Function fold_reduce 

pub fn fold_reduce<T, I, F, M>(items: I, fold_fn: F, merge_fn: M) -> T
where
    T: Send + Default,
    I: ParallelIterator,
    F: Fn(T, I::Item) -> T + Sync + Send,
    M: Fn(T, T) -> T + Sync + Send,

Execute parallel fold-reduce with custom accumulator.

This is the most general form of parallel aggregation:

1. Each thread gets its own accumulator (created by T::default)
2. fold_fn processes items into thread-local accumulators
3. merge_fn combines accumulators from different threads

Example

use grafeo_core::execution::parallel::fold::fold_reduce;
use rayon::prelude::*;

let items = vec![1, 2, 3, 4, 5];
let sum: i32 = fold_reduce(
    items.into_par_iter(),
    |acc: i32, item| acc + item,
    |a, b| a + b,
);
assert_eq!(sum, 15i32);