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Dynamic packet parsing within trees 🌲🌳🌴

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# peel
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## Dynamic parsing within trees 🌲 🌳 🌴
Target of this library is to provide a flexible approach in parsing data. This will mainly be done within
[arena](https://en.wikipedia.org/wiki/Region-based_memory_management) based
[parser trees](https://en.wikipedia.org/wiki/Parse_tree) which can be modified during runtime. Every parser is using the
[nom](https://github.com/Geal/nom) framework for the actual parsing work. A complete source code example can be found
within the [`src/example`](https://github.com/saschagrunert/peel/tree/master/src/example) directory of the crate.

## Architecture and usage
Every `Peel` instance can be seen as a parsing graph structure which has different states and transitions. In the
example within the crate the structure looks like this:

![Example parser diagram](https://raw.githubusercontent.com/saschagrunert/peel/master/.github/example.png)

Independently of what these parser do, the creation of this structure is done within the `peel_example` function:

```rust
pub fn peel_example() -> Peel<()> {
    // Create a tree
    let mut p = Peel::new();

// Create some parsers
    let parser_1 = p.new_parser(Parser1);
    let parser_2 = p.new_parser(Parser2);
    let parser_3 = p.new_parser(Parser3);
    let parser_4 = p.new_parser(Parser4);

// Link the parsers together
    p.link_nodes(&[(parser_1, parser_2),
                   (parser_1, parser_3),
                   (parser_2, parser_3),
                   (parser_3, parser_3),
                   (parser_3, parser_4)]);

p
}
```

The first created parser will automatically be the root parser and the entry point for the tree traversal. Every
succeeding parser returns a certain result, which will be pushed into a vector as a `Box<Any>` which can be downcasted
to a certain other type.

This means that the traversal method of `Peel` will try to find the deepest possible valid path within the tree
structure. After the creation of the structure the traversal can begin:

```rust
let mut peel = peel_example();
peel.set_log_level(LogLevel::Trace);
let result = peel.traverse(b"1234", vec![]).unwrap();

assert_eq!(result.len(), 4);
println!("{:?}", result);
```

With the help of the [log](https://crates.io/crates/log) crate it will output:
```
[peel] [DEBUG] Parser 1 parsing succeed, left input length: 3
[peel] [DEBUG] Failed parser: Parser 3
[peel] [DEBUG] Parser 2 parsing succeed, left input length: 2
[peel] [DEBUG] Parser 3 parsing succeed, left input length: 1
[peel] [DEBUG] Parser 4 parsing succeed, left input length: 0
```

A minimal parser has to implement the `Parser` trait which could look like this:
```rust
use example::prelude::*;

/// The first example parser
#[derive(Debug)]
pub struct Parser1;

#[derive(Debug, PartialEq)]
/// The result of the first example parser
pub struct Parser1Result;

impl Parsable<()> for Parser1 {
    /// The actual parsing entry point
    fn parse<'a>(&mut self,
                 input: &'a [u8],                    // The input for the parser
                 result: Option<&ParserResultVec>,   // The current parsing result
                 data: Option<&mut ()>)              // Additional data which will
                                                     // be shared accross parsers
                 -> IResult<&'a [u8], ParserResult> {

do_parse!(input, tag!("1") >> (Box::new(Parser1Result)))
    }
}
```

It is possible to access the current parsing `result` for a more advanced behavior like dependency checks during the
parsing. Furthermore, additional data `data` can be used to share data between parsers.

## Contributing
You want to contribute to this project? Wow, thanks! So please just fork it and send me a pull request.

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