Import linked-hash-map 0.5.6upstream/0.5.6upstream

10 files changed, 2097 insertions, 0 deletions

diff --git a/.cargo_vcs_info.json b/.cargo_vcs_info.json
new file mode 100644
index 0000000..2810244
--- /dev/null
+++ b/.cargo_vcs_info.json
@@ -0,0 +1,6 @@
+{
+  "git": {
+    "sha1": "dfc4a57278b79314b0c1892d6c3918f2e00e249b"
+  },
+  "path_in_vcs": ""
+}
\ No newline at end of file
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..a9d37c5
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+target
+Cargo.lock
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..51deccd
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,45 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+name = "linked-hash-map"
+version = "0.5.6"
+authors = [
+    "Stepan Koltsov <stepan.koltsov@gmail.com>",
+    "Andrew Paseltiner <apaseltiner@gmail.com>",
+]
+exclude = [
+    ".github",
+    "src/tests",
+]
+description = "A HashMap wrapper that holds key-value pairs in insertion order"
+homepage = "https://github.com/contain-rs/linked-hash-map"
+documentation = "https://docs.rs/linked-hash-map"
+readme = "README.md"
+keywords = ["data-structures"]
+license = "MIT/Apache-2.0"
+repository = "https://github.com/contain-rs/linked-hash-map"
+
+[dependencies.heapsize]
+version = "0.4"
+optional = true
+
+[dependencies.serde]
+version = "1.0"
+optional = true
+
+[dev-dependencies.serde_test]
+version = "1.0"
+
+[features]
+heapsize_impl = ["heapsize"]
+nightly = []
+serde_impl = ["serde"]
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
new file mode 100644
index 0000000..4400e6e
--- /dev/null
+++ b/Cargo.toml.orig
@@ -0,0 +1,29 @@
+[package]
+
+name = "linked-hash-map"
+version = "0.5.6"
+license = "MIT/Apache-2.0"
+description = "A HashMap wrapper that holds key-value pairs in insertion order"
+authors = [
+    "Stepan Koltsov <stepan.koltsov@gmail.com>",
+    "Andrew Paseltiner <apaseltiner@gmail.com>",
+]
+
+repository = "https://github.com/contain-rs/linked-hash-map"
+homepage = "https://github.com/contain-rs/linked-hash-map"
+documentation = "https://docs.rs/linked-hash-map"
+keywords = ["data-structures"]
+readme = "README.md"
+exclude = [".github", "src/tests"]
+
+[features]
+nightly = []
+serde_impl = ["serde"]
+heapsize_impl = ["heapsize"]
+
+[dependencies]
+serde = { version = "1.0", optional = true }
+heapsize = { version = "0.4", optional = true }
+
+[dev-dependencies]
+serde_test = { version = "1.0" }
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..11069ed
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,201 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+1. Definitions.
+
+   "License" shall mean the terms and conditions for use, reproduction,
+   and distribution as defined by Sections 1 through 9 of this document.
+
+   "Licensor" shall mean the copyright owner or entity authorized by
+   the copyright owner that is granting the License.
+
+   "Legal Entity" shall mean the union of the acting entity and all
+   other entities that control, are controlled by, or are under common
+   control with that entity. For the purposes of this definition,
+   "control" means (i) the power, direct or indirect, to cause the
+   direction or management of such entity, whether by contract or
+   otherwise, or (ii) ownership of fifty percent (50%) or more of the
+   outstanding shares, or (iii) beneficial ownership of such entity.
+
+   "You" (or "Your") shall mean an individual or Legal Entity
+   exercising permissions granted by this License.
+
+   "Source" form shall mean the preferred form for making modifications,
+   including but not limited to software source code, documentation
+   source, and configuration files.
+
+   "Object" form shall mean any form resulting from mechanical
+   transformation or translation of a Source form, including but
+   not limited to compiled object code, generated documentation,
+   and conversions to other media types.
+
+   "Work" shall mean the work of authorship, whether in Source or
+   Object form, made available under the License, as indicated by a
+   copyright notice that is included in or attached to the work
+   (an example is provided in the Appendix below).
+
+   "Derivative Works" shall mean any work, whether in Source or Object
+   form, that is based on (or derived from) the Work and for which the
+   editorial revisions, annotations, elaborations, or other modifications
+   represent, as a whole, an original work of authorship. For the purposes
+   of this License, Derivative Works shall not include works that remain
+   separable from, or merely link (or bind by name) to the interfaces of,
+   the Work and Derivative Works thereof.
+
+   "Contribution" shall mean any work of authorship, including
+   the original version of the Work and any modifications or additions
+   to that Work or Derivative Works thereof, that is intentionally
+   submitted to Licensor for inclusion in the Work by the copyright owner
+   or by an individual or Legal Entity authorized to submit on behalf of
+   the copyright owner. For the purposes of this definition, "submitted"
+   means any form of electronic, verbal, or written communication sent
+   to the Licensor or its representatives, including but not limited to
+   communication on electronic mailing lists, source code control systems,
+   and issue tracking systems that are managed by, or on behalf of, the
+   Licensor for the purpose of discussing and improving the Work, but
+   excluding communication that is conspicuously marked or otherwise
+   designated in writing by the copyright owner as "Not a Contribution."
+
+   "Contributor" shall mean Licensor and any individual or Legal Entity
+   on behalf of whom a Contribution has been received by Licensor and
+   subsequently incorporated within the Work.
+
+2. Grant of Copyright License. Subject to the terms and conditions of
+   this License, each Contributor hereby grants to You a perpetual,
+   worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+   copyright license to reproduce, prepare Derivative Works of,
+   publicly display, publicly perform, sublicense, and distribute the
+   Work and such Derivative Works in Source or Object form.
+
+3. Grant of Patent License. Subject to the terms and conditions of
+   this License, each Contributor hereby grants to You a perpetual,
+   worldwide, non-exclusive, no-charge, royalty-free, irrevocable
+   (except as stated in this section) patent license to make, have made,
+   use, offer to sell, sell, import, and otherwise transfer the Work,
+   where such license applies only to those patent claims licensable
+   by such Contributor that are necessarily infringed by their
+   Contribution(s) alone or by combination of their Contribution(s)
+   with the Work to which such Contribution(s) was submitted. If You
+   institute patent litigation against any entity (including a
+   cross-claim or counterclaim in a lawsuit) alleging that the Work
+   or a Contribution incorporated within the Work constitutes direct
+   or contributory patent infringement, then any patent licenses
+   granted to You under this License for that Work shall terminate
+   as of the date such litigation is filed.
+
+4. Redistribution. You may reproduce and distribute copies of the
+   Work or Derivative Works thereof in any medium, with or without
+   modifications, and in Source or Object form, provided that You
+   meet the following conditions:
+
+   (a) You must give any other recipients of the Work or
+       Derivative Works a copy of this License; and
+
+   (b) You must cause any modified files to carry prominent notices
+       stating that You changed the files; and
+
+   (c) You must retain, in the Source form of any Derivative Works
+       that You distribute, all copyright, patent, trademark, and
+       attribution notices from the Source form of the Work,
+       excluding those notices that do not pertain to any part of
+       the Derivative Works; and
+
+   (d) If the Work includes a "NOTICE" text file as part of its
+       distribution, then any Derivative Works that You distribute must
+       include a readable copy of the attribution notices contained
+       within such NOTICE file, excluding those notices that do not
+       pertain to any part of the Derivative Works, in at least one
+       of the following places: within a NOTICE text file distributed
+       as part of the Derivative Works; within the Source form or
+       documentation, if provided along with the Derivative Works; or,
+       within a display generated by the Derivative Works, if and
+       wherever such third-party notices normally appear. The contents
+       of the NOTICE file are for informational purposes only and
+       do not modify the License. You may add Your own attribution
+       notices within Derivative Works that You distribute, alongside
+       or as an addendum to the NOTICE text from the Work, provided
+       that such additional attribution notices cannot be construed
+       as modifying the License.
