@rimbu/multimap

Fast, immutable multimaps (multi‑value maps) for TypeScript & JavaScript.

@rimbu/multimap provides efficient, type‑safe MultiMap implementations: data structures where
each key can be associated with one or more unique values. Values for a key are stored in a
set‑like collection, so duplicates are automatically removed, while all operations remain immutable
and persistent.

Use it whenever you need to model one‑to‑many relationships such as tags, roles, inverted
indexes, or adjacency lists.

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Table of Contents

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

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Why @rimbu/multimap?

Plain maps give you key → value mappings, but many real‑world use cases are one‑to‑many:

- Users → roles, groups, or permissions.
- Documents → tags or keywords.
- Graphs → adjacency lists (node → neighbours).
- Indices → all items matching a category or property.

@rimbu/multimap focuses on:

- Multiple values per key – each key can have a set of unique values.
- Immutable operations – updates return new instances, sharing structure internally.
- Flexible underlying storage – hash‑based or sorted keys and values.
- Ergonomic API – map‑like operations, plus multi‑value‑aware helpers.

If you ever keep a map from keys to sets of values manually, a MultiMap is usually a better fit.

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Feature Highlights

- One‑to‑many mappings – each key can be associated with multiple unique values.
- Uniqueness per key – values for a given key are stored in a set; duplicates are dropped.
- Hash & sorted variants – choose hashing for speed or sorted variants for deterministic order.
- Immutable & persistent – structural sharing for fast copies and history‑friendly updates.
- Configurable contexts – build custom configurations via createContext for advanced use cases.
- Rich operations – add/remove values, bulk updates, streaming, traversal utilities.

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Quick Start

``ts
import { HashMultiMapHashValue } from '@rimbu/multimap';

// Create from entry tuples: key -> value
const multi = HashMultiMapHashValue.of([1, 'a'], [1, 'b'], [2, 'a']);

// Each key maps to a set of unique values
console.log(multi.getValues(1).toArray());
// ['a', 'b']

// Adding values returns a new multimap
const updated = multi.add(2, 'c');
console.log(updated.getValues(2).toArray());
// ['a', 'c']

// Removing keys or entries is also immutable
const withoutKey1 = updated.removeKey(1);
const withoutEntry = updated.removeEntry(2, 'a');
`

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

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Core Concepts & Types

$3

From @rimbu/multimap you get the following core types:

| Name | Description |
| ------------------------------------------- | --------------------------------------------------------------------------------------------------- |
| MultiMap | Generic, type‑invariant multimap interface: keys of type K mapping to sets of values V. |
| MultiMap.NonEmpty | Non‑empty refinement of MultiMap with stronger type guarantees. |
| MultiMap.Context | Context/factory for creating MultiMap instances with configurable underlying map & set contexts. |
| MultiMap.Builder | Mutable builder for efficiently constructing or mutating a MultiMap before freezing it. |
| VariantMultiMap | Read‑only, type‑variant multimap interface; supports safe type‑widening but no mutating operations. |
| VariantMultiMap.NonEmpty | Non‑empty refinement of VariantMultiMap. |
| HashMultiMapHashValue | Multimap with hashed keys and hashed value sets (HashMap + HashSet). |
| HashMultiMapSortedValue | Multimap with hashed keys and sorted value sets (HashMap + SortedSet). |
| SortedMultiMapHashValue | Multimap with sorted keys and hashed value sets (SortedMap + HashSet). |
| SortedMultiMapSortedValue | Multimap with sorted keys and sorted value sets (SortedMap + SortedSet). |
| HashMultiMapHashValue.Context | Context for HashMultiMapHashValue, exposing configuration and factories. |
| HashMultiMapSortedValue.Context | Context for HashMultiMapSortedValue. |
| SortedMultiMapHashValue.Context | Context for SortedMultiMapHashValue. |
| SortedMultiMapSortedValue.Context | Context for SortedMultiMapSortedValue. |

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`ts
import { HashMultiMapHashValue } from '@rimbu/multimap';

