[go: up one dir, main page]

ordermap 0.2.2

A hash table with consistent order and fast iteration.
Documentation 
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96


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

This is inpired by Python 3.6's new dict implementation which remembers
the insertion order and is fast to iterate.

This implementation corresponds to their "compact hash map" idea as it is now,
and has consistent order.

Please read the `API documentation here`__

__ https://docs.rs/ordermap/

|build_status|_ |crates|_

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

.. |build_status| image:: https://travis-ci.org/bluss/ordermap.svg
.. _build_status: https://travis-ci.org/bluss/ordermap


Using robin hood hashing just like Rust's libstd HashMap.

- Has insert, lookup, grow
- Remove is implemented, but it perturbs the insertion order.
  It's the usual backwards shift deletion, but only on the index vector, so
  it's cheaper.
  Order-preserving removal would want to be implemented with tombstones,
  but I'm hesitant and not sure if it can be performant.

Performance:

- 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``.


Where to go from here?

- 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).
- Ideas and PRs for how to implement insertion-order preserving remove (for example tombstones)
  are welcome.

- 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
--------------

- 0.2.2

  - 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