## 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.