indexmap 1.6.1

A hash table with consistent order and fast iteration. The indexmap is a hash table where the iteration order of the key-value pairs is independent of the hash values of the keys. It has the usual hash table functionality, it preserves insertion order except after removals, and it allows lookup of its elements by either hash table key or numerical index. A corresponding hash set type is also provided. This crate was initially published under the name ordermap, but it was renamed to indexmap.
Documentation 
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use rayon::prelude::*;

#[cfg(not(has_std))]
use alloc::collections::LinkedList;

#[cfg(has_std)]
use std::collections::LinkedList;

use crate::vec::Vec;

// generate `ParallelIterator` methods by just forwarding to the underlying
// self.entries and mapping its elements.
macro_rules! parallel_iterator_methods {
    // $map_elt is the mapping function from the underlying iterator's element
    ($map_elt:expr) => {
        fn drive_unindexed<C>(self, consumer: C) -> C::Result
        where
            C: UnindexedConsumer<Self::Item>,
        {
            self.entries
                .into_par_iter()
                .map($map_elt)
                .drive_unindexed(consumer)
        }

        // NB: This allows indexed collection, e.g. directly into a `Vec`, but the
        // underlying iterator must really be indexed.  We should remove this if we
        // start having tombstones that must be filtered out.
        fn opt_len(&self) -> Option<usize> {
            Some(self.entries.len())
        }
    };
}

// generate `IndexedParallelIterator` methods by just forwarding to the underlying
// self.entries and mapping its elements.
macro_rules! indexed_parallel_iterator_methods {
    // $map_elt is the mapping function from the underlying iterator's element
    ($map_elt:expr) => {
        fn drive<C>(self, consumer: C) -> C::Result
        where
            C: Consumer<Self::Item>,
        {
            self.entries.into_par_iter().map($map_elt).drive(consumer)
        }

        fn len(&self) -> usize {
            self.entries.len()
        }

        fn with_producer<CB>(self, callback: CB) -> CB::Output
        where
            CB: ProducerCallback<Self::Item>,
        {
            self.entries
                .into_par_iter()
                .map($map_elt)
                .with_producer(callback)
        }
    };
}

pub mod map;
pub mod set;

// This form of intermediate collection is also how Rayon collects `HashMap`.
// Note that the order will also be preserved!
fn collect<I: IntoParallelIterator>(iter: I) -> LinkedList<Vec<I::Item>> {
    iter.into_par_iter()
        .fold(Vec::new, |mut vec, elem| {
            vec.push(elem);
            vec
        })
        .map(|vec| {
            let mut list = LinkedList::new();
            list.push_back(vec);
            list
        })
        .reduce(LinkedList::new, |mut list1, mut list2| {
            list1.append(&mut list2);
            list1
        })
}