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https://github.com/evanw/kiwi

A schema-based binary format for efficiently encoding trees of data
https://github.com/evanw/kiwi

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A schema-based binary format for efficiently encoding trees of data

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# Kiwi Message Format



Kiwi is a schema-based binary format for efficiently encoding trees of data.

It's inspired by Google's [Protocol Buffer](https://developers.google.com/protocol-buffers/) format but is simpler, has a more compact encoding, and has better support for optional fields.



Goals:



* **Efficient encoding of common values:** Variable-length encoding is used for numeric values where small values take up less space.

* **Efficient encoding of compound objects:** The `struct` feature supports nested objects with zero encoding overhead.

* **Presence of optional fields is detectable:** This is not possible with Protocol Buffers, especially for repeated fields.

* **Linearly serializable:** Reading and writing are both single-scan operations so they are cache-efficient and have guaranteed time complexity.

* **Backwards compatibility:** New versions of the schema can still read old data.

* **Forwards compatibility:** Old versions of the schema can optionally read new data if a copy of the new schema is bundled with the data (the new schema lets the decoder skip over unknown fields).

* **Simple implementation:** The API is very minimal and the generated C++ code only depends on a single file.



Non-goals:



* **Optimal bit-packing:** Compression can be used after encoding for more space savings if needed.



## Native Types



* **bool:** A value that stores either `true` or `false`. Will use 1 byte.

* **byte:** An unsigned 8-bit integer value. Uses 1 byte, obviously.

* **int:** A 32-bit integer value stored using a variable-length encoding optimized for storing numbers with a small magnitude. Will use at most 5 bytes.

* **uint:** A 32-bit integer value stored using a variable-length encoding optimized for storing small non-negative numbers. Will use at most 5 bytes.

* **float:** A 32-bit floating-point number. Normally uses 4 bytes but a value of zero uses 1 byte ([denormal numbers](https://en.wikipedia.org/wiki/Denormal_number) become zero when encoded).

* **string:** A UTF-8 null-terminated string. Will use at least 1 byte.

* **int64:** A 64-bit integer value stored using a variable-length encoding optimized for storing numbers with a small magnitude. Will use at most 9 bytes.

* **uint64:** A 64-bit integer value stored using a variable-length encoding optimized for storing small non-negative numbers. Will use at most 9 bytes.

* **T[]:** Any type can be made into an array using the `[]` suffix.



## User Types



* **enum:** A `uint` with a restricted set of values that are identified by name. New fields can be added to any message while maintaining backwards compatibility.

* **struct:** A compound value with a fixed set of fields that are always required and written out in order. New fields cannot be added to a struct once that struct is in use.

* **message:** A compound value with optional fields. New fields can be added to any message while maintaining backwards compatibility.



## Example Schema



```proto

enum Type {

  FLAT = 0;

  ROUND = 1;

  POINTED = 2;

}



struct Color {

  byte red;

  byte green;

  byte blue;

  byte alpha;

}



message Example {

  uint clientID = 1;

  Type type = 2;

  Color[] colors = 3;

}

```



## Live Demo



See [http://evanw.github.io/kiwi/](http://evanw.github.io/kiwi/) for a live demo of the schema compiler.



## Usage Examples



Pick a language:



* [JavaScript](./examples/js.md)

* [C++](./examples/cpp.md)

* [Rust](./examples/rust.md)

* [Skew](./examples/skew.md)