qp-trie-rs: A QP-trie implementation in pure Rust

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.

Serialization/deserialization through Serde

Optionally, the qp_trie::Trie type supports (de-)serialization through serde. Enabling the serde feature will enable compilation of Deserialize and Serialize implementations for Trie.

When should I use a QP-trie?

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.

Examples

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":

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:

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:

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]);

assert_eq!(iter.next(), Some((&[1, 0], &1)));
assert_eq!(iter.next(), Some((&[1, 1], &2)));
assert_eq!(iter.next(), Some((&[1, 2], &3)));
assert_eq!(iter.next(), None);

Differences from the qptrie crate

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

Benchmarks are run against the qptrie crate, the Rust stdlib BTreeMap, and the stdlib HashMap with both default and FNV hashing. qp_trie::Trie consistently outperforms the std::collections BTreeMap and HashMap, as well as the qptrie crate's implementation.

Benchmarks named exotrie are using the qptrie::Trie implementation.

test bench_btreemap_get      ... bench: 111,468,098 ns/iter (+/- 10,103,247)
test bench_btreemap_insert   ... bench: 112,124,846 ns/iter (+/- 14,296,195)
test bench_exotrie_get       ... bench:  46,195,582 ns/iter (+/- 16,943,561)
test bench_exotrie_insert    ... bench:  52,886,847 ns/iter (+/- 15,574,538)
test bench_fnvhashmap_get    ... bench:   9,530,109 ns/iter (+/- 820,763)
test bench_fnvhashmap_insert ... bench:  21,281,107 ns/iter (+/- 7,254,084)
test bench_hashmap_get       ... bench:  49,653,426 ns/iter (+/- 7,004,051)
test bench_hashmap_insert    ... bench:  47,771,824 ns/iter (+/- 4,979,606)
test bench_trie_get          ... bench:  40,898,914 ns/iter (+/- 13,400,062)
test bench_trie_insert       ... bench:  50,966,392 ns/iter (+/- 18,077,240)

Future work

• Add wrapper types for String and str to make working with strings easier.

License

The qp-trie-rs crate is licensed under the MPL v2.0.