+
+   You may add Your own copyright statement to Your modifications and
+   may provide additional or different license terms and conditions
+   for use, reproduction, or distribution of Your modifications, or
+   for any such Derivative Works as a whole, provided Your use,
+   reproduction, and distribution of the Work otherwise complies with
+   the conditions stated in this License.
+
+5. Submission of Contributions. Unless You explicitly state otherwise,
+   any Contribution intentionally submitted for inclusion in the Work
+   by You to the Licensor shall be under the terms and conditions of
+   this License, without any additional terms or conditions.
+   Notwithstanding the above, nothing herein shall supersede or modify
+   the terms of any separate license agreement you may have executed
+   with Licensor regarding such Contributions.
+
+6. Trademarks. This License does not grant permission to use the trade
+   names, trademarks, service marks, or product names of the Licensor,
+   except as required for reasonable and customary use in describing the
+   origin of the Work and reproducing the content of the NOTICE file.
+
+7. Disclaimer of Warranty. Unless required by applicable law or
+   agreed to in writing, Licensor provides the Work (and each
+   Contributor provides its Contributions) on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+   implied, including, without limitation, any warranties or conditions
+   of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+   PARTICULAR PURPOSE. You are solely responsible for determining the
+   appropriateness of using or redistributing the Work and assume any
+   risks associated with Your exercise of permissions under this License.
+
+8. Limitation of Liability. In no event and under no legal theory,
+   whether in tort (including negligence), contract, or otherwise,
+   unless required by applicable law (such as deliberate and grossly
+   negligent acts) or agreed to in writing, shall any Contributor be
+   liable to You for damages, including any direct, indirect, special,
+   incidental, or consequential damages of any character arising as a
+   result of this License or out of the use or inability to use the
+   Work (including but not limited to damages for loss of goodwill,
+   work stoppage, computer failure or malfunction, or any and all
+   other commercial damages or losses), even if such Contributor
+   has been advised of the possibility of such damages.
+
+9. Accepting Warranty or Additional Liability. While redistributing
+   the Work or Derivative Works thereof, You may choose to offer,
+   and charge a fee for, acceptance of support, warranty, indemnity,
+   or other liability obligations and/or rights consistent with this
+   License. However, in accepting such obligations, You may act only
+   on Your own behalf and on Your sole responsibility, not on behalf
+   of any other Contributor, and only if You agree to indemnify,
+   defend, and hold each Contributor harmless for any liability
+   incurred by, or claims asserted against, such Contributor by reason
+   of your accepting any such warranty or additional liability.
+
+END OF TERMS AND CONDITIONS
+
+APPENDIX: How to apply the Apache License to your work.
+
+   To apply the Apache License to your work, attach the following
+   boilerplate notice, with the fields enclosed by brackets "[]"
+   replaced with your own identifying information. (Don't include
+   the brackets!)  The text should be enclosed in the appropriate
+   comment syntax for the file format. We also recommend that a
+   file or class name and description of purpose be included on the
+   same "printed page" as the copyright notice for easier
+   identification within third-party archives.
+
+Copyright [yyyy] [name of copyright owner]
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
diff --git a/LICENSE-MIT b/LICENSE-MIT
new file mode 100644
index 0000000..e69282e
--- /dev/null
+++ b/LICENSE-MIT
@@ -0,0 +1,25 @@
+Copyright (c) 2015 The Rust Project Developers
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..31b40b3
--- /dev/null
+++ b/README.md
@@ -0,0 +1,19 @@
+![Rust CI](https://github.com/contain-rs/linked-hash-map/workflows/Rust/badge.svg?branch=master) [![crates.io](https://img.shields.io/crates/v/linked-hash-map.svg)](https://crates.io/crates/linked-hash-map) [![](https://docs.rs/linked-hash-map/badge.svg)](https://docs.rs/linked-hash-map)
+
+
+**WARNING: THIS PROJECT IS IN MAINTENANCE MODE, DUE TO INSUFFICIENT MAINTAINER RESOURCES**
+
+It works fine, but will generally no longer be improved.
+
+We are currently only accepting changes which:
+
+* fix correctness issues
+* keep this compiling with the latest versions of Rust or its dependencies.
+* have minimal review requirements, such as documentation changes (so not totally new APIs).
+
+------
+
+
+A HashMap wrapper that holds key-value pairs in insertion order.
+
+Documentation is available at https://docs.rs/linked-hash-map.
diff --git a/src/heapsize.rs b/src/heapsize.rs
new file mode 100644
index 0000000..59f4629
--- /dev/null
+++ b/src/heapsize.rs
@@ -0,0 +1,51 @@
+extern crate heapsize;
+
+use self::heapsize::{heap_size_of, HeapSizeOf};
+use std::hash::{BuildHasher, Hash};
+
+use {KeyRef, LinkedHashMap, Node};
+
+impl<K> HeapSizeOf for KeyRef<K> {
+    fn heap_size_of_children(&self) -> usize {
+        0
+    }
+}
+
+impl<K, V> HeapSizeOf for Node<K, V>
+where
+    K: HeapSizeOf,
+    V: HeapSizeOf,
+{
+    fn heap_size_of_children(&self) -> usize {
+        self.key.heap_size_of_children() + self.value.heap_size_of_children()
+    }
+}
+
+impl<K, V, S> HeapSizeOf for LinkedHashMap<K, V, S>
+where
+    K: HeapSizeOf + Hash + Eq,
+    V: HeapSizeOf,
+    S: BuildHasher,
+{
+    fn heap_size_of_children(&self) -> usize {
+        unsafe {
+            let mut size = self.map.heap_size_of_children();
+            for &value in self.map.values() {
+                size += (*value).heap_size_of_children();
+                size += heap_size_of(value as *const _ as *const _);
+            }
+
+            if !self.head.is_null() {
+                size += heap_size_of(self.head as *const _ as *const _);
+            }
+
+            let mut free = self.free;
+            while !free.is_null() {
+                size += heap_size_of(free as *const _ as *const _);
+                free = (*free).next
+            }
+
+            size
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..d34d5ac
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,1618 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A `HashMap` wrapper that holds key-value pairs in insertion order.
+//!
+//! # Examples
+//!
+//! ```
+//! use linked_hash_map::LinkedHashMap;
+//!
+//! let mut map = LinkedHashMap::new();
+//! map.insert(2, 20);
+//! map.insert(1, 10);
+//! map.insert(3, 30);
+//! assert_eq!(map[&1], 10);
+//! assert_eq!(map[&2], 20);
+//! assert_eq!(map[&3], 30);
+//!
+//! let items: Vec<(i32, i32)> = map.iter().map(|t| (*t.0, *t.1)).collect();
+//! assert_eq!(items, [(2, 20), (1, 10), (3, 30)]);
+//! ```
+
+#![forbid(missing_docs)]
+#![cfg_attr(all(feature = "nightly", test), feature(test))]
+
+// Optional Serde support
+#[cfg(feature = "serde_impl")]
+pub mod serde;
+// Optional Heapsize support
+#[cfg(feature = "heapsize_impl")]
+mod heapsize;
+#[cfg(test)]
+mod tests;
+
+use std::borrow::Borrow;
+use std::cmp::Ordering;
+use std::collections::hash_map::{self, HashMap};
+use std::fmt;
+use std::hash::{BuildHasher, Hash, Hasher};
+use std::iter;
+use std::marker;
+use std::mem;
+use std::ops::{Index, IndexMut};
+use std::ptr::{self, addr_of_mut};
+
+struct KeyRef<K> {
+    k: *const K,
+}
+
+struct Node<K, V> {
+    next: *mut Node<K, V>,
+    prev: *mut Node<K, V>,
+    key: K,
+    value: V,
+}
+
+/// A linked hash map.
