chain-map

The ChainMap type groups a chain of HashMaps together in precedence order and provides a single, unified view into the values. The semantics for keys are the same as for a HashMap, however the value associated with a given key is the value of that key in the highest-precedence map that contains the key.

Rust Version

This version of chain-map requires Rust 1.31 or later.

Precedence

Maps added to the ChainMap earlier have precedence over those added later. So the first map added to the chain will have the highest precedence, while the most recent map added will have the lowest.

Performance

Each read of the ChainMap will read the chain of maps in order, so each operation will complete in worst-case O(N), with N the number of maps in the chain. As a result, this should only be used for cases where the number of reads is low compared to the number of elements in each map.

Examples

use std::collections::HashMap;
use chain_map::ChainMap;

let mut first_map = HashMap::new();
first_map.insert("first", 10);

let mut second_map = HashMap::new();
second_map.insert("first", 20);
second_map.insert("second", 20);

let mut third_map = HashMap::new();
third_map.insert("first", 30);
third_map.insert("second", 30);
third_map.insert("third", 30);

let mut chain: ChainMap<_, _> =
    vec![first_map, second_map, third_map].into_iter().collect();
assert_eq!(chain.get("first"), Some(&10));
assert_eq!(chain["second"], 20);
assert!(chain.contains_key("third"));
assert!(!chain.contains_key("fourth"));

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.