@rimbu/bimultimap

Immutable many‑to‑many bidirectional multimaps for TypeScript & JavaScript.

@rimbu/bimultimap provides an efficient, type‑safe BiMultiMap: a data structure that maintains a many‑to‑many relationship between keys and values. Each key can map to multiple values, each value can map to multiple keys, and all associations are kept in sync in both directions with immutable, persistent semantics.

Use it whenever you need reverse lookups over multi‑valued relationships (e.g. tags ↔ items, users ↔ groups, roles ↔ permissions) without manually juggling two separate multimaps.

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Table of Contents

1. Why @rimbu/bimultimap?
2. Feature Highlights
3. Quick Start
4. Core Concepts & Types
5. Working with Hash & Sorted BiMultiMaps
6. Performance Notes
7. Installation
8. FAQ
9. Ecosystem & Integration
10. Contributing
11. License
12. Attributions

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Why @rimbu/bimultimap?

Plain multimaps give you key → values lookups, but in many domains you also need efficient values → keys:

- Tagging & categorization – item ↔ tags, user ↔ roles, post ↔ topics.
- Bidirectional indices – id ↔ features, entity ↔ relations.
- Graph‑like structures – modelling adjacency where both node → neighbors and neighbor → nodes are interesting.

@rimbu/bimultimap focuses on:

- True two‑way navigation – every key‑value association is mirrored as a value‑key association.
- Immutable operations – updates return new instances, sharing most of their structure internally.
- Multi‑valued associations – both keys and values can be associated with multiple counterparts.
- Ergonomic API – familiar multimap‑like operations plus BiMultiMap‑specific helpers.

If you find yourself maintaining two multimaps in sync, a BiMultiMap is usually the better fit.

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Feature Highlights

- Bidirectional many‑to‑many lookups – efficient key → values and value → keys access.
- Rich operations – add/remove by key, value, or entry; bulk updates; streaming; traversal utilities.
- Immutable & persistent – structural sharing for fast copies and snapshots.
- Multiple views – access keyValueMultiMap and valueKeyMultiMap as regular Rimbu multimaps.
- Hash & sorted variants – choose HashBiMultiMap for speed, SortedBiMultiMap for ordering.

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Quick Start

``ts
import { HashBiMultiMap } from '@rimbu/bimultimap';

// Create from entry tuples
const biMultiMap = HashBiMultiMap.of([1, 'a'], [1, 'b'], [2, 'b']);

// Forward lookup: key -> values
console.log(biMultiMap.getValues(1).toArray()); // ['a', 'b']

// Reverse lookup: value -> keys
console.log(biMultiMap.getKeys('b').toArray()); // [1, 2]

// Add more associations immutably
const updated = biMultiMap.add(2, 'c');
console.log(updated.getValues(2).toArray()); // ['b', 'c']
`

Try Rimbu (including @rimbu/bimultimap) live in the browser using the
Rimbu Sandbox on CodeSandbox.

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Core Concepts & Types

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| Name | Description |
| --------------------------- | ----------------------------------------------------------------------------------------------------------------------- |
| BiMultiMap | Immutable, type‑invariant bidirectional MultiMap with a many‑to‑many relationship between K and V. |
| BiMultiMap.NonEmpty | Non‑empty refinement of BiMultiMap with stronger type guarantees. |
| HashBiMultiMap | BiMultiMap implementation backed by hashed multimaps – great default for most use cases. |
| SortedBiMultiMap | BiMultiMap implementation backed by sorted multimaps – keeps entries ordered by key / value depending on configuration. |

$3

`ts
import { HashBiMultiMap } from '@rimbu/bimultimap';

// Construction
const empty = HashBiMultiMap.empty();
const fromEntries = HashBiMultiMap.of([1, 'one'], [1, 'uno'], [2, 'two']);

// Size & emptiness
empty.isEmpty; // true
fromEntries.size; // 3 (three key-value associations)
fromEntries.keySize; // 2 (keys: 1 and 2)

// Lookups
fromEntries.hasKey(1); // true
fromEntries.hasValue('two'); // true
fromEntries.hasEntry(1, 'uno'); // true

fromEntries.getValues(1).toArray(); // ['one', 'uno']
fromEntries.getKeys('two').toArray(); // [2]

// Updating (returns new HashBiMultiMap)
const withMore = fromEntries.add(2, 'dos');

// Removing by key or value
const withoutKey = withMore.removeKey(1);
const withoutValue = withMore.removeValue('two');
`

See the full BiMultiMap docs and
API reference for all operations.

