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rayon 1.5.3

Simple work-stealing parallelism for Rust
Documentation 
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use super::plumbing::*;
use super::ParallelIterator;

use std::iter::{self, Product};
use std::marker::PhantomData;

pub(super) fn product<PI, P>(pi: PI) -> P
where
    PI: ParallelIterator,
    P: Send + Product<PI::Item> + Product,
{
    pi.drive_unindexed(ProductConsumer::new())
}

fn mul<T: Product>(left: T, right: T) -> T {
    iter::once(left).chain(iter::once(right)).product()
}

struct ProductConsumer<P: Send> {
    _marker: PhantomData<*const P>,
}

unsafe impl<P: Send> Send for ProductConsumer<P> {}

impl<P: Send> ProductConsumer<P> {
    fn new() -> ProductConsumer<P> {
        ProductConsumer {
            _marker: PhantomData,
        }
    }
}

impl<P, T> Consumer<T> for ProductConsumer<P>
where
    P: Send + Product<T> + Product,
{
    type Folder = ProductFolder<P>;
    type Reducer = Self;
    type Result = P;

    fn split_at(self, _index: usize) -> (Self, Self, Self) {
        (
            ProductConsumer::new(),
            ProductConsumer::new(),
            ProductConsumer::new(),
        )
    }

    fn into_folder(self) -> Self::Folder {
        ProductFolder {
            product: iter::empty::<T>().product(),
        }
    }

    fn full(&self) -> bool {
        false
    }
}

impl<P, T> UnindexedConsumer<T> for ProductConsumer<P>
where
    P: Send + Product<T> + Product,
{
    fn split_off_left(&self) -> Self {
        ProductConsumer::new()
    }

    fn to_reducer(&self) -> Self::Reducer {
        ProductConsumer::new()
    }
}

impl<P> Reducer<P> for ProductConsumer<P>
where
    P: Send + Product,
{
    fn reduce(self, left: P, right: P) -> P {
        mul(left, right)
    }
}

struct ProductFolder<P> {
    product: P,
}

impl<P, T> Folder<T> for ProductFolder<P>
where
    P: Product<T> + Product,
{
    type Result = P;

    fn consume(self, item: T) -> Self {
        ProductFolder {
            product: mul(self.product, iter::once(item).product()),
        }
    }

    fn consume_iter<I>(self, iter: I) -> Self
    where
        I: IntoIterator<Item = T>,
    {
        ProductFolder {
            product: mul(self.product, iter.into_iter().product()),
        }
    }

    fn complete(self) -> P {
        self.product
    }

    fn full(&self) -> bool {
        false
    }
}