Experimental hash table implementation in just Rust (stable, no unsafe code).

Please read the API documentation here__

__ https://docs.rs/ordermap/

|buildstatus| |crates|_

.. |crates| image:: https://img.shields.io/crates/v/ordermap.svg .. _crates: https://crates.io/crates/ordermap

.. |buildstatus| image:: https://travis-ci.org/bluss/ordermap.svg .. _buildstatus: https://travis-ci.org/bluss/ordermap

Background

This was inpired 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

• 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.

  • 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

• .reserve exists but does not have a full implementation

Performance

(Figures from late 2016)

• Iteration is very fast
• Lookup is faster than libstd HashMap for "small" tables (below something like 100 000 key-value pairs), but suffers under load more due to the index vec to entries vec indirection still.
• Growing the map is faster than libstd HashMap, doesn't need to move keys and values at all, only the index vec

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
• No unsafe.
• Supports IndexMut.

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

Ideas that we already did

• 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)

  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 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.

Recent Changes

• 0.3.0

  • 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
  • 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.

__ 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

• 0.2.12

  • 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

• 0.2.10

  • Add iterator .drain(..) by @stevej

• 0.2.9

  • 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

• 0.2.7

  • Add .retain()

• 0.2.6

  • Add OccupiedEntry::remove_entry and other minor entry methods, so that it now has all the features of HashMap's entries.

• 0.2.5

  • Improved .pop() slightly

• 0.2.4

  • Improved performance of .insert() (#3) by pczarn

• 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 does not parameterize its entries by the hasher (S typarm).
  • Special case some iterator methods like .nth()

• 0.2.2

  • Disable the verbose Debug impl by default.

• 0.2.1

  • 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()
  • Changed .insert() to swap value if the entry already exists, and 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().

• 0.1.2

  • Implement the 32/32 split idea for Pos which improves cache utilization and lookup performance

• 0.1.1

  • Initial release