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Merge pull request rust-lang#50 from hcpl/readme-fixes
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Tweak README
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bluss authored Dec 1, 2017
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Expand Up @@ -17,56 +17,56 @@ __ https://docs.rs/ordermap/
Background
==========

This was inpired by Python 3.6's new dict implementation (which remembers
This was inspired by Python 3.6's new dict implementation (which remembers
the insertion order and is fast to iterate, and is compact in memory).

Some of those features were translated to Rust, and some were not. The result
was ordermap, a hash table that is
was ordermap, a hash table that has following properties:

- Order is **independent of hash function** and hash values of keys
- Fast to iterate
- Indexed in compact space
- Preserves insertion order **as long** as you don't call ``remove``.
- Using robin hood hashing just like Rust's libstd HashMap.
- Order is **independent of hash function** and hash values of keys.
- Fast to iterate.
- Indexed in compact space.
- Preserves insertion order **as long** as you don't call ``.remove()``.
- Uses robin hood hashing just like Rust's libstd ``HashMap``.

- It's the usual backwards shift deletion, but only on the index vector, so
it's cheaper because it's moving less memory around.

Does not implement (Yet)
------------------------

- ``.reserve`` exists but does not have a full implementation
- ``.reserve()`` exists but does not have a full implementation

Performance
-----------

OrderMap derives a couple of performance facts directly from how it is constructed,
``OrderMap`` derives a couple of performance facts directly from how it is constructed,
which is roughly:

Two vectors, the first, sparse, with hashes and key-value indices, and the
second, dense, the key-value pairs.

- Iteration is very fast since it is on the dense key-values.
- Removal is fast since it moves memory areas only in the first vector,
and uses a single swap in the second vector
and uses a single swap in the second vector.
- Lookup is fast-ish because the hashes and indices are densely stored.
Lookup also is slow-ish since hashes and key-value pairs are stored in
separate places. (Visible when cpu caches size is limiting.)

- In practice, OrderMap has been tested out as the hashmap in rustc in PR45282_ and
the perforamnce was roughly on par across the whole workload.
- If you want the properties of OrderMap, or its strongest performance points
- In practice, ``OrderMap`` has been tested out as the hashmap in rustc in PR45282_ and
the performance was roughly on par across the whole workload.
- If you want the properties of ``OrderMap``, or its strongest performance points
fits your workload, it might be the best hash table implementation.

.. _PR45282: https://github.com/rust-lang/rust/pull/45282

Interesting Features
--------------------

- Insertion order is preserved (swap_remove perturbs the order, like the method name says)
- Implements .pop() -> Option<(K, V)> in O(1) time
- OrderMap::new() is empty and uses no allocation until you insert something
- Lookup key-value pairs by index and vice versa
- Insertion order is preserved (``.swap_remove()`` perturbs the order, like the method name says).
- Implements ``.pop() -> Option<(K, V)>`` in O(1) time.
- ``OrderMap::new()`` is empty and uses no allocation until you insert something.
- Lookup key-value pairs by index and vice versa.
- No ``unsafe``.
- Supports ``IndexMut``.

Expand All @@ -76,19 +76,19 @@ Where to go from here?

- Ideas and PRs for how to implement insertion-order preserving remove (for example tombstones)
are welcome. The plan is to split the crate into two hash table implementations
a) the current compact index space version and b) the full insertion order version
a) the current compact index space version and b) the full insertion order version.


Ideas that we already did
-------------------------

- It can be an *indexable* ordered map in the current fashion.
- It can be an *indexable* ordered map in the current fashion
(This was implemented in 0.2.0, for potential use as a graph datastructure).

- Idea for more cache efficient lookup (This was implemented in 0.1.2)
- Idea for more cache efficient lookup (This was implemented in 0.1.2).

Current ``indices: Vec<Pos>``. ``Pos`` is interpreted as ``(u32, u32)`` more
or less when raw_capacity() fits in 32 bits. Pos then stores both the lower
or less when ``.raw_capacity()`` fits in 32 bits. ``Pos`` then stores both the lower
half of the hash and the entry index.
This means that the hash values in ``Bucket`` don't need to be accessed
while scanning for an entry.
Expand All @@ -100,62 +100,62 @@ Recent Changes
- 0.3.2

- Add ``OrderSet`` by @cuviper!
- ``OrderMap::drain`` is now (too) a double ended iterator
- ``OrderMap::drain`` is now (too) a double ended iterator.

- 0.3.1

- In all ordermap iterators, forward the ``collect`` method to the underlying
iterator as well.
- Add crates.io categories
- Add crates.io categories.