+pub struct LinkedHashMap<K, V, S = hash_map::RandomState> {
+    map: HashMap<KeyRef<K>, *mut Node<K, V>, S>,
+    head: *mut Node<K, V>,
+    free: *mut Node<K, V>,
+}
+
+impl<K: Hash> Hash for KeyRef<K> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        unsafe { (*self.k).hash(state) }
+    }
+}
+
+impl<K: PartialEq> PartialEq for KeyRef<K> {
+    fn eq(&self, other: &Self) -> bool {
+        unsafe { (*self.k).eq(&*other.k) }
+    }
+}
+
+impl<K: Eq> Eq for KeyRef<K> {}
+
+// This type exists only to support borrowing `KeyRef`s, which cannot be borrowed to `Q` directly
+// due to conflicting implementations of `Borrow`. The layout of `&Qey<Q>` must be identical to
+// `&Q` in order to support transmuting in the `Qey::from_ref` method.
+#[derive(Hash, PartialEq, Eq)]
+#[repr(transparent)]
+struct Qey<Q: ?Sized>(Q);
+
+impl<Q: ?Sized> Qey<Q> {
+    fn from_ref(q: &Q) -> &Self {
+        unsafe { mem::transmute(q) }
+    }
+}
+
+impl<K, Q: ?Sized> Borrow<Qey<Q>> for KeyRef<K>
+where
+    K: Borrow<Q>,
+{
+    fn borrow(&self) -> &Qey<Q> {
+        Qey::from_ref(unsafe { (*self.k).borrow() })
+    }
+}
+
+impl<K, V> Node<K, V> {
+    fn new(k: K, v: V) -> Self {
+        Node {
+            key: k,
+            value: v,
+            next: ptr::null_mut(),
+            prev: ptr::null_mut(),
+        }
+    }
+}
+
+// drop empty node without dropping its key and value
+unsafe fn drop_empty_node<K, V>(the_box: *mut Node<K, V>) {
+    // Safety:
+    // In this crate all `Node` is allocated via `Box` or `alloc`, and `Box` uses the
+    // Global allocator for its allocation,
+    // (https://doc.rust-lang.org/std/boxed/index.html#memory-layout) so we can safely
+    // deallocate the pointer to `Node` by calling `dealloc` method
+    let layout = std::alloc::Layout::new::<Node<K, V>>();
+    std::alloc::dealloc(the_box as *mut u8, layout);
+}
+
+impl<K: Hash + Eq, V> LinkedHashMap<K, V> {
+    /// Creates a linked hash map.
+    pub fn new() -> Self {
+        Self::with_map(HashMap::new())
+    }
+
+    /// Creates an empty linked hash map with the given initial capacity.
+    pub fn with_capacity(capacity: usize) -> Self {
+        Self::with_map(HashMap::with_capacity(capacity))
+    }
+}
+
+impl<K, V, S> LinkedHashMap<K, V, S> {
+    #[inline]
+    fn detach(&mut self, node: *mut Node<K, V>) {
+        unsafe {
+            (*(*node).prev).next = (*node).next;
+            (*(*node).next).prev = (*node).prev;
+        }
+    }
+
+    #[inline]
+    fn attach(&mut self, node: *mut Node<K, V>) {
+        unsafe {
+            (*node).next = (*self.head).next;
+            (*node).prev = self.head;
+            (*self.head).next = node;
+            (*(*node).next).prev = node;
+        }
+    }
+
+    // Caller must check `!self.head.is_null()`
+    unsafe fn drop_entries(&mut self) {
+        let mut cur = (*self.head).next;
+        while cur != self.head {
+            let next = (*cur).next;
+            Box::from_raw(cur);
+            cur = next;
+        }
+    }
+
+    fn clear_free_list(&mut self) {
+        unsafe {
+            let mut free = self.free;
+            while !free.is_null() {
+                let next_free = (*free).next;
+                drop_empty_node(free);
+                free = next_free;
+            }
+            self.free = ptr::null_mut();
+        }
+    }
+
+    fn ensure_guard_node(&mut self) {
+        if self.head.is_null() {
+            // allocate the guard node if not present
+            unsafe {
+                let node_layout = std::alloc::Layout::new::<Node<K, V>>();
+                self.head = std::alloc::alloc(node_layout) as *mut Node<K, V>;
+                (*self.head).next = self.head;
+                (*self.head).prev = self.head;
+            }
+        }
+    }
+}
+
+impl<K: Hash + Eq, V, S: BuildHasher> LinkedHashMap<K, V, S> {
+    fn with_map(map: HashMap<KeyRef<K>, *mut Node<K, V>, S>) -> Self {
+        LinkedHashMap {
+            map,
+            head: ptr::null_mut(),
+            free: ptr::null_mut(),
+        }
+    }
+
+    /// Creates an empty linked hash map with the given initial hash builder.
+    pub fn with_hasher(hash_builder: S) -> Self {
+        Self::with_map(HashMap::with_hasher(hash_builder))
+    }
+
+    /// Creates an empty linked hash map with the given initial capacity and hash builder.
+    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> Self {
+        Self::with_map(HashMap::with_capacity_and_hasher(capacity, hash_builder))
+    }
+
+    /// Reserves capacity for at least `additional` more elements to be inserted into the map. The
+    /// map may reserve more space to avoid frequent allocations.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new allocation size overflows `usize.`
+    pub fn reserve(&mut self, additional: usize) {
+        self.map.reserve(additional);
+    }
+
+    /// Shrinks the capacity of the map as much as possible. It will drop down as much as possible
+    /// while maintaining the internal rules and possibly leaving some space in accordance with the
+    /// resize policy.
+    pub fn shrink_to_fit(&mut self) {
+        self.map.shrink_to_fit();
+        self.clear_free_list();
+    }
+
+    /// Gets the given key's corresponding entry in the map for in-place manipulation.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut letters = LinkedHashMap::new();
+    ///
+    /// for ch in "a short treatise on fungi".chars() {
+    ///     let counter = letters.entry(ch).or_insert(0);
+    ///     *counter += 1;
+    /// }
+    ///
+    /// assert_eq!(letters[&'s'], 2);
+    /// assert_eq!(letters[&'t'], 3);
+    /// assert_eq!(letters[&'u'], 1);
+    /// assert_eq!(letters.get(&'y'), None);
+    /// ```
+    pub fn entry(&mut self, k: K) -> Entry<K, V, S> {
+        let self_ptr: *mut Self = self;
+
+        if let Some(entry) = self.map.get_mut(&KeyRef { k: &k }) {
+            return Entry::Occupied(OccupiedEntry {
+                entry: *entry,
+                map: self_ptr,
+                marker: marker::PhantomData,
+            });
+        }
+
+        Entry::Vacant(VacantEntry { key: k, map: self })
+    }
+
+    /// Returns an iterator visiting all entries in insertion order.
+    /// Iterator element type is `OccupiedEntry<K, V, S>`. Allows for removal
+    /// as well as replacing the entry.
+    ///
+    /// # Examples
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert("a", 10);
+    /// map.insert("c", 30);
+    /// map.insert("b", 20);
+    ///
+    /// {
+    ///     let mut iter = map.entries();
+    ///     let mut entry = iter.next().unwrap();
+    ///     assert_eq!(&"a", entry.key());
+    ///     *entry.get_mut() = 17;
+    /// }
+    ///
+    /// assert_eq!(&17, map.get(&"a").unwrap());
+    /// ```
+    pub fn entries(&mut self) -> Entries<K, V, S> {
+        let head = if !self.head.is_null() {
+            unsafe { (*self.head).prev }
+        } else {
+            ptr::null_mut()
+        };
+        Entries {
+            map: self,
+            head,
+            remaining: self.len(),
+            marker: marker::PhantomData,
+        }
+    }
+
+    /// Inserts a key-value pair into the map. If the key already existed, the old value is
+    /// returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    ///
+    /// map.insert(1, "a");
+    /// map.insert(2, "b");
+    /// assert_eq!(map[&1], "a");
+    /// assert_eq!(map[&2], "b");
+    /// ```
+    pub fn insert(&mut self, k: K, v: V) -> Option<V> {
+        self.ensure_guard_node();
+
+        let (node, old_val) = match self.map.get(&KeyRef { k: &k }) {
+            Some(node) => {
+                let old_val = unsafe { ptr::replace(&mut (**node).value, v) };
+                (*node, Some(old_val))
+            }
+            None => {
+                let node = if self.free.is_null() {
+                    Box::into_raw(Box::new(Node::new(k, v)))
+                } else {
+                    // use a recycled box
+                    unsafe {
+                        let free = self.free;
+                        self.free = (*free).next;
+                        ptr::write(free, Node::new(k, v));
+                        free
+                    }
+                };
+                (node, None)
+            }
+        };
+        match old_val {
+            Some(_) => {
+                // Existing node, just update LRU position
+                self.detach(node);
+                self.attach(node);
+            }
+            None => {
+                let keyref = unsafe { &(*node).key };
+                self.map.insert(KeyRef { k: keyref }, node);
+                self.attach(node);
+            }
+        }
+        old_val
+    }
+
+    /// Checks if the map contains the given key.
