mapam

A bidirectional Map/WeakMap implementation with the same API as an ES6 Map/WeakMap!

Features

• Tiny: less than 720 bytes minzipped including dependencies!
• Familiar: uses the same API as an ES6 Map/WeakMap
• Compliant: maintains all the invariants of Map/WeakMap including method return values and even iteration order!
• Correct: handles edge cases correctly

Install

$ npm i mapam

Huh?

A bidirectional map is like a regular map except it preserves uniqueness of its values (in addition to its keys) and supports an inverse view, which is another bidirectional map containing and backed by the same entries, but with reversed keys and values.

For a weak bidirectional map specifically, its values (in addition to its keys) must be objects.

Usage

The API of one side/view of the bidirectional map is identical to the Map/WeakMap API. Just use like a normal Map or WeakMap and call the inverse map to get the other side of the bidirectional map!

import { BiMap, WeakBiMap } from 'mapam'

const biMap = new BiMap()

biMap.set(1, `one`)
biMap.set(2, `two`)
biMap.inverse().set(`three`, 3)

console.log([...biMap])
//=> [ [ 1, 'one' ], [ 2, 'two' ], [ 3, 'three' ] ]

console.log([...biMap.inverse()])
//=> [ [ 'one', 1 ], [ 'two', 2 ], [ 'three', 3 ] ]

console.log(biMap.get(2))
//=> two

console.log(biMap.inverse().get(`two`))
//=> 2

biMap.inverse().delete(`two`)

console.log(biMap.has(2))
//=> false

console.log([...biMap])
//=> [ [ 1, 'one' ], [ 3, 'three' ] ]

try {
  // Throws!
  biMap.set(10, `three`)
} catch (e) {
  console.log(e.message)
  //=> value already bound to another key
}

// Doesn't throw!
// Conveys intention of changing the value associated with the given key
biMap.inverse().set(`three`, 10)

// Also doesn't throw
biMap.set(10, `three`, { force: true })

const weakBiMap = new WeakBiMap()

const a = {}
const b = {}
weakBiMap.set(a, b)

console.log(weakBiMap.get(a) === b)
//=> true

console.log(weakBiMap.inverse().get(b) === a)
//=> true

See the type definitions for more documentation.

Correctness

Category	mapam	bimap@0.0.15	bim@1.3.3	mnemonist@0.38.3	@rimbu/bimap@0.7.2
Iteration order	✔️	❌	❌	❌	❌
Negative zero	✔️	❌	❌	❌	❌

Iteration order

Map (and many other "unordered" data structures) allow iteration in insertion order so it's only logical for a bidirectional map to support iteration in a logical order like insertion order.

const biMap = new BiMap()

biMap.set(1, `one`)
biMap.set(2, `two`)
biMap.set(3, `three`)
biMap.set(4, `four`)

biMap.inverse().set(`two`, -2)
biMap.delete(3)

console.log([...biMap])

// Correct behavior:
// mapam => [ [ 1, 'one' ], [ -2, 'two' ], [ 4, 'four' ] ]

// Incorrect behaviors:
// bimap@0.0.15 => Doesn't support iteration
// bim@1.3.3 => [ [ 1, 'one' ], [ 4, 'four' ], [ -2, 'two' ] ]
// mnemonist@0.38.3 => [ [ 1, 'one' ], [ 4, 'four' ], [ -2, 'two' ] ]
// @rimbu/bimap@0.7.2 => [ [ 1, 'one' ], [ 4, 'four' ], [ -2, 'two' ] ]

Negative zero

A key in a Map can only occur once. But how is the key's uniqueness determined? JavaScript's Map uses the sameValueZero algorithm when checking if two keys are equal. The algorithm considers +0 and -0 to be equal, but they are actually two different values due to how IEEE floating point numbers work.

This means that Map coerces -0 to +0 for its keys, but not for its values. So if a bidirectional map implementation uses Map internally, then it must either coerce -0 to +0 for its values as well or somehow broaden its keys to support both -0 and +0.

const biMap = new BiMap()

biMap.set(`negative-zero`, -0)
biMap.set(`zero`, 0)

console.log(
  biMap.get(`negative-zero`),
  biMap.get(`zero`),
  biMap.inverse().get(-0),
  biMap.inverse().get(0),
)

// Correct behavior:
// mapam => -0 0 negative-zero zero

// Incorrect behaviors:
// bimap@0.0.15 => -0 0 negative-zero negative-zero
// bim@1.3.3 => undefined 0 zero zero
// mnemonist@0.38.3 => undefined 0 zero zero
// @rimbu/bimap@0.7.2 => -0 0 negative-zero negative-zero

Contributing

Stars are always welcome!

For bugs and feature requests, please create an issue.

License

Apache 2.0

This is not an official Google product.