MultiIndexMap

Also available on crates.io.

Rust library useful for storing structs that needs to be accessed through various different indexes of the fields of the struct. Inspired by C++/Boost Multi-index Containers but redesigned for a more idiomatic Rust API.

Current implementation supports:

• Hashed indexes using FxHashMap from rustc-hash.
• Sorted indexes using BTreeMap from std::collections.
• Unique and non-unique indexes.
• Unindexed fields.
• Iterators for each indexed field.
• Iterators for the underlying backing storage.

Performance characteristics

Unique Indexes

• Hashed index retrievals are constant-time. (FxHashMap + Slab).
• Sorted indexes retrievals are logarithmic-time. (BTreeMap + Slab).
• Iteration over hashed index is same as FxHashMap, plus a retrieval from the backing storage for each element.
• Iteration over ordered index is same as BTreeMap, plus a retrieval from the backing storage for each element.
• Iteration over the backing store is the same as Slab, so contiguous memory but with potentially vacant slots.
• Insertion, removal, and modification complexity grows as the number of indexed fields grow. All indexes must be updated during these operations so these are slower.
• Modification of unindexed fields through unsafe mut methods is the same as regular retrieval time.
• Insertion or modification such that uniqueness is violated, will result in a panic.

Non-Unique Indexes

• Hashed index retrievals are still constant-time with the total number of elements, but linear-time with the number of matching elements. (FxHashMap + (Slab * num_matches)).
• Sorted indexes retrievals are still logarithmic-time with total number of elements, but linear-time with the number of matching elements. (BTreeMap + (Slab * num_matches)).
• Iteration within an equal range of a non-unique index is fast, as the matching elements are stored contiguously in memory. Otherwise iteration is the same as unique indexes.

How to use

This crate provides a derive macro MultiIndexMap, which when applied to the struct representing an element will generate a map to store and access these elements. Annotations are used to specify which fields to index. Currently hashed_unique, hashed_non_unique, ordered_unique, and ordered_non_unique are supported. The element must implement Clone.

Example

use multi_index_map::MultiIndexMap;

#[derive(MultiIndexMap, Clone, Debug)]
struct Order {
    #[multi_index(hashed_unique)]
    order_id: u32,
    #[multi_index(ordered_unique)]
    timestamp: u64,
    #[multi_index(hashed_non_unique)]
    trader_name: String,
}

fn main() {
    let order1 = Order {
        order_id: 1,
        timestamp: 1656145181,
        trader_name: "JohnDoe".into(),
    };

    let order2 = Order {
        order_id: 2,
        timestamp: 1656145182,
        trader_name: "JohnDoe".into(),
    };

    let mut map = MultiIndexOrderMap::default();

    map.insert(order1);
    map.insert(order2);

    let orders = map.get_by_trader_name(&"JohnDoe".to_string());
    assert_eq!(orders.len(), 2);
    println!("Found 2 orders for JohnDoe: [{orders:?}]");

    let order1_ref = map.get_by_order_id(&1).unwrap();
    assert_eq!(order1_ref.timestamp, 1656145181);

    let order2_ref = map
        .modify_by_order_id(&2, |o| {
            o.timestamp = 1656145183;
            o.order_id = 42;
        })
        .unwrap();
    assert_eq!(order2_ref.timestamp, 1656145183);
    assert_eq!(order2_ref.order_id, 42);
    assert_eq!(order2_ref.trader_name, "JohnDoe".to_string());

    let orders = map.get_by_trader_name(&"JohnDoe".to_string());
    assert_eq!(orders.len(), 2);
    println!("Found 2 orders for JohnDoe: [{orders:?}]");

    let orders = map.remove_by_trader_name(&"JohnDoe".to_string());
    for (_idx, order) in map.iter() {
        assert_eq!(order.trader_name, "JohnDoe");
    }
    assert_eq!(orders.len(), 2);

    // See examples and tests directories for more in depth usage.
}

Under the hood

The above example will generate the following MultiIndexMap and associated Iterators. The Orders are stored in a Slab, in contiguous memory, which allows for fast lookup and quick iteration. A lookup table is created for each indexed field, which maps the index key to a index in the Slab. The exact type used for these depends on the annotations. For hashed_unique and hashed_non_unique a FxHashMap is used, for ordered_unique and ordered_non_unique a BTreeMap is used.

• When inserting an element, we add it to the backing store, then add elements to each lookup table pointing to the index in the backing store.
• When retrieving elements for a given key, we lookup the key in the lookup table, then retrieve the item at that index in the backing store.
• When removing an element for a given key, we do the same, but we then must also remove keys from all the other lookup tables before returning the element.
• When iterating over an index, we use the default iterators for the lookup table, then simply retrieve the element at the given index in the backing store.
• When modifying an element, we lookup the element through the given key, then apply the closure to modify the element in-place. We then return a reference to the modified element. We must then update all the lookup tables to account for any changes to indexed fields. If we only want to modify an unindexed field then it is much faster to just mutate that field directly. This is why the unsafe methods are provided. These can be used to modify unindexed fields quickly, but must not be used to modify indexed fields.

struct MultiIndexOrderMap {
    _store: slab::Slab<Order>,
    _order_id_index: rustc_hash::FxHashMap<u32, usize>,
    _timestamp_index: std::collections::BTreeMap<u64, usize>,
    _trader_name_index: rustc_hash::FxHashMap<String, Vec<usize>>,
}

struct MultiIndexOrderMapOrderIdIter<'a> {
    ...
}

struct MultiIndexOrderMapTimestampIter<'a> {
    ...
}

struct MultiIndexOrderMapTraderNameIter<'a> {
    ...
}

impl MultiIndexOrderMap {
    fn insert(&mut self, elem: Order);
    
    fn len(&self) -> usize;
    fn is_empty(&self) -> bool;
    fn clear(&mut self);
    
    fn get_by_order_id(&self) -> Option<&Order>;
    fn get_by_timestamp(&self) -> Option<&Order>;
    fn get_by_trader_name(&self) -> Vec<&Order>;
    
    unsafe fn get_mut_by_order_id(&mut self) -> Option<&mut Order>;
    unsafe fn get_mut_by_timestamp(&mut self) -> Option<&mut Order>;
    unsafe fn get_mut_by_trader_name(&mut self) -> Vec<&mut Order>;
    
    fn modify_by_order_id(&mut self, f: impl FnOnce(&mut Order)) -> Option<&Order>;
    fn modify_by_timestamp(&mut self, f: impl FnOnce(&mut Order)) -> Option<&Order>;
    
    fn remove_by_order_id(&mut self) -> Option<Order>;
    fn remove_by_timestamp(&mut self) -> Option<Order>;
    fn remove_by_trader_name(&mut self) -> Vec<Order>;
    
    fn iter(&self) -> slab::Iter<Order>;
    unsafe fn iter_mut(&mut self) -> slab::IterMut<Order>;
    
    fn iter_by_order_id(&self) -> MultiIndexOrderMapOrderIdIter;
    fn iter_by_timestamp(&self) -> MultiIndexOrderMapTimestampIter;
    fn iter_by_trader_name(&self) -> MultiIndexOrderMapTraderNameIter;
}

Dependencies

See Cargo.toml for information on each dependency.

Future work

• Allow users to specify which hash function to use, rather than always using an FxHashMap.
• Potentially a vector-map style lookup table would be very quick for small tables with integer indexes.
• Allow overwriting behaviour upon inserting a duplicate unique index, returning a Vec of the overwritten elements.
• Implement clever tricks used in boost::multi_index_containers to improve performance.