rayon 0.7.1

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

/// Specifies a "reduce operator". This is the combination of a start
/// value and a reduce function. The reduce function takes two items
/// and computes a reduced version. The start value `S` is a kind of
/// "zero" or "identity" value that may be intermingled as needed;
/// ideally, `reduce(S, X)` for any item `X` yields `X`.
///
/// Example: to sum up the values, use a `start_value` of `0` and a
/// reduce function of `reduce(a, b) = a + b`.
///
/// The order in which the reduce function will be applied is not
/// specified. For example, the input `[ 0 1 2 ]` might be reduced in a
/// sequential fashion:
///
/// ```ignore
/// reduce(reduce(reduce(S, 0), 1), 2)
/// ```
///
/// or it might be reduced in a tree-like way:
///
/// ```ignore
/// reduce(reduce(0, 1), reduce(S, 2))
/// ```
///
/// etc.
///
/// Implementing this trait is not permitted outside of `rayon`.
pub trait ReduceOp<T>: Sync {
    fn start_value(&self) -> T;
    fn reduce(&self, value1: T, value2: T) -> T;
    fn reduce_iter<I>(&self, value: T, iter: I) -> T where I: Iterator<Item = T>;
    private_decl!{}
}

pub fn reduce<PI, R, T>(pi: PI, reduce_op: &R) -> T
    where PI: ParallelIterator<Item = T>,
          R: ReduceOp<T>,
          T: Send
{
    let consumer = ReduceConsumer { reduce_op: reduce_op };
    pi.drive_unindexed(consumer)
}

struct ReduceConsumer<'r, R: 'r> {
    reduce_op: &'r R,
}

impl<'r, R> Copy for ReduceConsumer<'r, R> {}

impl<'r, R> Clone for ReduceConsumer<'r, R> {
    fn clone(&self) -> Self {
        *self
    }
}

impl<'r, R, T> Consumer<T> for ReduceConsumer<'r, R>
    where R: ReduceOp<T>,
          T: Send
{
    type Folder = ReduceFolder<'r, R, T>;
    type Reducer = Self;
    type Result = T;

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

    fn into_folder(self) -> ReduceFolder<'r, R, T> {
        ReduceFolder {
            reduce_op: self.reduce_op,
            item: self.reduce_op.start_value(),
        }
    }
}

impl<'r, R, T> UnindexedConsumer<T> for ReduceConsumer<'r, R>
    where R: ReduceOp<T>,
          T: Send
{
    fn split_off_left(&self) -> Self {
        ReduceConsumer { reduce_op: self.reduce_op }
    }

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

impl<'r, R, T> Reducer<T> for ReduceConsumer<'r, R>
    where R: ReduceOp<T>
{
    fn reduce(self, left: T, right: T) -> T {
        self.reduce_op.reduce(left, right)
    }
}

struct ReduceFolder<'r, R: 'r, T> {
    reduce_op: &'r R,
    item: T,
}

impl<'r, R, T> Folder<T> for ReduceFolder<'r, R, T>
    where R: ReduceOp<T>
{
    type Result = T;

    fn consume(self, item: T) -> Self {
        let item = self.reduce_op.reduce(self.item, item);
        ReduceFolder {
            reduce_op: self.reduce_op,
            item: item,
        }
    }

    fn consume_iter<I>(self, iter: I) -> Self
        where I: IntoIterator<Item = T>
    {
        let iter = iter.into_iter();
        let item = self.reduce_op.reduce_iter(self.item, iter);
        ReduceFolder {
            reduce_op: self.reduce_op,
            item: item,
        }
    }

    fn complete(self) -> T {
        self.item
    }
}

/// ////////////////////////////////////////////////////////////////////////
/// Specific operations

pub struct ReduceWithIdentityOp<'r, ID: 'r, OP: 'r> {
    identity: &'r ID,
    op: &'r OP,
}

impl<'r, ID, OP> ReduceWithIdentityOp<'r, ID, OP> {
    pub fn new(identity: &'r ID, op: &'r OP) -> ReduceWithIdentityOp<'r, ID, OP> {
        ReduceWithIdentityOp {
            identity: identity,
            op: op,
        }
    }
}

impl<'r, ID, OP, T> ReduceOp<T> for ReduceWithIdentityOp<'r, ID, OP>
    where OP: Fn(T, T) -> T + Sync,
          ID: Fn() -> T + Sync,
          T: 'r
{
    fn start_value(&self) -> T {
        (self.identity)()
    }

    fn reduce(&self, value1: T, value2: T) -> T {
        (self.op)(value1, value2)
    }

    fn reduce_iter<I>(&self, value: T, iter: I) -> T
        where I: Iterator<Item = T>
    {
        iter.fold(value, self.op)
    }

    private_impl!{}
}