+    pub fn contains_key<Q: ?Sized>(&self, k: &Q) -> bool
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash,
+    {
+        self.map.contains_key(Qey::from_ref(k))
+    }
+
+    /// Returns the value corresponding to the key in the map.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    ///
+    /// map.insert(1, "a");
+    /// map.insert(2, "b");
+    /// map.insert(2, "c");
+    /// map.insert(3, "d");
+    ///
+    /// assert_eq!(map.get(&1), Some(&"a"));
+    /// assert_eq!(map.get(&2), Some(&"c"));
+    /// ```
+    pub fn get<Q: ?Sized>(&self, k: &Q) -> Option<&V>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash,
+    {
+        self.map
+            .get(Qey::from_ref(k))
+            .map(|e| unsafe { &(**e).value })
+    }
+
+    /// Returns the mutable reference corresponding to the key in the map.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    ///
+    /// map.insert(1, "a");
+    /// map.insert(2, "b");
+    ///
+    /// *map.get_mut(&1).unwrap() = "c";
+    /// assert_eq!(map.get(&1), Some(&"c"));
+    /// ```
+    pub fn get_mut<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash,
+    {
+        self.map
+            .get(Qey::from_ref(k))
+            .map(|e| unsafe { &mut (**e).value })
+    }
+
+    /// Returns the value corresponding to the key in the map.
+    ///
+    /// If value is found, it is moved to the end of the list.
+    /// This operation can be used in implemenation of LRU cache.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    ///
+    /// map.insert(1, "a");
+    /// map.insert(2, "b");
+    /// map.insert(3, "d");
+    ///
+    /// assert_eq!(map.get_refresh(&2), Some(&mut "b"));
+    ///
+    /// assert_eq!((&2, &"b"), map.iter().rev().next().unwrap());
+    /// ```
+    pub fn get_refresh<Q: ?Sized>(&mut self, k: &Q) -> Option<&mut V>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash,
+    {
+        let (value, node_ptr_opt) = match self.map.get(Qey::from_ref(k)) {
+            None => (None, None),
+            Some(node) => (Some(unsafe { &mut (**node).value }), Some(*node)),
+        };
+        if let Some(node_ptr) = node_ptr_opt {
+            self.detach(node_ptr);
+            self.attach(node_ptr);
+        }
+        value
+    }
+
+    /// Removes and returns the value corresponding to the key from the map.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    ///
+    /// map.insert(2, "a");
+    ///
+    /// assert_eq!(map.remove(&1), None);
+    /// assert_eq!(map.remove(&2), Some("a"));
+    /// assert_eq!(map.remove(&2), None);
+    /// assert_eq!(map.len(), 0);
+    /// ```
+    pub fn remove<Q: ?Sized>(&mut self, k: &Q) -> Option<V>
+    where
+        K: Borrow<Q>,
+        Q: Eq + Hash,
+    {
+        let removed = self.map.remove(Qey::from_ref(k));
+        removed.map(|node| {
+            self.detach(node);
+            unsafe {
+                // add to free list
+                (*node).next = self.free;
+                self.free = node;
+                // drop the key and return the value
+                drop(ptr::read(&(*node).key));
+                ptr::read(&(*node).value)
+            }
+        })
+    }
+
+    /// Returns the maximum number of key-value pairs the map can hold without reallocating.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map: LinkedHashMap<i32, &str> = LinkedHashMap::new();
+    /// let capacity = map.capacity();
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.map.capacity()
+    }
+
+    /// Removes the first entry.
+    ///
+    /// Can be used in implementation of LRU cache.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert(1, 10);
+    /// map.insert(2, 20);
+    /// map.pop_front();
+    /// assert_eq!(map.get(&1), None);
+    /// assert_eq!(map.get(&2), Some(&20));
+    /// ```
+    #[inline]
+    pub fn pop_front(&mut self) -> Option<(K, V)> {
+        if self.is_empty() {
+            return None;
+        }
+        let lru = unsafe { (*self.head).prev };
+        self.detach(lru);
+        self.map
+            .remove(&KeyRef {
+                k: unsafe { &(*lru).key },
+            })
+            .map(|e| {
+                let e = *unsafe { Box::from_raw(e) };
+                (e.key, e.value)
+            })
+    }
+
+    /// Gets the first entry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert(1, 10);
+    /// map.insert(2, 20);
+    /// assert_eq!(map.front(), Some((&1, &10)));
+    /// ```
+    #[inline]
+    pub fn front(&self) -> Option<(&K, &V)> {
+        if self.is_empty() {
+            return None;
+        }
+        let lru = unsafe { (*self.head).prev };
+        self.map
+            .get(&KeyRef {
+                k: unsafe { &(*lru).key },
+            })
+            .map(|e| unsafe { (&(**e).key, &(**e).value) })
+    }
+
+    /// Removes the last entry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert(1, 10);
+    /// map.insert(2, 20);
+    /// map.pop_back();
+    /// assert_eq!(map.get(&1), Some(&10));
+    /// assert_eq!(map.get(&2), None);
+    /// ```
+    #[inline]
+    pub fn pop_back(&mut self) -> Option<(K, V)> {
+        if self.is_empty() {
+            return None;
+        }
+        let mru = unsafe { (*self.head).next };
+        self.detach(mru);
+        self.map
+            .remove(&KeyRef {
+                k: unsafe { &(*mru).key },
+            })
+            .map(|e| {
+                let e = *unsafe { Box::from_raw(e) };
+                (e.key, e.value)
+            })
+    }
+
+    /// Gets the last entry.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert(1, 10);
+    /// map.insert(2, 20);
+    /// assert_eq!(map.back(), Some((&2, &20)));
+    /// ```
+    #[inline]
+    pub fn back(&self) -> Option<(&K, &V)> {
+        if self.is_empty() {
+            return None;
+        }
+        let mru = unsafe { (*self.head).next };
+        self.map
+            .get(&KeyRef {
+                k: unsafe { &(*mru).key },
+            })
+            .map(|e| unsafe { (&(**e).key, &(**e).value) })
+    }
+
+    /// Returns the number of key-value pairs in the map.
+    pub fn len(&self) -> usize {
+        self.map.len()
+    }
+
+    /// Returns whether the map is currently empty.
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Returns a reference to the map's hasher.
+    pub fn hasher(&self) -> &S {
+        self.map.hasher()
+    }
+
+    /// Clears the map of all key-value pairs.
+    pub fn clear(&mut self) {
+        self.map.clear();
+        // update the guard node if present
+        if !self.head.is_null() {
+            unsafe {
+                self.drop_entries();
+                (*self.head).prev = self.head;
+                (*self.head).next = self.head;
+            }
+        }
+    }
+
+    /// Returns a double-ended iterator visiting all key-value pairs in order of insertion.
+    /// Iterator element type is `(&'a K, &'a V)`
+    ///
+    /// # Examples
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert("a", 10);
+    /// map.insert("c", 30);
+    /// map.insert("b", 20);
+    ///
+    /// let mut iter = map.iter();
+    /// assert_eq!((&"a", &10), iter.next().unwrap());
+    /// assert_eq!((&"c", &30), iter.next().unwrap());
+    /// assert_eq!((&"b", &20), iter.next().unwrap());
+    /// assert_eq!(None, iter.next());
+    /// ```
+    pub fn iter(&self) -> Iter<K, V> {
+        let head = if self.head.is_null() {
+            ptr::null_mut()
+        } else {
+            unsafe { (*self.head).prev }
+        };
+        Iter {
+            head,
+            tail: self.head,
+            remaining: self.len(),
+            marker: marker::PhantomData,
+        }
+    }
+
+    /// Returns a double-ended iterator visiting all key-value pairs in order of insertion.
