# roto

Zero-allocation Rust protobuf reader and writer.

## Overview

Instead of deserializing binary protobuf data into Rust structs, roto scans a message _once_ on
construction — recording the byte offset of each field — then reads fields on demand directly from
the original bytes. No heap allocation, no data copying, no full deserialization upfront.

Writing works the same way: you provide a fixed buffer and a builder writes fields directly into it,
returning a slice of the bytes written.

## Design

`protoc` generates a `CodeGeneratorRequest` message; `protoc-gen-roto` (in
`src/bin/protoc-gen-roto.rs`) reads this from stdin, generates Rust source files, and writes a
`CodeGeneratorResponse` to stdout. `protoc` then writes those `.rs` files to disk. The generated
files are included directly in the crate that uses the protobuffers.

## Generated code

For each protobuf message roto generates two types:

- **Reader struct** `MessageName<'a>` — borrows the original byte slice, zero-copy.
- **Builder struct** `MessageNameBuilder<'b>` — writes into a caller-provided `&mut [u8]`.

Nested message types are placed in a `pub mod message_name { ... }` module (snake_case of the
parent message name) within the same generated file.

## Sample usage

Given this proto definition:

```proto
message Hello {
    string hello_world = 1;
    message InnerWorld {
        string thought = 1;
    }
    InnerWorld inner_world = 2;
}
```

### Reading

```rust
fn parse_proto(data: &[u8]) -> roto::Result<String> {
    // Scan the data once, recording field offsets
    let hello = Hello::new(data)?;

    // String fields return &str borrowed from the original bytes (zero-copy)
    let hello_world: &str = hello.hello_world()?;

    // Nested message fields return &[u8]; construct the nested reader from those bytes
    let inner_bytes: &[u8] = hello.inner_world()?;
    let inner_world = hello::InnerWorld::new(inner_bytes)?;
    let thought: &str = inner_world.thought()?;

    Ok(format!("{} is about {}", hello_world, thought))
}
```

Fields absent from the binary data return `Err(roto::RotoError::FieldNotFound)`.

### Writing

Nested messages must be serialized into a scratch buffer first, then embedded as raw bytes in the
outer builder.

```rust
fn build_proto(buf: &mut [u8]) -> roto::Result<&[u8]> {
    // Serialize the inner message first
    let mut inner_buf = [0u8; 256];
    let inner_bytes = hello::InnerWorldBuilder::builder(&mut inner_buf)
        .thought("some thought")?
        .finish()?;

    // Build the outer message, embedding the serialized inner bytes
    HelloBuilder::builder(buf)
        .hello_world("some world")?
        .inner_world(inner_bytes)?
        .finish() // returns Result<&'b mut [u8]> — the written portion of buf
}
```

Builder methods consume `self` and return `Result<Self>`, enabling `?`-based chaining.
`finish()` returns `Result<&'b mut [u8]>` — a slice of the portion of the buffer that was written.

### Repeated fields

Repeated fields return a `RepeatedFieldIterator<'a>`. Each item yields `Result<(&[u8], WireType)>`.

```rust
let hello = Hello::new(data)?;
for item in hello.tags() {
    let (value_bytes, _wire_type) = item?;
    // decode value_bytes according to the expected wire type
}
```

## Runtime API

The core runtime in `src/lib.rs` provides:

- `ProtoAccessor<'a>` — scans a message's fields and reads values at recorded offsets.
- `ProtoBuilder<'a>` — writes fields into a provided `&mut [u8]` buffer.
- `FieldIterator<'a>` / `RepeatedFieldIterator<'a>` — iterators over fields and repeated fields.
- `Tag`, `WireType` — protobuf encoding primitives.
- `read_varint`, `write_varint`, `skip_value` — low-level wire-format helpers.
- `RotoError`, `Result<T>` — error type and alias.

## High-level design

On construction (`MessageName::new(data)`), the generated reader struct iterates the binary once
using `FieldIterator` and records the byte offset of each field's tag. Subsequent field accesses
call `ProtoAccessor::get_value_at(offset)` — no re-scanning. For repeated fields, the start and
end offsets of the field range are recorded to bound iteration efficiently.

## Literature

https://protobuf.dev/programming-guides/encoding/