# Elixir Protocol Buffer 

**Warning:** only protocol buffers 3 is supported. Use [protobuf-elixir](https://github.com/tony612/protobuf-elixir) if you need support for version 2 (protobuf-elixir was a major inspiration for this project).

This is a protocol buffer encoder and decoder. Its goal is to be fast at the cost of larger generated files. This is achieved by generating a significant part of the encoding and decoding logic at generation time with the protoc plugin.

Encoding and decoding performance is ~3-4x times faster than protobuf-elixir. For example, if we take the `%Everything` structure used in our tests, which has all field types, including all array types (with 2 values per array) and a few maps, `pbuf` takes ~14µs to encode and ~24µs to decode, versus 66µs and 67µs. However, the .beam file is quite a bit larger: 19K vs 7K.

(Note that there _is_ limited support for version 2 syntax, but only enough to allow the protoc plugin to bootstrap itself. This may may or may not provide all the version 2 support you need).

## Installation
Assuming you already have protoc installed, you'll want to run:

    $ mix escript.install hex pbuf

to install the pbuf elixir generator. This will place `protoc-gen-fast-elixir` in your `~/.mix/escript/` folder. This must be on your $PATH.

You can then generate elixir files using the `protoc` command with the `-fast-elixir_out=PATH` flag:

    protoc --fast-elixir_out=generated/ myschema.proto 

Note the name `fast-elixir_out`. This allows you to also have [protobuf](https://github.com/tony612/protobuf-elixir) installed in order to support proto2 syntax.

## Encoding
The generated code is normal Elixir modules with a `defstruct`. Use `new/1` to create new instances:

    user = Models.User.new(name: "leto", age: 2000)

And `Pbuf.encode!/1` and `Pbuf.encode_to_iodata!/1` to encode them:

    data = Pbuf.encode!(user)

Only structures generated by `protoc` can be passed to `encode!/1` and `encode_to_iodata!/1`; you cannot pass maps or other structures.

These functions will raise a `Pbuf.Encoder.Error` on invalid data (such as assigning a `float` to a `bool` field). There are currently no non-raising functions.

## Decoding
Decoding is done via `Pbuf.decode!/2`:
    
    user = Pbuf.decode!(Models.User, data)

As an alternative, you can also use: `Models.User.decode!(data)`.

Unlike encoding, there **are** non-raising versions of `decode!`:
  
    # or use Models.user.decode(data)
    case Pbuf.decode(Models.User, data) do
      {:ok, user} -> ...
      {:error, err} -> # err is a %Pbuf.Decode.Error{}
    end

Decoding truly invalid data (as opposed to simply unexpected types) can raise.

### Enumerations
A field declared as an `enum` should be set to the atom representation of the protocol buffer name, or the integer value. For example, a message defined as:

    message User {
      UserType type = 1;
    }

    enum UserType {
      USER_TYPE_UNKNOWN = 0;
      USER_TYPE_PENDING = 1;
      USER_TYPE_NORMAL = 2;
      USER_TYPE_DELETED = 3;
    }

Should be used as:

    user = User.new(type: :USER_TYPE_PENDING)
    # OR
    user = User.new(type: 1)

(casing is preserved from the proto file)

#### Advanced Enums
You'll likely want to map your protocol buffer enums to specific atoms. With a bit of work, the generator can do this for you.

First, you'll need to specify a custom option, say in `options.proto`:

```
syntax = "proto2";

import 'google/protobuf/descriptor.proto';

extend google.protobuf.EnumValueOptions {
  optional ErlangEnumValueOptions erlang = 4369;
}

message ErlangEnumValueOptions {
  optional string atom = 1;
}
```

You can them import this .proto file like any other and use the option:

```
import 'options.proto';

enum HTTPMethod {
  HTTP_METHOD_GET = 0 [(erlang).atom = 'get'];
  HTTP_METHOD_POST = 1 [(erlang).atom = 'post'];
}
```

The value will now be `:get` and `:post` rather than `:HTTP_METHOD_GET` and `:HTTP_METHOD_POST`.

For this to work, Google's proto definitions must be available when you run `protoc`:

```
protoc -I=/usr/local/include/proto/ -I=. ...
```

They are available from the protocol buffer source: [https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protoc-3.6.1-osx-x86_64.zip](https://github.com/protocolbuffers/protobuf/releases/download/v3.6.1/protoc-3.6.1-osx-x86_64.zip).


### Oneofs
The value of a oneof field must be set to a tuple where the first element is the name of the field and the second is the value. Given:

    message Event {
      oneof event_oneof {
        Commit commit = 1;
        Wiki wiki = 2;
      }
    }

Then valid values for `event_oneof` are: nil, `{:commit, Commit.t}` or `{:wiki, Wiki.t}`.


## What's Ugly?
There are two distinctly ugly parts of the code. The first is pretty much anything to do with `oneof` fields. The second is the decoding of maps.