+    /// Iterator element type is `(&'a K, &'a mut V)`
+    /// # Examples
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert("a", 10);
+    /// map.insert("c", 30);
+    /// map.insert("b", 20);
+    ///
+    /// {
+    ///     let mut iter = map.iter_mut();
+    ///     let mut entry = iter.next().unwrap();
+    ///     assert_eq!(&"a", entry.0);
+    ///     *entry.1 = 17;
+    /// }
+    ///
+    /// assert_eq!(&17, map.get(&"a").unwrap());
+    /// ```
+    pub fn iter_mut(&mut self) -> IterMut<K, V> {
+        let head = if self.head.is_null() {
+            ptr::null_mut()
+        } else {
+            unsafe { (*self.head).prev }
+        };
+        IterMut {
+            head,
+            tail: self.head,
+            remaining: self.len(),
+            marker: marker::PhantomData,
+        }
+    }
+
+    /// Clears the map, returning all key-value pairs as an iterator. Keeps the
+    /// allocated memory for reuse.
+    ///
+    /// If the returned iterator is dropped before being fully consumed, it
+    /// drops the remaining key-value pairs. The returned iterator keeps a
+    /// mutable borrow on the vector to optimize its implementation.
+    ///
+    /// Current performance implications (why to use this over into_iter()):
+    ///
+    /// * Clears the inner HashMap instead of dropping it
+    /// * Puts all drained nodes in the free-list instead of deallocating them
+    /// * Avoids deallocating the sentinel node
+    pub fn drain(&mut self) -> Drain<K, V> {
+        let len = self.len();
+        // Map should be empty now, regardless of current state
+        self.map.clear();
+        let (head, tail) = if len != 0 {
+            // This is basically the same as IntoIter's impl, but we don't
+            // deallocate/drop anything. Instead we make the sentinel head node
+            // point at itself (same state you get from removing the last element from a map),
+            // and then append the entire list to the free list. At this point all the entries
+            // have essentially been fed into mem::forget. The Drain iterator will then iterate
+            // over those nodes in the freelist (using  `len` to know where to stop) and `read`
+            // the values out of the nodes, "unforgetting" them.
+            //
+            // This design results in no observable consequences for mem::forgetting the
+            // drain iterator, because the drain iterator has no responsibility to "fix up"
+            // things during iteration/destruction. That said, you will effectively mem::forget
+            // any elements that weren't yielded yet.
+            unsafe {
+                debug_assert!(!self.head.is_null());
+                debug_assert!(!(*self.head).prev.is_null());
+                debug_assert!((*self.head).prev != self.head);
+                let head = (*self.head).prev;
+                let tail = (*self.head).next;
+                (*self.head).prev = self.head;
+                (*self.head).next = self.head;
+                (*head).next = self.free;
+                (*tail).prev = ptr::null_mut();
+                self.free = tail;
+                (head, tail)
+            }
+        } else {
+            (ptr::null_mut(), ptr::null_mut())
+        };
+
+        Drain {
+            head,
+            tail,
+            remaining: len,
+            marker: marker::PhantomData,
+        }
+    }
+
+    /// Returns a double-ended iterator visiting all key in order of insertion.
+    ///
+    /// # Examples
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert('a', 10);
+    /// map.insert('c', 30);
+    /// map.insert('b', 20);
+    ///
+    /// let mut keys = map.keys();
+    /// assert_eq!(&'a', keys.next().unwrap());
+    /// assert_eq!(&'c', keys.next().unwrap());
+    /// assert_eq!(&'b', keys.next().unwrap());
+    /// assert_eq!(None, keys.next());
+    /// ```
+    pub fn keys(&self) -> Keys<K, V> {
+        Keys { inner: self.iter() }
+    }
+
+    /// Returns a double-ended iterator visiting all values in order of insertion.
+    ///
+    /// # Examples
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::new();
+    /// map.insert('a', 10);
+    /// map.insert('c', 30);
+    /// map.insert('b', 20);
+    ///
+    /// let mut values = map.values();
+    /// assert_eq!(&10, values.next().unwrap());
+    /// assert_eq!(&30, values.next().unwrap());
+    /// assert_eq!(&20, values.next().unwrap());
+    /// assert_eq!(None, values.next());
+    /// ```
+    pub fn values(&self) -> Values<K, V> {
+        Values { inner: self.iter() }
+    }
+}
+
+impl<'a, K, V, S, Q: ?Sized> Index<&'a Q> for LinkedHashMap<K, V, S>
+where
+    K: Hash + Eq + Borrow<Q>,
+    S: BuildHasher,
+    Q: Eq + Hash,
+{
+    type Output = V;
+
+    fn index(&self, index: &'a Q) -> &V {
+        self.get(index).expect("no entry found for key")
+    }
+}
+
+impl<'a, K, V, S, Q: ?Sized> IndexMut<&'a Q> for LinkedHashMap<K, V, S>
+where
+    K: Hash + Eq + Borrow<Q>,
+    S: BuildHasher,
+    Q: Eq + Hash,
+{
+    fn index_mut(&mut self, index: &'a Q) -> &mut V {
+        self.get_mut(index).expect("no entry found for key")
+    }
+}
+
+impl<K: Hash + Eq + Clone, V: Clone, S: BuildHasher + Clone> Clone for LinkedHashMap<K, V, S> {
+    fn clone(&self) -> Self {
+        let mut map = Self::with_hasher(self.map.hasher().clone());
+        map.extend(self.iter().map(|(k, v)| (k.clone(), v.clone())));
+        map
+    }
+}
+
+impl<K: Hash + Eq, V, S: BuildHasher + Default> Default for LinkedHashMap<K, V, S> {
+    fn default() -> Self {
+        Self::with_hasher(S::default())
+    }
+}
+
+impl<K: Hash + Eq, V, S: BuildHasher> Extend<(K, V)> for LinkedHashMap<K, V, S> {
+    fn extend<I: IntoIterator<Item = (K, V)>>(&mut self, iter: I) {
+        for (k, v) in iter {
+            self.insert(k, v);
+        }
+    }
+}
+
+impl<'a, K, V, S> Extend<(&'a K, &'a V)> for LinkedHashMap<K, V, S>
+where
+    K: 'a + Hash + Eq + Copy,
+    V: 'a + Copy,
+    S: BuildHasher,
+{
+    fn extend<I: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: I) {
+        for (&k, &v) in iter {
+            self.insert(k, v);
+        }
+    }
+}
+
+impl<K: Hash + Eq, V, S: BuildHasher + Default> iter::FromIterator<(K, V)>
+    for LinkedHashMap<K, V, S>
+{
+    fn from_iter<I: IntoIterator<Item = (K, V)>>(iter: I) -> Self {
+        let iter = iter.into_iter();
+        let mut map = Self::with_capacity_and_hasher(iter.size_hint().0, S::default());
+        map.extend(iter);
+        map
+    }
+}
+
+impl<A: fmt::Debug + Hash + Eq, B: fmt::Debug, S: BuildHasher> fmt::Debug
+    for LinkedHashMap<A, B, S>
+{
+    /// Returns a string that lists the key-value pairs in insertion order.
