# stuff

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A crate for stuffing things into a pointer.

This crate is tested using miri (with `-Zmiri-tag-raw-pointers`).

This crate consists of three parts:
* The type `StuffedPtr`
* The trait `StuffingStrategy`
* The trait `Backend`
`StuffedPtr` is the main type of this crate. You it's a type whose size depends on the
choice of `Backend` (defaults to `usize`, `u64` and `u128` are also possible). It can store a
pointer or some extra data.

except that the extra data is bitstuffed into the pointer. You can chose any arbitrary bitstuffing
depending on the `StuffingStrategy`, an unsafe trait that governs how the extra data
(or the pointer itself) will be packed into the backend.

# Example: NaN-Boxing
Pointers are hidden in the NaN values of floats. NaN boxing often involves also hiding booleans
or null in there, but we stay with floats and pointers (pointers to a `HashMap` that servers
as our "object" type).
See [crafting interpreters](https://craftinginterpreters.com/optimization.html#nan-boxing)
for more details.
```rust
use std::collections::HashMap;
use stuff::{StuffedPtr, StuffingStrategy};

// Create a unit struct for our strategy
struct NanBoxStrategy;

const QNAN: u64 = 0x7ffc000000000000; // implementation detail of NaN boxing, a quiet NaN mask

const SIGN_BIT: u64 = 0x8000000000000000; // implementation detail of NaN boxing, the sign bit of an f64

unsafe impl StuffingStrategy<u64> for NanBoxStrategy {
    type Extra = f64;
    
    fn is_extra(data: u64) -> bool {
        (data & QNAN) != QNAN
    }
    
    fn stuff_extra(inner: Self::Extra) -> u64 {
        unsafe { std::mem::transmute(inner) } // both are 64 bit POD's
    }
    
    unsafe fn extract_extra(data: u64) -> Self::Extra {
        std::mem::transmute(data) // both are 64 bit POD's
    }
    
    fn stuff_ptr(addr: usize) -> u64 {
        // add the QNAN and SIGN_BIT
        SIGN_BIT | QNAN | u64::try_from(addr).unwrap()
    }
    
    fn extract_ptr(inner: u64) -> usize {
        // keep everything except for QNAN and SIGN_BIT
        (inner & !(SIGN_BIT | QNAN)).try_into().unwrap()
    }
}

type Object = HashMap<String, u32>; // a very, very crude representation of an object

type Value = StuffedPtr<Object, NanBoxStrategy, u64>; // our value type

fn main() {
    let float: Value = StuffedPtr::new_extra(123.5);
    assert_eq!(float.copy_extra(), Some(123.5));
    
    let object: Object = HashMap::from([("a".to_owned(), 457)]);
    
    let boxed = Box::new(object);
    let ptr: Value = StuffedPtr::new_ptr(Box::into_raw(boxed));
    
    let object = unsafe { &*ptr.get_ptr().unwrap() };
    assert_eq!(object.get("a"), Some(&457));
    
    drop(unsafe { Box::from_raw(ptr.get_ptr().unwrap()) });
    // `ptr` is a dangling pointer now!
}
```