- 0.3.0

- The methods ``get_pair, get_pair_index`` were both replaced by
- The methods ``get_pair``, ``get_pair_index`` were both replaced by
``get_full`` (and the same for the mutable case).
- Method ``swap_remove_pair`` replaced by ``swap_remove_full``
- Method ``swap_remove_pair`` replaced by ``swap_remove_full``.
- Add trait ``MutableKeys`` for opt-in mutable key access. Mutable key access
is only possible through the methods of this extension trait.
- Add new trait ``Equivalent`` for key equivalence. This extends the
``Borrow`` trait mechanism for ``OrderMap::get`` in a backwards compatible
way, just some minor type inference related issues may become apparent.
See `#10`__ for more information.
- Implement ``Extend<(&K, &V)>`` by @xfix
- Implement ``Extend<(&K, &V)>`` by @xfix.

__ https://github.com/bluss/ordermap/pull/10

- 0.2.13

- Fix deserialization to support custom hashers by @Techcable
- Add methods ``.index()`` on the entry types by @garro95
- Fix deserialization to support custom hashers by @Techcable.
- Add methods ``.index()`` on the entry types by @garro95.

- 0.2.12

- Add methods ``.with_hasher()``, ``.hasher()``
- Add methods ``.with_hasher()``, ``.hasher()``.

- 0.2.11

- Support ``ExactSizeIterator`` for the iterators. By @Binero
- Use ``Box<[Pos]>`` internally, saving a word in the OrderMap struct.
- Serde support, with crate feature ``"serde-1"``. By @xfix
- Support ``ExactSizeIterator`` for the iterators. By @Binero.
- Use ``Box<[Pos]>`` internally, saving a word in the ``OrderMap`` struct.
- Serde support, with crate feature ``"serde-1"``. By @xfix.

- 0.2.10

- Add iterator ``.drain(..)`` by @stevej
- Add iterator ``.drain(..)`` by @stevej.

- 0.2.9

- Add method ``.is_empty()`` by @overvenus
- Implement ``PartialEq, Eq`` by @overvenus
- Add method ``.sorted_by()``
- Add method ``.is_empty()`` by @overvenus.
- Implement ``PartialEq, Eq`` by @overvenus.
- Add method ``.sorted_by()``.

- 0.2.8

- Add iterators ``values()`` and ``.values_mut()``
- Fix compatibility with 32-bit platforms
- Add iterators ``.values()`` and ``.values_mut()``.
- Fix compatibility with 32-bit platforms.

- 0.2.7

- Add ``.retain()``
- Add ``.retain()``.

- 0.2.6

Expand All @@ -164,44 +164,46 @@ __ https://github.com/bluss/ordermap/pull/10

- 0.2.5

- Improved .pop() slightly
- Improved ``.pop()`` slightly.

- 0.2.4

- Improved performance of .insert() (#3) by pczarn
- Improved performance of ``.insert()`` (`#3`__) by @pczarn.

__ https://github.com/bluss/ordermap/pull/3

- 0.2.3

- Generalize ``Entry`` for now, so that it works on hashmaps with non-default
hasher. However, there's a lingering compat issue since libstd HashMap
hasher. However, there's a lingering compat issue since libstd ``HashMap``
does not parameterize its entries by the hasher (``S`` typarm).
- Special case some iterator methods like ``.nth()``
- Special case some iterator methods like ``.nth()``.

- 0.2.2

- Disable the verbose Debug impl by default.
- Disable the verbose ``Debug`` impl by default.

- 0.2.1

- Fix doc links and clarify docs
- Fix doc links and clarify docs.

- 0.2.0

- Add more HashMap methods & compat with its API
- Experimental support for ``.entry()`` (the simplest parts of the API)
- Add ``.reserve()`` (placeholder impl)
- Add ``.remove()`` as synonym for ``.swap_remove()``
- Add more ``HashMap`` methods & compat with its API.
- Experimental support for ``.entry()`` (the simplest parts of the API).
- Add ``.reserve()`` (placeholder impl).
- Add ``.remove()`` as synonym for ``.swap_remove()``.
- Changed ``.insert()`` to swap value if the entry already exists, and
return Option.
return ``Option``.
- Experimental support as an *indexed* hash map! Added methods
``.get_index(), .get_index_mut(), .swap_remove_index()``,
``.get_pair_index(), .get_pair_index_mut()``.
``.get_index()``, ``.get_index_mut()``, ``.swap_remove_index()``,
``.get_pair_index()``, ``.get_pair_index_mut()``.

- 0.1.2

- Implement the 32/32 split idea for ``Pos`` which improves cache utilization
and lookup performance
and lookup performance.

- 0.1.1

- Initial release
- Initial release.

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