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_map().entries(self).finish()
+    }
+}
+
+impl<K: Hash + Eq, V: PartialEq, S: BuildHasher> PartialEq for LinkedHashMap<K, V, S> {
+    fn eq(&self, other: &Self) -> bool {
+        self.len() == other.len() && self.iter().eq(other)
+    }
+}
+
+impl<K: Hash + Eq, V: Eq, S: BuildHasher> Eq for LinkedHashMap<K, V, S> {}
+
+impl<K: Hash + Eq + PartialOrd, V: PartialOrd, S: BuildHasher> PartialOrd
+    for LinkedHashMap<K, V, S>
+{
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        self.iter().partial_cmp(other)
+    }
+
+    fn lt(&self, other: &Self) -> bool {
+        self.iter().lt(other)
+    }
+
+    fn le(&self, other: &Self) -> bool {
+        self.iter().le(other)
+    }
+
+    fn ge(&self, other: &Self) -> bool {
+        self.iter().ge(other)
+    }
+
+    fn gt(&self, other: &Self) -> bool {
+        self.iter().gt(other)
+    }
+}
+
+impl<K: Hash + Eq + Ord, V: Ord, S: BuildHasher> Ord for LinkedHashMap<K, V, S> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.iter().cmp(other)
+    }
+}
+
+impl<K: Hash + Eq, V: Hash, S: BuildHasher> Hash for LinkedHashMap<K, V, S> {
+    fn hash<H: Hasher>(&self, h: &mut H) {
+        for e in self.iter() {
+            e.hash(h);
+        }
+    }
+}
+
+unsafe impl<K: Send, V: Send, S: Send> Send for LinkedHashMap<K, V, S> {}
+
+unsafe impl<K: Sync, V: Sync, S: Sync> Sync for LinkedHashMap<K, V, S> {}
+
+impl<K, V, S> Drop for LinkedHashMap<K, V, S> {
+    fn drop(&mut self) {
+        if !self.head.is_null() {
+            unsafe {
+                self.drop_entries();
+                drop_empty_node(self.head);
+            }
+        }
+        self.clear_free_list();
+    }
+}
+
+/// An insertion-order iterator over a `LinkedHashMap`'s entries, with immutable references to the
+/// values.
+pub struct Iter<'a, K: 'a, V: 'a> {
+    head: *const Node<K, V>,
+    tail: *const Node<K, V>,
+    remaining: usize,
+    marker: marker::PhantomData<(&'a K, &'a V)>,
+}
+
+/// An insertion-order iterator over a `LinkedHashMap`'s entries, with mutable references to the
+/// values.
+pub struct IterMut<'a, K: 'a, V: 'a> {
+    head: *mut Node<K, V>,
+    tail: *mut Node<K, V>,
+    remaining: usize,
+    marker: marker::PhantomData<(&'a K, &'a mut V)>,
+}
+
+/// A consuming insertion-order iterator over a `LinkedHashMap`'s entries.
+pub struct IntoIter<K, V> {
+    head: *mut Node<K, V>,
+    tail: *mut Node<K, V>,
+    remaining: usize,
+    marker: marker::PhantomData<(K, V)>,
+}
+
+/// A draining insertion-order iterator over a `LinkedHashMap`'s entries.
+pub struct Drain<'a, K, V> {
+    head: *mut Node<K, V>,
+    tail: *mut Node<K, V>,
+    remaining: usize,
+    marker: marker::PhantomData<&'a mut (K, V)>,
+}
+
+/// An insertion-order iterator over a `LinkedHashMap`'s entries represented as
+/// an `OccupiedEntry`.
+pub struct Entries<'a, K: 'a, V: 'a, S: 'a = hash_map::RandomState> {
+    map: *mut LinkedHashMap<K, V, S>,
+    head: *mut Node<K, V>,
+    remaining: usize,
+    marker: marker::PhantomData<(&'a K, &'a mut V, &'a S)>,
+}
+
+unsafe impl<'a, K, V> Send for Iter<'a, K, V>
+where
+    K: Send,
+    V: Send,
+{
+}
+
+unsafe impl<'a, K, V> Send for IterMut<'a, K, V>
+where
+    K: Send,
+    V: Send,
+{
+}
+
+unsafe impl<'a, K, V> Send for Drain<'a, K, V>
+where
+    K: Send,
+    V: Send,
+{
+}
+
+unsafe impl<K, V> Send for IntoIter<K, V>
+where
+    K: Send,
+    V: Send,
+{
+}
+
+unsafe impl<'a, K, V, S> Send for Entries<'a, K, V, S>
+where
+    K: Send,
+    V: Send,
+    S: Send,
+{
+}
+
+unsafe impl<'a, K, V> Sync for Iter<'a, K, V>
+where
+    K: Sync,
+    V: Sync,
+{
+}
+
+unsafe impl<'a, K, V> Sync for IterMut<'a, K, V>
+where
+    K: Sync,
+    V: Sync,
+{
+}
+
+unsafe impl<'a, K, V> Sync for Drain<'a, K, V>
+where
+    K: Sync,
+    V: Sync,
+{
+}
+unsafe impl<K, V> Sync for IntoIter<K, V>
+where
+    K: Sync,
+    V: Sync,
+{
+}
+
+unsafe impl<'a, K, V, S> Sync for Entries<'a, K, V, S>
+where
+    K: Sync,
+    V: Sync,
+    S: Sync,
+{
+}
+
+impl<'a, K, V> Clone for Iter<'a, K, V> {
+    fn clone(&self) -> Self {
+        Iter { ..*self }
+    }
+}
+
+impl<K, V> Clone for IntoIter<K, V>
+where
+    K: Clone,
+    V: Clone,
+{
+    fn clone(&self) -> Self {
+        if self.remaining == 0 {
+            return IntoIter { ..*self };
+        }
+
+        fn clone_node<K, V>(e: *mut Node<K, V>) -> *mut Node<K, V>
+        where
+            K: Clone,
+            V: Clone,
+        {
+            Box::into_raw(Box::new(Node::new(unsafe { (*e).key.clone() }, unsafe {
+                (*e).value.clone()
+            })))
+        }
+
+        let mut cur = self.head;
+        let head = clone_node(cur);
+        let mut tail = head;
+        for _ in 1..self.remaining {
+            unsafe {
+                (*tail).prev = clone_node((*cur).prev);
+                (*(*tail).prev).next = tail;
+                tail = (*tail).prev;
+                cur = (*cur).prev;
+            }
+        }
+
+        IntoIter {
+            head,
+            tail,
+            remaining: self.remaining,
+            marker: marker::PhantomData,
+        }
+    }
+}
+
+impl<'a, K, V> Iterator for Iter<'a, K, V> {
+    type Item = (&'a K, &'a V);
+
+    fn next(&mut self) -> Option<(&'a K, &'a V)> {
+        if self.head == self.tail {
+            None
+        } else {
+            self.remaining -= 1;
+            unsafe {
+                let r = Some((&(*self.head).key, &(*self.head).value));
+                self.head = (*self.head).prev;
+                r
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+impl<'a, K, V> Iterator for IterMut<'a, K, V> {
+    type Item = (&'a K, &'a mut V);
+
+    fn next(&mut self) -> Option<(&'a K, &'a mut V)> {
+        if self.head == self.tail {
+            None
+        } else {
+            self.remaining -= 1;
+            unsafe {
+                let r = Some((&(*self.head).key, &mut (*self.head).value));
+                self.head = (*self.head).prev;
+                r
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+impl<K, V> Iterator for IntoIter<K, V> {
+    type Item = (K, V);
+
+    fn next(&mut self) -> Option<(K, V)> {
+        if self.remaining == 0 {
+            return None;
+        }
+        self.remaining -= 1;
+        unsafe {
+            let prev = (*self.head).prev;
+            let e = *Box::from_raw(self.head);
+            self.head = prev;
+            Some((e.key, e.value))
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+impl<'a, K, V> Iterator for Drain<'a, K, V> {
+    type Item = (K, V);
+
+    fn next(&mut self) -> Option<(K, V)> {
+        if self.remaining == 0 {
+            return None;
+        }
+        self.remaining -= 1;
+        unsafe {
+            let prev = (*self.head).prev;
+            // Read the values out, the node is in the free-list already so these will
+            // be treated as uninit memory.
+            let k = addr_of_mut!((*self.head).key).read();
+            let v = addr_of_mut!((*self.head).value).read();
+            self.head = prev;
+            Some((k, v))
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+impl<'a, K, V> DoubleEndedIterator for Drain<'a, K, V> {
+    fn next_back(&mut self) -> Option<(K, V)> {
+        if self.remaining == 0 {
+            return None;
+        }
+        self.remaining -= 1;
+        unsafe {
+            let next = (*self.tail).next;
+            // Read the values out, the node is in the free-list already so these will
+            // be treated as uninit memory.
