Memory-efficient mapping from values to integer identifiers (AKA a lexicon or symbol table), with options for fast bidirectional lookup




This crate provides the means for associating values of an arbitrary type T with values of another arbitrary type S ("symbols"). Typical use cases involve values of T that are less convenient to deal with than values of S, in which the identity of a T is important but its value is not. For example, the first step in many natural language handling pipelines is construct a mapping from token strings to a dense range of integers starting at 0, so that they may be compared very quickly and used to index simple data structures like vectors and arrays.

You can get a lot of this package's functionality with a structure like:

pub struct Table<T> where T: Hash + Eq {
    // Mapping from T to usize.
    by_symbol: HashMap<T, usize>,
    // Mapping from usize to T.
    by_id: Vec<T>,

But then you're maintaining two copies of each T. You can resort to a HashMap<Rc<T>, usize> and Vec<Rc<T>>, but that forces you to pay the cost of reference counting and isn't easily shared across threads. Using Arc instead of Rc makes your type Send-able, but that has even more overhead than using Rc.

Or you could use symbol_table::HashIndexing<Data=T, SymbolId=usize> and get a type that is Send and Sync when T is and owns only one T per association in the table.

See the rustdoc for example usage and further technical details.


Copyright 2016, Donald S. Black.

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.