parapop/src/lib.rs

#![allow(dead_code)]

use std::sync::atomic::{
    self,
    AtomicBool,
    AtomicUsize,
};
use std::mem::{self, MaybeUninit};
use std::cell::UnsafeCell;
use std::ops::Drop;

/// A parallel, atomic populator of items
pub struct Populator<T>
{
    values: Box<[UnsafeCell<MaybeUninit<T>>]>,
    populates: Box<[AtomicBool]>, // 
    populated: AtomicUsize, // number of populated items
}

impl<T> Drop for Populator<T>
{
    fn drop(&mut self)
    {
	if mem::needs_drop::<T>() {
	    if self.populated() == self.values.len() {
		// Fully populated, drop whole slice in place
		unsafe {
		    std::ptr::drop_in_place(self.values.as_mut() as *mut [UnsafeCell<MaybeUninit<T>>] as *mut [T])
		}
	    } else if self.values.len() > 0 { // If values is 0, then that means `[try_]complete()` has been called.
		// Partially populated, drop individual parts
		for value in self.values.iter_mut().zip(self.populates.iter().map(|x| x.load(atomic::Ordering::Acquire))).filter_map(|(v, prod)| prod.then(move || v.get_mut().as_mut_ptr()))
		{
		    unsafe {
			std::ptr::drop_in_place(value)
		    }
		}
	    }
	}
	// Both boxes will be dealloced after this, the values are dropped.
    }
}

unsafe impl<T> Send for Populator<T> where Box<T>: Send {}
unsafe impl<T> Sync for Populator<T>{} // Populator is always sync

impl<T> Populator<T>
{
    /// How many items are populated
    pub fn populated(&self) -> usize
    {
	self.populated.load(atomic::Ordering::Acquire)
    }
    /// Is the populator full?
    pub fn is_full(&self) -> bool
    {
	self.populated() == self.values.len()
    }
    /// Number of items held by the populator
    pub fn len(&self) -> usize
    {
	self.values.len()
    }
    /// Create a new, empty populator with this size
    pub fn new(size: usize) -> Self
    {
	Self {
	    // SAFETY: MaybeUninit is not Copy, so instead we allocate the space for uninitialised memory and then .set_len(). 
	    values: unsafe {
		let mut uninit = Vec::with_capacity(size);
		uninit.set_len(size);
		uninit
	    }.into_boxed_slice(),
	    populates: std::iter::repeat_with(|| false.into()).take(size).collect(),
	    populated: 0usize.into(),
	}
    }

    /// Try to insert `value` at `idx`.
    ///
    /// If `idx` already has a value, then `Err(value)` is returned, otherwise, `value` is inserted into the table and the number of items now populated is returned.
    pub fn try_insert(&self, idx: usize, value: T) -> Result<usize, T>
    {
	//TODO: XXX: Should we use SeqCst -> Acquire, or Acquire -> Relaxed?
	if let Ok(false) = self.populates[idx].compare_exchange(false, true, atomic::Ordering::SeqCst, atomic::Ordering::Acquire) {
	    // The value at idx hasn't been set
	    
	    if cfg!(debug_assertions) {
		match std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
		    let ptr = self.values[idx].get();
		    unsafe {
			*ptr = MaybeUninit::new(value);
		    }
		})) {
		    Err(p) => std::panic::resume_unwind(p),
		    Ok(_) => (),
		}
	    } else {
		// SAFETY: This operation will never panic, since `values` and `populates` are always the same size
		// SAFETY: We have already ensured that `values[idx]` does not contain a value.
		unsafe {
		    *self.values[idx].get() = MaybeUninit::new(value);
		}
	    }
	    // Value is inserted, increment `populated`
	    Ok(self.populated.fetch_add(1, atomic::Ordering::SeqCst) + 1)
	} else {
	    Err(value)
	}
    }

    /// Insert `value` into `idx`.
    ///
    /// # Panics
    /// If `idx` already has a value inserted.
    pub fn insert(&self, idx: usize, value: T) -> usize
    {
	#[inline(never)]
	#[cold]
	fn panic_inserted(i: usize) -> !
	{
	    panic!("There is already a value at {}", i)
	}

	match self.try_insert(idx, value) {
	    Ok(v) => v,
	    Err(_) => panic_inserted(idx),
	}
    }

    /// If all values are populated, then convert it into a boxed slice and return it.
    pub fn try_complete(mut self) -> Result<Box<[T]>, Self>
    {
	if *self.populated.get_mut() == self.values.len() {
	    let ptr = Box::into_raw(std::mem::replace(&mut self.values, vec![].into_boxed_slice()));
	    Ok(unsafe {
		Box::from_raw(ptr as *mut [T])
	    })
	} else {
	    Err(self)
	}
    }

    /// Returns the completed population.
    ///
    /// # Panics
    /// If the collection is not fully populated.
    pub fn complete(self) -> Box<[T]>
    {
	#[inline(never)]
	#[cold]
	fn panic_uncomplete() -> !
	{
	    panic!("Not all values had been populated")
	}
	
	match self.try_complete() {
	    Ok(v) => v,
	    Err(_) => panic_uncomplete(),
	}
    }
}

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        let result = 2 + 2;
        assert_eq!(result, 4);
    }
}