+            let k = addr_of_mut!((*self.tail).key).read();
+            let v = addr_of_mut!((*self.tail).value).read();
+            self.tail = next;
+            Some((k, v))
+        }
+    }
+}
+
+impl<'a, K, V> ExactSizeIterator for Drain<'a, K, V> {
+    fn len(&self) -> usize {
+        self.remaining
+    }
+}
+
+impl<'a, K, V, S: BuildHasher> Iterator for Entries<'a, K, V, S> {
+    type Item = OccupiedEntry<'a, K, V, S>;
+
+    fn next(&mut self) -> Option<OccupiedEntry<'a, K, V, S>> {
+        if self.remaining == 0 {
+            None
+        } else {
+            self.remaining -= 1;
+            unsafe {
+                let r = Some(OccupiedEntry {
+                    map: self.map,
+                    entry: self.head,
+                    marker: marker::PhantomData,
+                });
+
+                self.head = (*self.head).prev;
+                r
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.remaining, Some(self.remaining))
+    }
+}
+
+impl<'a, K, V> DoubleEndedIterator for Iter<'a, K, V> {
+    fn next_back(&mut self) -> Option<(&'a K, &'a V)> {
+        if self.head == self.tail {
+            None
+        } else {
+            self.remaining -= 1;
+            unsafe {
+                self.tail = (*self.tail).next;
+                Some((&(*self.tail).key, &(*self.tail).value))
+            }
+        }
+    }
+}
+
+impl<'a, K, V> DoubleEndedIterator for IterMut<'a, K, V> {
+    fn next_back(&mut self) -> Option<(&'a K, &'a mut V)> {
+        if self.head == self.tail {
+            None
+        } else {
+            self.remaining -= 1;
+            unsafe {
+                self.tail = (*self.tail).next;
+                Some((&(*self.tail).key, &mut (*self.tail).value))
+            }
+        }
+    }
+}
+
+impl<K, V> DoubleEndedIterator for IntoIter<K, V> {
+    fn next_back(&mut self) -> Option<(K, V)> {
+        if self.remaining == 0 {
+            return None;
+        }
+        self.remaining -= 1;
+        unsafe {
+            let next = (*self.tail).next;
+            let e = *Box::from_raw(self.tail);
+            self.tail = next;
+            Some((e.key, e.value))
+        }
+    }
+}
+
+impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> {
+    fn len(&self) -> usize {
+        self.remaining
+    }
+}
+
+impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> {
+    fn len(&self) -> usize {
+        self.remaining
+    }
+}
+
+impl<K, V> ExactSizeIterator for IntoIter<K, V> {
+    fn len(&self) -> usize {
+        self.remaining
+    }
+}
+
+impl<K, V> Drop for IntoIter<K, V> {
+    fn drop(&mut self) {
+        for _ in 0..self.remaining {
+            unsafe {
+                let next = (*self.tail).next;
+                Box::from_raw(self.tail);
+                self.tail = next;
+            }
+        }
+    }
+}
+
+impl<'a, K, V> Drop for Drain<'a, K, V> {
+    fn drop(&mut self) {
+        for _ in self {}
+    }
+}
+
+/// An insertion-order iterator over a `LinkedHashMap`'s keys.
+pub struct Keys<'a, K: 'a, V: 'a> {
+    inner: Iter<'a, K, V>,
+}
+
+impl<'a, K, V> Clone for Keys<'a, K, V> {
+    fn clone(&self) -> Self {
+        Keys {
+            inner: self.inner.clone(),
+        }
+    }
+}
+
+impl<'a, K, V> Iterator for Keys<'a, K, V> {
+    type Item = &'a K;
+
+    #[inline]
+    fn next(&mut self) -> Option<&'a K> {
+        self.inner.next().map(|e| e.0)
+    }
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl<'a, K, V> DoubleEndedIterator for Keys<'a, K, V> {
+    #[inline]
+    fn next_back(&mut self) -> Option<&'a K> {
+        self.inner.next_back().map(|e| e.0)
+    }
+}
+
+impl<'a, K, V> ExactSizeIterator for Keys<'a, K, V> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+/// An insertion-order iterator over a `LinkedHashMap`'s values.
+pub struct Values<'a, K: 'a, V: 'a> {
+    inner: Iter<'a, K, V>,
+}
+
+impl<'a, K, V> Clone for Values<'a, K, V> {
+    fn clone(&self) -> Self {
+        Values {
+            inner: self.inner.clone(),
+        }
+    }
+}
+
+impl<'a, K, V> Iterator for Values<'a, K, V> {
+    type Item = &'a V;
+
+    #[inline]
+    fn next(&mut self) -> Option<&'a V> {
+        self.inner.next().map(|e| e.1)
+    }
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl<'a, K, V> DoubleEndedIterator for Values<'a, K, V> {
+    #[inline]
+    fn next_back(&mut self) -> Option<&'a V> {
+        self.inner.next_back().map(|e| e.1)
+    }
+}
+
+impl<'a, K, V> ExactSizeIterator for Values<'a, K, V> {
+    fn len(&self) -> usize {
+        self.inner.len()
+    }
+}
+
+impl<'a, K: Hash + Eq, V, S: BuildHasher> IntoIterator for &'a LinkedHashMap<K, V, S> {
+    type Item = (&'a K, &'a V);
+    type IntoIter = Iter<'a, K, V>;
+    fn into_iter(self) -> Iter<'a, K, V> {
+        self.iter()
+    }
+}
+
+impl<'a, K: Hash + Eq, V, S: BuildHasher> IntoIterator for &'a mut LinkedHashMap<K, V, S> {
+    type Item = (&'a K, &'a mut V);
+    type IntoIter = IterMut<'a, K, V>;
+    fn into_iter(self) -> IterMut<'a, K, V> {
+        self.iter_mut()
+    }
+}
+
+impl<K: Hash + Eq, V, S: BuildHasher> IntoIterator for LinkedHashMap<K, V, S> {
+    type Item = (K, V);
+    type IntoIter = IntoIter<K, V>;
+    fn into_iter(mut self) -> IntoIter<K, V> {
+        let (head, tail) = if !self.head.is_null() {
+            unsafe { ((*self.head).prev, (*self.head).next) }
+        } else {
+            (ptr::null_mut(), ptr::null_mut())
+        };
+        let len = self.len();
+
+        if !self.head.is_null() {
+            unsafe { drop_empty_node(self.head) }
+        }
+        self.clear_free_list();
+        // drop the HashMap but not the LinkedHashMap
+        unsafe {
+            ptr::drop_in_place(&mut self.map);
+        }
+        mem::forget(self);
+
+        IntoIter {
+            head,
+            tail,
+            remaining: len,
+            marker: marker::PhantomData,
+        }
+    }
+}
+
+/// A view into a single location in a map, which may be vacant or occupied.
+pub enum Entry<'a, K: 'a, V: 'a, S: 'a = hash_map::RandomState> {
+    /// An occupied Entry.
+    Occupied(OccupiedEntry<'a, K, V, S>),
+    /// A vacant Entry.
+    Vacant(VacantEntry<'a, K, V, S>),
+}
+
+/// A view into a single occupied location in a `LinkedHashMap`.
+pub struct OccupiedEntry<'a, K: 'a, V: 'a, S: 'a = hash_map::RandomState> {
+    entry: *mut Node<K, V>,
+    map: *mut LinkedHashMap<K, V, S>,
+    marker: marker::PhantomData<&'a K>,
+}
+
+/// A view into a single empty location in a `LinkedHashMap`.