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Working with Hash & Sorted BiMultiMaps

`ts
import { HashBiMultiMap, SortedBiMultiMap } from '@rimbu/bimultimap';

// HashBiMultiMap: fast hash-based implementation
const hashBiMultiMap = HashBiMultiMap.of(
['alice', 'admin'],
['alice', 'editor'],
['bob', 'viewer']
);

// SortedBiMultiMap: ordered by keys and/or values
const sortedBiMultiMap = SortedBiMultiMap.of(
['alice', 'admin'],
['alice', 'editor'],
['bob', 'viewer']
);

// Both support the same BiMultiMap interface:
hashBiMultiMap.getValues('alice').toArray(); // ['admin', 'editor']
sortedBiMultiMap.getKeys('viewer').toArray(); // ['bob']

// Iterate like any other Rimbu collection
for (const [user, role] of sortedBiMultiMap) {
console.log(user, role);
}
`

- Choose HashBiMultiMap when you care about raw lookup/update performance.
- Choose SortedBiMultiMap when stable ordering (e.g. for UI rendering, serialization, or range‑like operations) matters.

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Performance Notes

- BiMultiMaps in Rimbu are built on persistent data structures – updates are typically \\(O(\log n)\\) and share most of their structure.
- Lookups by key or value are designed to behave similarly to their underlying multimap implementations (hashed / sorted).
- Many bulk operations accept generic StreamSource inputs, letting you construct and transform BiMultiMaps efficiently from arrays, iterables, or streams.

For detailed performance characteristics and benchmarks, see the main Rimbu documentation at rimbu.org.

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Installation

$3

`sh
npm install @rimbu/bimultimap


or

yarn add @rimbu/bimultimap

or

bun add @rimbu/bimultimap

or

deno add npm:@rimbu/bimultimap
`

$3

@rimbu/bimultimap ships both ESM and CJS builds. Use it with any modern bundler
(Vite, Webpack, esbuild, Bun, etc.) or directly in Node ESM projects.

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FAQ

Q: How is a BiMultiMap different from a regular Map or MultiMap?
A BiMultiMap maintains two synchronized multimaps under the hood: one for key → values
and one for value → keys. You get efficient lookups in both directions without manually
maintaining two separate structures.

Q: Can a key or value appear multiple times?
Yes. Unlike a BiMap, a BiMultiMap allows a key to be associated with multiple values and a
value with multiple keys. Operations like add, getValues, and getKeys are designed
around this many‑to‑many model.

Q: Is the structure mutable?
No. All updates return new BiMultiMap instances; existing ones remain unchanged and can be safely
shared across your application.

Q: Can I iterate keys or values separately?
Yes. Use keyValueMultiMap and valueKeyMultiMap to access standard Rimbu multimap views
with all their APIs.

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Ecosystem & Integration

- Part of the broader Rimbu collection ecosystem – interoperates with @rimbu/hashed,
@rimbu/sorted, @rimbu/collection-types, and @rimbu/stream`.
- Ideal for modelling multi‑valued relationships in domain models, tagging systems, permission
models, or graph‑like structures.
- Works seamlessly with other Rimbu collections and utilities for building rich, immutable data models.

Explore more at the Rimbu documentation and the
BiMultiMap API docs.

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Contributing

We welcome contributions! See the
Contributing guide for details.

_Made with contributors-img._

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License

MIT © Rimbu contributors. See LICENSE for details.

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Attributions

Created and maintained by Arvid Nicolaas. Logo © Rimbu.