://docs.mongodb.org/manual/reference/object-id/
+
+- 4-byte value representing the seconds since the Unix epoch,
+- 3-byte machine identifier,
+- 2-byte process id, and
+- 3-byte counter, starting with a random value.
+
+The binary representation of the id is compatible with Mongo 12 bytes Object IDs.
+The string representation is using base32 hex (w/o padding) for better space efficiency
+when stored in that form (20 bytes). The hex variant of base32 is used to retain the
+sortable property of the id.
+
+Xid doesn't use base64 because case sensitivity and the 2 non alphanum chars may be an
+issue when transported as a string between various systems. Base36 wasn't retained either
+because 1/ it's not standard 2/ the resulting size is not predictable (not bit aligned)
+and 3/ it would not remain sortable. To validate a base32 `xid`, expect a 20 chars long,
+all lowercase sequence of `a` to `v` letters and `0` to `9` numbers (`[0-9a-v]{20}`).
+
+UUIDs are 16 bytes (128 bits) and 36 chars as string representation. Twitter Snowflake
+ids are 8 bytes (64 bits) but require machine/data-center configuration and/or central
+generator servers. xid stands in between with 12 bytes (96 bits) and a more compact
+URL-safe string representation (20 chars). No configuration or central generator server
+is required so it can be used directly in server's code.
+
+| Name        | Binary Size | String Size    | Features
+|-------------|-------------|----------------|----------------
+| [UUID]      | 16 bytes    | 36 chars       | configuration free, not sortable
+| [shortuuid] | 16 bytes    | 22 chars       | configuration free, not sortable
+| [Snowflake] | 8 bytes     | up to 20 chars | needs machine/DC configuration, needs central server, sortable
+| [MongoID]   | 12 bytes    | 24 chars       | configuration free, sortable
+| xid         | 12 bytes    | 20 chars       | configuration free, sortable
+
+[UUID]: https://en.wikipedia.org/wiki/Universally_unique_identifier
+[shortuuid]: https://github.com/stochastic-technologies/shortuuid
+[Snowflake]: https://blog.twitter.com/2010/announcing-snowflake
+[MongoID]: https://docs.mongodb.org/manual/reference/object-id/
+
+Features:
+
+- Size: 12 bytes (96 bits), smaller than UUID, larger than snowflake
+- Base32 hex encoded by default (20 chars when transported as printable string, still sortable)
+- Non configured, you don't need set a unique machine and/or data center id
+- K-ordered
+- Embedded time with 1 second precision
+- Unicity guaranteed for 16,777,216 (24 bits) unique ids per second and per host/process
+- Lock-free (i.e.: unlike UUIDv1 and v2)
+
+Best used with [zerolog](https://github.com/rs/zerolog)'s
+[RequestIDHandler](https://godoc.org/github.com/rs/zerolog/hlog#RequestIDHandler).
+
+Notes:
+
+- Xid is dependent on the system time, a monotonic counter and so is not cryptographically secure. If unpredictability of IDs is important, you should not use Xids. It is worth noting that most other UUID-like implementations are also not cryptographically secure. You should use libraries that rely on cryptographically secure sources (like /dev/urandom on unix, crypto/rand in golang), if you want a truly random ID generator.
+
+References:
+
+- http://www.slideshare.net/davegardnerisme/unique-id-generation-in-distributed-systems
+- https://en.wikipedia.org/wiki/Universally_unique_identifier
+- https://blog.twitter.com/2010/announcing-snowflake
+- Python port by [Graham Abbott](https://github.com/graham): https://github.com/graham/python_xid
+- Scala port by [Egor Kolotaev](https://github.com/kolotaev): https://github.com/kolotaev/ride
+- Rust port by [Jérôme Renard](https://github.com/jeromer/): https://github.com/jeromer/libxid
+- Ruby port by [Valar](https://github.com/valarpirai/): https://github.com/valarpirai/ruby_xid
+- Java port by [0xShamil](https://github.com/0xShamil/): https://github.com/0xShamil/java-xid
+- Dart port by [Peter Bwire](https://github.com/pitabwire): https://pub.dev/packages/xid
+- PostgreSQL port by [Rasmus Holm](https://github.com/crholm): https://github.com/modfin/pg-xid
+- Swift port by [Uditha Atukorala](https://github.com/uditha-atukorala): https://github.com/uditha-atukorala/swift-xid
+- C++ port by [Uditha Atukorala](https://github.com/uditha-atukorala): https://github.com/uditha-atukorala/libxid
+
+## Install
+
+    go get github.com/rs/xid
+
+## Usage
+
+```go
+guid := xid.New()
+
+println(guid.String())
+// Output: 9m4e2mr0ui3e8a215n4g
+```
+
+Get `xid` embedded info:
+
+```go
+guid.Machine()
+guid.Pid()
+guid.Time()
+guid.Counter()
+```
+
+## Benchmark
+
+Benchmark against Go [Maxim Bublis](https://github.com/satori)'s [UUID](https://github.com/satori/go.uuid).
+
+```
+BenchmarkXID            20000000                91.1 ns/op            32 B/op          1 allocs/op
+BenchmarkXID-2          20000000                55.9 ns/op            32 B/op          1 allocs/op
+BenchmarkXID-4          50000000                32.3 ns/op            32 B/op          1 allocs/op
+BenchmarkUUIDv1         10000000               204 ns/op              48 B/op          1 allocs/op
+BenchmarkUUIDv1-2       10000000               160 ns/op              48 B/op          1 allocs/op
+BenchmarkUUIDv1-4       10000000               195 ns/op              48 B/op          1 allocs/op
+BenchmarkUUIDv4          1000000              1503 ns/op              64 B/op          2 allocs/op
+BenchmarkUUIDv4-2        1000000              1427 ns/op              64 B/op          2 allocs/op
+BenchmarkUUIDv4-4        1000000              1452 ns/op              64 B/op          2 allocs/op
+```
+
+Note: UUIDv1 requires a global lock, hence the performance degradation as we add more CPUs.
+
+## Licenses
+
+All source code is licensed under the [MIT License](https://raw.github.com/rs/xid/master/LICENSE).