+pub struct VacantEntry<'a, K: 'a, V: 'a, S: 'a = hash_map::RandomState> {
+    key: K,
+    map: &'a mut LinkedHashMap<K, V, S>,
+}
+
+impl<'a, K: Hash + Eq, V, S: BuildHasher> Entry<'a, K, V, S> {
+    /// Returns the entry key
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::<String, u32>::new();
+    ///
+    /// assert_eq!("hello", map.entry("hello".to_string()).key());
+    /// ```
+    pub fn key(&self) -> &K {
+        match *self {
+            Entry::Occupied(ref e) => e.key(),
+            Entry::Vacant(ref e) => e.key(),
+        }
+    }
+
+    /// Ensures a value is in the entry by inserting the default if empty, and returns
+    /// a mutable reference to the value in the entry.
+    pub fn or_insert(self, default: V) -> &'a mut V {
+        match self {
+            Entry::Occupied(entry) => entry.into_mut(),
+            Entry::Vacant(entry) => entry.insert(default),
+        }
+    }
+
+    /// Ensures a value is in the entry by inserting the result of the default function if empty,
+    /// and returns a mutable reference to the value in the entry.
+    pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
+        match self {
+            Entry::Occupied(entry) => entry.into_mut(),
+            Entry::Vacant(entry) => entry.insert(default()),
+        }
+    }
+
+    /// Provides in-place mutable access to an occupied entry before any
+    /// potential inserts into the map.
+    pub fn and_modify<F>(self, f: F) -> Self
+    where
+        F: FnOnce(&mut V),
+    {
+        match self {
+            Entry::Occupied(mut entry) => {
+                f(entry.get_mut());
+                Entry::Occupied(entry)
+            }
+            Entry::Vacant(entry) => Entry::Vacant(entry),
+        }
+    }
+
+    /// Ensures a value is in the entry by inserting the default value if empty,
+    /// and returns a mutable reference to the value in the entry.
+    pub fn or_default(self) -> &'a mut V
+    where
+        V: Default,
+    {
+        match self {
+            Entry::Occupied(entry) => entry.into_mut(),
+            Entry::Vacant(entry) => entry.insert(V::default()),
+        }
+    }
+}
+
+impl<'a, K: Hash + Eq, V, S: BuildHasher> OccupiedEntry<'a, K, V, S> {
+    /// Gets a reference to the entry key
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::new();
+    ///
+    /// map.insert("foo".to_string(), 1);
+    /// assert_eq!("foo", map.entry("foo".to_string()).key());
+    /// ```
+    pub fn key(&self) -> &K {
+        unsafe { &(*self.entry).key }
+    }
+
+    /// Gets a reference to the value in the entry.
+    pub fn get(&self) -> &V {
+        unsafe { &(*self.entry).value }
+    }
+
+    /// Gets a mutable reference to the value in the entry.
+    pub fn get_mut(&mut self) -> &mut V {
+        unsafe { &mut (*self.entry).value }
+    }
+
+    /// Converts the OccupiedEntry into a mutable reference to the value in the entry
+    /// with a lifetime bound to the map itself
+    pub fn into_mut(self) -> &'a mut V {
+        unsafe { &mut (*self.entry).value }
+    }
+
+    /// Sets the value of the entry, and returns the entry's old value
+    pub fn insert(&mut self, value: V) -> V {
+        unsafe {
+            (*self.map).ensure_guard_node();
+
+            let old_val = mem::replace(&mut (*self.entry).value, value);
+            let node_ptr: *mut Node<K, V> = self.entry;
+
+            // Existing node, just update LRU position
+            (*self.map).detach(node_ptr);
+            (*self.map).attach(node_ptr);
+
+            old_val
+        }
+    }
+
+    /// Takes the value out of the entry, and returns it
+    pub fn remove(self) -> V {
+        unsafe { (*self.map).remove(&(*self.entry).key) }.unwrap()
+    }
+}
+
+impl<'a, K: 'a + Hash + Eq, V: 'a, S: BuildHasher> VacantEntry<'a, K, V, S> {
+    /// Gets a reference to the entry key
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use linked_hash_map::LinkedHashMap;
+    ///
+    /// let mut map = LinkedHashMap::<String, u32>::new();
+    ///
+    /// assert_eq!("foo", map.entry("foo".to_string()).key());
+    /// ```
+    pub fn key(&self) -> &K {
+        &self.key
+    }
+
+    /// Sets the value of the entry with the VacantEntry's key,
+    /// and returns a mutable reference to it
+    pub fn insert(self, value: V) -> &'a mut V {
+        self.map.ensure_guard_node();
+
+        let node = if self.map.free.is_null() {
+            Box::into_raw(Box::new(Node::new(self.key, value)))
+        } else {
+            // use a recycled box
+            unsafe {
+                let free = self.map.free;
+                self.map.free = (*free).next;
+                ptr::write(free, Node::new(self.key, value));
+                free
+            }
+        };
+
+        let keyref = unsafe { &(*node).key };
+
+        self.map.attach(node);
+
+        let ret = self.map.map.entry(KeyRef { k: keyref }).or_insert(node);
+        unsafe { &mut (**ret).value }
+    }
+}
diff --git a/src/serde.rs b/src/serde.rs
new file mode 100644
index 0000000..0690d11
--- /dev/null
+++ b/src/serde.rs
@@ -0,0 +1,101 @@
+//! An optional implementation of serialization/deserialization.
+
+extern crate serde;
+
+use std::fmt::{Formatter, Result as FmtResult};
+use std::hash::{BuildHasher, Hash};
+use std::marker::PhantomData;
+
+use super::LinkedHashMap;
+
+use self::serde::de::{Error, MapAccess, Visitor};
+use self::serde::ser::SerializeMap;
+use self::serde::{Deserialize, Deserializer, Serialize, Serializer};
+
+impl<K, V, S> Serialize for LinkedHashMap<K, V, S>
+where
+    K: Serialize + Eq + Hash,
+    V: Serialize,
+    S: BuildHasher,
+{
+    #[inline]
+    fn serialize<T>(&self, serializer: T) -> Result<T::Ok, T::Error>
+    where
+        T: Serializer,
+    {
+        let mut map_serializer = serializer.serialize_map(Some(self.len()))?;
+        for (k, v) in self {
+            map_serializer.serialize_key(k)?;
+            map_serializer.serialize_value(v)?;
+        }
+        map_serializer.end()
+    }
+}
+
+#[derive(Debug)]
+/// `serde::de::Visitor` for a linked hash map.
+pub struct LinkedHashMapVisitor<K, V> {
+    marker: PhantomData<LinkedHashMap<K, V>>,
+}
+
+impl<K, V> LinkedHashMapVisitor<K, V> {
+    /// Creates a new visitor for a linked hash map.
+    pub fn new() -> Self {
+        LinkedHashMapVisitor {
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<K, V> Default for LinkedHashMapVisitor<K, V> {
+    fn default() -> Self {
+        LinkedHashMapVisitor::new()
+    }
+}
+
+impl<'de, K, V> Visitor<'de> for LinkedHashMapVisitor<K, V>
+where
+    K: Deserialize<'de> + Eq + Hash,
+    V: Deserialize<'de>,
+{
+    type Value = LinkedHashMap<K, V>;
+
+    fn expecting(&self, formatter: &mut Formatter) -> FmtResult {
+        write!(formatter, "a map")
+    }
+
+    #[inline]
+    fn visit_unit<E>(self) -> Result<Self::Value, E>
+    where
+        E: Error,
+    {
+        Ok(LinkedHashMap::new())
+    }
+
+    #[inline]
+    fn visit_map<M>(self, mut map: M) -> Result<Self::Value, M::Error>
+    where
+        M: MapAccess<'de>,
+    {
+        let mut values = LinkedHashMap::with_capacity(map.size_hint().unwrap_or(0));
+
+        while let Some((key, value)) = map.next_entry()? {
+            values.insert(key, value);
+        }
+
+        Ok(values)
+    }
+}
+
+impl<'de, K, V> Deserialize<'de> for LinkedHashMap<K, V>
+where
+    K: Deserialize<'de> + Eq + Hash,
+    V: Deserialize<'de>,
+{
+    fn deserialize<D>(deserializer: D) -> Result<LinkedHashMap<K, V>, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_map(LinkedHashMapVisitor::new())
+    }
+}