A QP-trie ("Quelques-bits Popcount trie" or "Quad-bit Popcount trie") is a radix trie for keys which can be interpreted as a string of nybbles (where a nybble is half a byte, or four bits.) QP-tries are essentially Patricia tries which branch on nybbles instead of individual bits; as such, a QP-trie has a branching factor (and radix) of 16.
QP-tries as implemented in this crate are key-value maps for any keys which
implement Borrow<[u8]>. They are useful whenever you might need the same
operations as a HashMap or BTreeMap, but need either a bit more speed
(QP-tries are as fast or a bit faster as Rust's HashMap with the default
hasher) and/or the ability to efficiently query for sets of elements with a
given prefix.
QP-tries support efficient lookup/insertion/removal of individual elements, lookup/removal of sets of values with keys with a given prefix.
Keys can be any type which implements Borrow<[u8]>. Unfortunately at the
moment, this rules out String - while this trie can still be used to store
strings, it is necessary to manually convert them to byte slices and Vec<u8>s
for use as keys. Here's a naive, simple example of putting 9 2-element byte arrays
into the trie, and then removing all byte arrays which begin with "1":
```rust use qp_trie::Trie;
let mut trie = Trie::new();
for i in 0u8..3 { for j in 0u8..3 { trie.insert([i, j], i + j); } }
for i in 0u8..3 { trie.remove([1, i]); }
assert!(trie.iter().all(|(&key, _)| key[0] != 1)); ```
Here's a slightly less naive method, which is actually vastly more efficient:
```rust use qp_trie::Trie;
let mut trie = Trie::new();
for i in 0u8..3 { trie.extend((0u8..3).map(|j| ([i, j], i + j))); }
trie.remove_prefix([1]);
assert!(trie.iter().all(|(&key, _)| key[0] != 1)); ```
Although the extend bit really isn't any more efficient (it's difficult to
preallocate space for n elements in a trie) we're guaranteed that
trie.remove_prefix([1]) only actually removes a single node in the trie - the
parent node of all nodes with the given prefix. QP-tries, like all radix tries,
are extremely efficient when dealing with anything related to prefixes. This
extends to iteration over prefixes:
```rust use qp_trie::Trie;
let mut trie = Trie::new();
for i in 0u8..3 { trie.extend((0u8..3).map(|k| ([i, j], i + j))); }
let mut iter = trie.iter_prefix([1]);
asserteq!(iter.next(), Some((&[1, 0], &1))); asserteq!(iter.next(), Some((&[1, 1], &2))); asserteq!(iter.next(), Some((&[1, 2], &3))); asserteq!(iter.next(), None); ```
This crate originally started as a fork of the qptrie crate; however, I found
the code difficult to work with and full of unsafe pointer manipulation which I
felt could be avoided. To avoid a pull request which would essentially rewrite
the entire library I decided to write my own instead.
Several notable idiomatic features are provided which were missing from the qptrie crate:
- .iter() and .iter_mut() for immutable and mutable iteration over the key/value pairs of the trie
- qp_trie::Trie implements Extend and IntoIterator
- qp_trie::Trie implements Index and IndexMut
- qp_trie::Trie provides an "Entry API" with type signatures almost identical
to that provided by the std::collections BTreeMap and HashMap.
In addition to being written using safer code (failures which would otherwise
cause undefined behavior will cause panics when compiled with debug assertions
enabled) qp_trie::Trie is slightly faster than qptrie::Trie according to
benchmarks based on those shown in the qptrie repository.
Benchmarks are run against the qptrie crate and the Rust stdlib BTreeMap
and HashMap. qp_trie::Trie consistently outperforms the std::collections
BTreeMap and HashMap and also the qptrie crate's implementation on my
machine - a Chromebook Pixel 2.0 running GalliumOS.
Benchmarks can be reproduced using cargo bench. The Rust version used was
rustc 1.19.0-nightly (cfb5debbc 2017-06-12). Run several times, the
benchmarks are consistent in their outputs but I selected the lowest variance
results to display here.
Benchmarks named exotrie are using the qptrie::Trie implementation.
``` running 8 tests test benchbtreemapget ... bench: 114,172,574 ns/iter (+/- 10,890,962) test benchbtreemapinsert ... bench: 118,547,331 ns/iter (+/- 13,464,035) test benchexotrieget ... bench: 54,297,605 ns/iter (+/- 4,392,593) test benchexotrieinsert ... bench: 62,537,678 ns/iter (+/- 21,724,153) test benchhashmapget ... bench: 63,191,541 ns/iter (+/- 6,685,288) test benchhashmapinsert ... bench: 55,076,618 ns/iter (+/- 2,212,986) test benchtrieget ... bench: 48,232,553 ns/iter (+/- 6,583,801) test benchtrieinsert ... bench: 57,935,037 ns/iter (+/- 16,538,104)
test result: ok. 0 passed; 0 failed; 0 ignored; 8 measured; 0 filtered out ```
The qp-trie-rs crate is licensed under the MPL v2.0.