#index-map #container #map

macro multi_index_map_derive

MultiIndexMap: A generic multi index map inspired by boost multi index containers

10 releases (5 breaking)

0.11.0 Nov 2, 2023
0.10.0 Nov 2, 2023
0.9.0 Sep 14, 2023
0.8.1 Aug 30, 2023
0.6.0 Jun 23, 2023

#16 in #index-map

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Used in 14 crates (via multi_index_map)

MIT license

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MultiIndexMap Tests

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.
  • Modification such that uniqueness is violated, will result in a panic.
  • Insertion such that uniqueness would be violated does not mutate the map, instead the element is returned to the user wrapped in an Err variant.

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)).
  • Each equal range of any non-unique index is stored as a BTreeSet, which we must iterate through the length of when retrieving all matching elements, and also when iterating over the whole index.

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 types of all indexed fields must implement Clone.
  • Optionally, multi_index_derive can be used to derive traits on the generated MultiIndexMap, eg. #[multi_index_derive(Clone, Debug)] See examples/main.rs for more details.

Example

use multi_index_map::MultiIndexMap;

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

fn main() {
    let order1 = Order {
        order_id: 1,
        timestamp: 1656145181,
        trader_name: "JohnDoe".into(),
        filled: false,
        volume: 100,
    };

    let order2 = Order {
        order_id: 2,
        timestamp: 1656145182,
        trader_name: "JohnDoe".into(),
        filled: false,
        volume: 100,
    };

    let mut map = MultiIndexOrderMap::default();

    map.try_insert(order1).unwrap();
    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 order2_ref = map
        .update_by_order_id(&42, |filled: &mut bool, volume: &mut u64| {
            *filled = true;
            *volume = 0;
        })
        .unwrap();
    assert_eq!(order2_ref.filled, true);
    assert_eq!(order2_ref.volume, 0);

    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);

    println!("{map:?}");

    // 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 updating un-indexed fields, we lookup the element(s) through the given key, then apply the closure to modify just the unindexed fields in-place. We then return a reference to the modified element(s). If the key doesn't match, the closure won't be applied, and Option::None will be returned.
  • When modifying indexed fields of an element, we do the same process, but the closure takes a mutable reference to the whole element. Any fields, indexed and un-indexed can be modified. We must then update all the lookup tables to account for any changes to indexed fields, so this is slower than an un-indexed update.
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, BTreeSet<usize>>,
}

struct MultiIndexOrderMapOrderIdIter<'a> {
    ...
}

struct MultiIndexOrderMapTimestampIter<'a> {
    ...
}

struct MultiIndexOrderMapTraderNameIter<'a> {
    ...
}

impl MultiIndexOrderMap {
    fn try_insert(&mut self, elem: Order) -> Result<&Order, MultiIndexMapError<Order>>;
    fn insert(&mut self, elem: Order) -> &Order;
    
    fn len(&self) -> usize;
    fn is_empty(&self) -> bool;
    fn clear(&mut self);
    
    fn get_by_order_id(&self, key: &u32) -> Option<&Order>;
    fn get_by_timestamp(&self, key: &u64) -> Option<&Order>;
    fn get_by_trader_name(&self, key: &String) -> Vec<&Order>;

    fn update_by_order_id(&mut self, key: &u32, f: impl FnOnce(&mut bool, &mut u64)) -> Option<&Order>;
    fn update_by_timestamp(&mut self, key: &u64, f: impl FnOnce(&mut bool, &mut u64)) -> Option<&Order>;
    fn update_by_trader_name(&mut self, key: &String, f: impl FnMut(&mut bool, &mut u64)) -> Vec<&Order>;
    
    fn modify_by_order_id(&mut self, key: &u32, f: impl FnOnce(&mut Order)) -> Option<&Order>;
    fn modify_by_timestamp(&mut self, key: &u64, f: impl FnOnce(&mut Order)) -> Option<&Order>;
    fn modify_by_trader_name(&mut self, key: &String, f: impl FnMut(&mut Order)) -> Vec<&Order>;
    
    fn remove_by_order_id(&mut self, key: &u32) -> Option<Order>;
    fn remove_by_timestamp(&mut self, key: &u64) -> Option<Order>;
    fn remove_by_trader_name(&mut self, key: &String) -> 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.

Dependencies

~2MB
~43K SLoC