[](https://travis-ci.org/m4b/scroll)
## Scroll - cast some magic
```text
_______________
()==( (@==()
'______________'|
| |
| ἀρετή |
__)_____________|
()==( (@==()
'--------------'
```
### Documentation
https://docs.rs/scroll
### Usage
Add to your `Cargo.toml`
```toml, no_test
[dependencies]
scroll = "0.10"
```
### Overview
Scroll implements several traits for read/writing generic containers (byte buffers are currently implemented by default). Most familiar will likely be the `Pread` trait, which at its basic takes an immutable reference to self, an immutable offset to read at, (and a parsing context, more on that later), and then returns the deserialized value.
Because self is immutable, _**all** reads can be performed in parallel_ and hence are trivially parallelizable.
A simple example demonstrates its flexibility:
```rust
use scroll::{ctx, Pread, LE};
fn main() -> Result<(), scroll::Error> {
let bytes: [u8; 4] = [0xde, 0xad, 0xbe, 0xef];
// reads a u32 out of `b` with the endianness of the host machine, at offset 0, turbofish-style
let number: u32 = bytes.pread::<u32>(0)?;
// ...or a byte, with type ascription on the binding.
let byte: u8 = bytes.pread(0)?;
//If the type is known another way by the compiler, say reading into a struct field, we can omit the turbofish, and type ascription altogether!
// If we want, we can explicitly add a endianness to read with by calling `pread_with`.
// The following reads a u32 out of `b` with Big Endian byte order, at offset 0
let be_number: u32 = bytes.pread_with(0, scroll::BE)?;
// or a u16 - specify the type either on the variable or with the beloved turbofish
let be_number2 = bytes.pread_with::<u16>(2, scroll::BE)?;
// Scroll has core friendly errors (no allocation). This will have the type `scroll::Error::BadOffset` because it tried to read beyond the bound
let byte: scroll::Result<i64> = bytes.pread(0);
// Scroll is extensible: as long as the type implements `TryWithCtx`, then you can read your type out of the byte array!
// We can parse out custom datatypes, or types with lifetimes
// if they implement the conversion trait `TryFromCtx`; here we parse a C-style \0 delimited &str (safely)
let hello: &[u8] = b"hello_world\0more words";
let hello_world: &str = hello.pread(0)?;
assert_eq!("hello_world", hello_world);
// ... and this parses the string if its space separated!
use scroll::ctx::*;
let spaces: &[u8] = b"hello world some junk";
let world: &str = spaces.pread_with(6, StrCtx::Delimiter(SPACE))?;
assert_eq!("world", world);
Ok(())
}
```
### Deriving `Pread` and `Pwrite`
Scroll implements a custom derive that can provide `Pread` and `Pwrite` implementations for your structs.
```rust
use scroll::{Pread, Pwrite, BE};
#[derive(Pread, Pwrite)]
struct Data {
one: u32,
two: u16,
three: u8,
}
fn main() -> Result<(), scroll::Error> {
let bytes: [u8; 7] = [0xde, 0xad, 0xbe, 0xef, 0xfa, 0xce, 0xff];
// Read a single `Data` at offset zero in big-endian byte order.
let data: Data = bytes.pread_with(0, BE)?;
assert_eq!(data.one, 0xdeadbeef);
assert_eq!(data.two, 0xface);
assert_eq!(data.three, 0xff);
// Write it back to a buffer
let mut out: [u8; 7] = [0; 7];
out.pwrite_with(data, 0, BE)?;
assert_eq!(bytes, out);
Ok(())
}
```
This feature is **not** enabled by default, you must enable the `derive` feature in Cargo.toml to use it:
```toml, no_test
[dependencies]
scroll = { version = "0.10", features = ["derive"] }
```
# `std::io` API
Scroll can also read/write simple types from a `std::io::Read` or `std::io::Write` implementor. The built-in numeric types are taken care of for you. If you want to read a custom type, you need to implement the `FromCtx` (_how_ to parse) and `SizeWith` (_how_ big the parsed thing will be) traits. You must compile with default features. For example:
```rust
use std::io::Cursor;
use scroll::IOread;
fn main() -> Result<(), scroll::Error> {
let bytes_ = [0x01,0x00,0x00,0x00,0x00,0x00,0x00,0x00, 0xef,0xbe,0x00,0x00,];
let mut bytes = Cursor::new(bytes_);
// this will bump the cursor's Seek
let foo = bytes.ioread::<usize>()?;
// ..ditto
let bar = bytes.ioread::<u32>()?;
Ok(())
}
```
Similarly, we can write to anything that implements `std::io::Write` quite naturally:
```rust
use scroll::{IOwrite, LE, BE};
use std::io::{Write, Cursor};
fn main() -> Result<(), scroll::Error> {
let mut bytes = [0x0u8; 10];
let mut cursor = Cursor::new(&mut bytes[..]);
cursor.write_all(b"hello")?;
cursor.iowrite_with(0xdeadbeef as u32, BE)?;
assert_eq!(cursor.into_inner(), [0x68, 0x65, 0x6c, 0x6c, 0x6f, 0xde, 0xad, 0xbe, 0xef, 0x0]);
Ok(())
}
```
# Advanced Uses
Scroll is designed to be highly configurable - it allows you to implement various context (`Ctx`) sensitive traits, which then grants the implementor _automatic_ uses of the `Pread` and/or `Pwrite` traits.
For example, suppose we have a datatype and we want to specify how to parse or serialize this datatype out of some arbitrary
byte buffer. In order to do this, we need to provide a [TryFromCtx](trait.TryFromCtx.html) impl for our datatype.
In particular, if we do this for the `[u8]` target, using the convention `(usize, YourCtx)`, you will automatically get access to
calling `pread_with::<YourDatatype>` on arrays of bytes.
```rust
use scroll::{ctx, Pread, BE, Endian};
struct Data<'a> {
name: &'a str,
id: u32,
}
// note the lifetime specified here
impl<'a> ctx::TryFromCtx<'a, Endian> for Data<'a> {
type Error = scroll::Error;
// and the lifetime annotation on `&'a [u8]` here
fn try_from_ctx (src: &'a [u8], endian: Endian)
-> Result<(Self, usize), Self::Error> {
let offset = &mut 0;
let name = src.gread::<&str>(offset)?;
let id = src.gread_with(offset, endian)?;
Ok((Data { name: name, id: id }, *offset))
}
}
fn main() -> Result<(), scroll::Error> {
let bytes = b"UserName\x00\x01\x02\x03\x04";
let data = bytes.pread_with::<Data>(0, BE)?;
assert_eq!(data.id, 0x01020304);
assert_eq!(data.name.to_string(), "UserName".to_string());
Ok(())
}
```
Please see the official documentation, or a simple [example](examples/data_ctx.rs) for more.
# Contributing
Any ideas, thoughts, or contributions are welcome!