// Construction
const empty = HashMultiMapHashValue.empty();
const fromEntries = HashMultiMapHashValue.of([1, 'a'], [1, 'b'], [2, 'a']);

// Size & key count
empty.isEmpty; // true
fromEntries.keySize; // 2 (keys: 1, 2)
fromEntries.size; // 3 (entries: [1, 'a'], [1, 'b'], [2, 'a'])

// Lookups
fromEntries.hasKey(1); // true
fromEntries.hasEntry(1, 'b'); // true
fromEntries.getValues(1).toArray(); // ['a', 'b']

// Updating (returns new MultiMap)
const withMore = fromEntries.add(2, 'c');
const replaced = fromEntries.setValues(1, ['x', 'y']);

// Removing
const withoutKey = fromEntries.removeKey(2);
const withoutEntry = fromEntries.removeEntry(1, 'a');
`

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

---

Working with Hash & Sorted MultiMaps

All concrete variants share the same MultiMap semantics but differ in how keys and values are
stored internally:

`ts
import {
HashMultiMapHashValue,
HashMultiMapSortedValue,
SortedMultiMapHashValue,
SortedMultiMapSortedValue,
} from '@rimbu/multimap';

// Hash keys, hash value sets (fast, unordered)
const hashHash = HashMultiMapHashValue.of([1, 'a'], [1, 'b'], [2, 'a']);

// Hash keys, sorted value sets (deterministic value order per key)
const hashSorted = HashMultiMapSortedValue.of([1, 'b'], [1, 'a'], [2, 'c']);
hashSorted.getValues(1).toArray(); // ['a', 'b']

// Sorted keys, hash value sets (sorted key order, fast values)
const sortedHash = SortedMultiMapHashValue.of(['b', 1], ['a', 2]);
sortedHash.streamKeys().toArray(); // ['a', 'b']

// Sorted keys, sorted value sets
const sortedSorted = SortedMultiMapSortedValue.of(['b', 2], ['b', 1], ['a', 3]);
sortedSorted.stream().toArray();
// [['a', 3], ['b', 1], ['b', 2]] (keys and values sorted)
`

If you need custom underlying contexts (e.g. custom hashers or comparators), you can create them via
createContext:

`ts
import { HashMultiMapHashValue } from '@rimbu/multimap';

const context = HashMultiMapHashValue.createContext({
// optional: custom key/value contexts
});

const multi = context.of([1, 'a'], [1, 'b']);
`

For read‑only, type‑variant views that can be safely widened, use the VariantMultiMap interfaces
exported from this package.

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Performance Notes

- MultiMaps in Rimbu are built on persistent data structures – updates are typically
\\(O(\log n)\\) and share most of their structure.
- Lookups and updates behave similarly to the underlying HashMap / SortedMap and HashSet /
SortedSet implementations.
- Many bulk operations accept generic StreamSource inputs, letting you construct and transform
MultiMaps 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/multimap


or

yarn add @rimbu/multimap

or

bun add @rimbu/multimap

or

deno add npm:@rimbu/multimap
`

$3

@rimbu/multimap 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 MultiMap different from a regular Map?
A MultiMap allows multiple unique values per key. Retrieval is still keyed, but you work with
sets of values (getValues(key)), not a single value.

Q: Are values per key ordered?
That depends on the variant. Hash‑based value sets (HashMultiMapHashValue, SortedMultiMapHashValue)
are not ordered; sorted value sets (HashMultiMapSortedValue, SortedMultiMapSortedValue) are.

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

Q: Can I iterate keys or values separately?
Yes. Use stream, streamKeys, streamValues, or the underlying keyMap to traverse in different
ways.

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Ecosystem & Integration

- Part of the broader Rimbu collection ecosystem – interoperates with @rimbu/hashed,
@rimbu/ordered, @rimbu/collection-types, and @rimbu/stream`.
- Ideal for modelling tag systems, permission sets, inverted indices, and adjacency lists.
- Works seamlessly with other Rimbu collections and utilities for building rich, immutable data
models.

Explore more at the Rimbu documentation and the
MultiMap 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.