# Protox
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Protox is an Elixir library to work with Google's Protocol Buffers (version 2 and 3).
# Prerequisites
Protox uses Google's `protoc` (>= 3.0) to parse `.proto` files. It must be available in `$PATH`.
You can get it [here](https://github.com/google/protobuf).
# Usage
From files:
```elixir
defmodule Foo do
@external_resource "./defs/foo.proto"
@external_resource "./defs/bar.proto"
@external_resource "./defs/baz/fiz.proto"
use Protox, files: [
"./defs/foo.proto",
"./defs/bar.proto",
"./defs/baz/fiz.proto",
]
end
```
From a textual description:
```elixir
defmodule Bar do
use Protox, schema: """
syntax = "proto3";
package fiz;
message Baz {
}
message Foo {
int32 a = 1;
map<int32, Baz> b = 2;
}
"""
end
```
The previous example will generate two modules: `Fiz.Baz` and `Fiz.Foo`.
It's possible to prepend a namespace to all generated modules:
```elixir
defmodule Bar do
use Protox, schema: """
syntax = "proto3";
enum Enum {
FOO = 0;
BAR = 1;
}
""",
namespace: Namespace
end
```
In this case, the module `Namespace.Enum` will be generated.
Here's how to create a new message:
```elixir
iex> %Fiz.Foo{a: 3, b: %{1 => %Fiz.Baz{}}} |> Protox.Encode.encode()
[[[], "\b", <<3>>], <<18>>, <<4>>, "\b", <<1>>, <<18>>, <<0>>]
```
Note that `Protox.Encode.encode/1` creates an iolist, not a binary. Such iolists can be used
directly with files or sockets read/write operations.
However, you can use `:binary.list_to_bin/1` to get a binary:
```elixir
iex> %Fiz.Foo{a: 3, b: %{1 => %Fiz.Baz{}}} |> Protox.Encode.encode() |> :binary.list_to_bin()
<<8, 3, 18, 4, 8, 1, 18, 0>>
```
Finally, here's how to decode:
```elixir
iex> <<8, 3, 18, 4, 8, 1, 18, 0>> |> Fiz.Foo.decode()
{:ok, %Fiz.Foo{a: 3, b: %{1 => %Fiz.Baz{}}}}
```
# Unknown fields
If any unknown fields are encountered when decoding, they are kept in the decoded message.
It's possible to access them with the function `unknown_fields/1` defined with the message.
```elixir
iex> msg = <<8, 42, 42, 4, 121, 97, 121, 101, 136, 241, 4, 83>> |> Msg.decode!()
%Msg{a: 42, b: "", z: -42, __unknown_fields__: [{5, 2, <<121, 97, 121, 101>>}]}
iex> Msg.unknown_fields(msg)
[{5, 2, <<121, 97, 121, 101>>}]
```
You should always use `unknown_fields/1` as the name of the struct field
(e.g. `__unknown_fields__`) is generated at compile-time to avoid collision with the actual
fields of the protobuf message.
It returns a list of tuples `{tag, wire_type, bytes}`.
# Unsupported features
* protobuf 3 JSON mapping
* groups
* rpc
Furthermore, all options other than `packed` and `default` are ignored.
# Implementation choices
* Required fields (protobuf 2): an error is raised when decoding a message with a missing required
field.
* When decoding enum aliases, the last encountered constant will be used.
For instance, in the following example, `:BAR` will always be used if the value `1` is read
on the wire.
```
enum E {
option allow_alias = true;
FOO = 0;
BAZ = 1;
BAR = 1;
}
```
* Unset optionals
* For protobuf 2, unset optional fields are mapped to `nil`
* For protobuf 3, unset optional fields are mapped to their default values, as mandated by
the protobuf spec
# Types mapping
Protobuf | Elixir
-----------|--------------
int32 | integer()
int64 | integer()
uint32 | integer()
uint64 | integer()
sint32 | integer()
sint64 | integer()
fixed32 | integer()
fixed64 | integer()
sfixed32 | integer()
sfixed64 | integer()
float | float()
double | float()
bool | boolean()
string | String.t
bytes | binary()
map | %{}
oneof | {:field, value}
enum | atom()
message | struct()
# Performance
TODO. Do some benchmarks.
# Conformance
This library has been tested using the conformance checker provided by Google.
Note that only the protobuf part is tested: as protox doesn't support JSON
output, the corresponding tests are skipped.
Here's how to launch the conformance test:
* Get conformance-test-runner (https://github.com/google/protobuf/tree/master/conformance)
* `mix protox.conformance --runner=/path/to/conformance-test-runner`
A report will be generated in a file named `conformance_report.txt`.
If everything's fine, something like the following should be displayed:
```
CONFORMANCE TEST BEGIN ====================================
CONFORMANCE SUITE PASSED: 149 successes, 384 skipped, 0 expected failures, 0 unexpected failures.
```
# Credits
Both [gpb](https://github.com/tomas-abrahamsson/gpb) and
[exprotobuf](https://github.com/bitwalker/exprotobuf) were very useful in
understanding how to implement Protocol Buffers.