Crate prefix_trie 

Prefix-Trie

This crate provides prefix-map and prefix-set collections for IP prefixes and other fixed-width prefix types. PrefixMap is backed by a compact TreeBitMap-style trie and supports exact, longest-prefix, and shortest-prefix matches. The crate supports both IPv4 and IPv6 (from either ipnet, ipnetwork, or cidr). It also supports any tuple (R, u8), where R is any unsigned primitive integer (u8, u16, u32, u64, u128, or usize).

Prefixes are not stored verbatim. They are reconstructed from their trie position when returned from map and set operations, so host bits outside the prefix length are not preserved.

This crate also provides a JointPrefixMap and JointPrefixSet that contains two tables, one for IPv4 and one for IPv6.

Description of the Tree

PrefixMap stores the logical binary prefix trie in multi-bit nodes, as described by W. Eatherton, Z. Dittia, and G. Varghes. Each internal node covers five consecutive binary-trie levels. A node at depth d can hold values for prefixes with lengths d..=d+4, and it has up to 32 child slots for subtries rooted at depth d+5.

Each node stores two bitmaps: one for the value slots that are present in the node, and one for the child slots that are present below it. The allocators store multi-bit nodes and value cells in compact, linearized arrays, which improves cache locality and keeps lookup and traversal decisions local to a node. Physical slots are derived from the bitmaps with a popcount, avoiding one pointer per possible branch.

A stored entry is identified by its path through the trie and by a value bit inside the final multi-bit node. The prefix object passed to insert is not stored alongside the value. Returned prefixes are therefore reconstructed and canonicalized to the prefix length.

Trie Views and Set Operations

TrieView is a trait for immutable, mutable, and composed cursors into a trie. Concrete leaf views are TrieRef (created from &PrefixMap or &PrefixSet) and TrieRefMut (created from mutable references). Both are obtained through the AsView trait: call map.view() for a full-trie view or map.view_at(&prefix) for a non-empty subtrie.

Views and iterators traverse the logical prefix trie in lexicographic order and yield reconstructed owned prefixes together with references or owned values.

Set operations use the same view infrastructure. union, intersection, difference, covering_union, and covering_difference simultaneously traverse the involved trie views and yield results in lexicographic order. Covering variants also report longest-prefix matches from the opposite side where appropriate. The result of such a set operation is not the finished traversal, but a struct that implements TrieView and describes the traversal. Thus, you can chain multiple set operations and compute them in a single traversal of the original tries.

Aggregation

Both PrefixMap and PrefixSet can be aggregated, that is, finding a compact representation of the Trie to yield the same result for longest-prefix matches. For both the map and the set, this library has two versions of the aggregation: aggregate_consistent and aggregate. The consistent variant only drops nodes that are covered by an ancestor. Thus, get_lpm and is_covered always return the same value for any prefix. The non-consistent variant is also allowed to merge siblings with the same value. Thus, get_lpm and is_covered are only guaranteed to return the same value for addresses (i.e., /32 prefixes for IPv4). However, even with the non-consistent variant, the function is_covered_in_aggregate will always return the same result.

The PrefixMap::aggregate, PrefixMap::aggregate_fill, and PrefixMap::aggregate_fill_default implement Reduced Routing Table Construction (ORTC) by R. P. Draves, C. King, S. Venkatachary, and B. D. Zill. The result is a minimal routing table. The fill variants will also add the root prefix (i.e., 0.0.0.0/0), and fill holes with the default value.

Other operations on the Tree

Most operations are bounded by prefix width, not by the number of stored entries. Let w be the number of bits in the prefix representation, and let h = ceil((w + 1) / 5) be the maximum number of multi-bit nodes on a search path. For IPv4, h <= 7; for IPv6, h <= 26. Let n be the number of stored entries, and let v be the number of trie nodes visited by a traversal.

Operation	Complexity
len, is_empty, mem_size	O(1)
get, get_mut, contains_key	O(h)
get_lpm, get_spm, cover	O(h)
entry, insert	O(h)
remove, remove_keep_tree	O(h)
children, view_at	O(h) to create, then linear in the subtrie
iter, keys, values	O(n + v) for a complete traversal
retain, clear	O(n + v)
remove_children	O(h + m) with the removed subtrie size m
Operations on an occupied map::Entry	O(1) after the entry lookup
Inserting through a vacant map::Entry	O(h) worst case

There are three removal styles:

• PrefixMap::remove will remove an entry from the tree and modify the tree structure as if the value was never inserted before. It may remove now-empty multi-bit nodes and compact their allocator blocks.
• PrefixMap::remove_children will remove all entries that are contained within the given prefix, including entries stored in the same multi-bit node and in child nodes below it.
• PrefixMap::remove_keep_tree removes only the value and leaves the existing node structure in place, which can make reinserting the same prefix cheaper but may leave empty internal nodes for future traversals to pass through.

Zero-Copy Archives with rkyv

With the rkyv feature enabled, every map and set in this crate can be serialized into a byte buffer and read back without a deserialization step. Each owned collection has an archived counterpart: ArchivedPrefixMap, ArchivedPrefixSet, ArchivedJointPrefixMap, and ArchivedJointPrefixSet. These types are validated once and then queried directly out of the bytes. The archived types expose (or reimplement) all immutable access methods of their owned counterparts, such as get, get_lpm, get_spm, contains_key, and the various iterators.

Archives integrate with the rest of the crate through the same TrieView infrastructure used by the owned collections. &ArchivedPrefixMap and &ArchivedPrefixSet implement AsView, so any function written against a TrieView accepts an archive just as it accepts a borrowed PrefixMap or PrefixSet, and archives participate in the same set operations ( union, intersection, difference, and the covering variants) as owned tries. See the rkyv module for details and examples.

Re-exports

pub use map::PrefixMap;

pub use set::PrefixSet;

pub use trieview::AsView;

pub use trieview::TrieRef;

pub use trieview::TrieRefMut;

pub use trieview::TrieView;

Modules

joint

Module that defines the joint version of a prefix map and set, including all helper functions. You can access each individual table of the prefix map, allowing you to perform the usual operations set operations.

map

This module contains the implementation for the Dense Prefix Map.

rkyvrkyv

Zero-copy archives with rkyv

set

Prefix set implemented on top of PrefixMap.

trieview

Composable trie-view trait for crate::PrefixMap.

Traits

Prefix

A fixed-width prefix key.