Ruby's Array.permutation in TypeScript

How to use

Install via yarn add locutus and import: import { permutation } from 'locutus/ruby/Array/permutation'.

Or with CommonJS: const { permutation } = require('locutus/ruby/Array/permutation')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	permutation([1, 2, 3], 2)	[[1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2]]
2	permutation(['a', 'b'])	[['a', 'b'], ['b', 'a']]
3	permutation([1, 2, 3], 0)	[[]]

Ruby nil and TypeScript/JavaScript undefined

Ruby's nil and TypeScript/JavaScript undefined are semantically equivalent—both represent "no value" or "nothing here". However, they serialize differently: nil becomes null in JSON, while undefined is typically omitted or becomes undefined.

Locutus Ruby functions return undefined (idiomatic TypeScript/JavaScript) where Ruby would return nil. Our parity tests treat these as equivalent when verifying against native Ruby.

Notes

• Returns all r-length permutations without repetition (default r = array length).

Here's what our current TypeScript equivalent to Ruby's Array.permutation looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

Copy code

export function permutation<T>(arr: T[] | unknown, r?: number): T[][] {
  // parity verified: Ruby 3.3
  //      discuss at: https://locutus.io/ruby/Array/permutation/
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Returns all r-length permutations without repetition (default r = array length).
  //       example 1: permutation([1, 2, 3], 2)
  //       returns 1: [[1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2]]
  //       example 2: permutation(['a', 'b'])
  //       returns 2: [['a', 'b'], ['b', 'a']]
  //       example 3: permutation([1, 2, 3], 0)
  //       returns 3: [[]]

  if (!Array.isArray(arr)) {
    return []
  }

  const target = r === undefined ? arr.length : Math.trunc(Number(r))
  if (!Number.isFinite(target) || target < 0 || target > arr.length) {
    return []
  }
  if (target === 0) {
    return [[]]
  }

  const out: T[][] = []
  const used = new Array<boolean>(arr.length).fill(false)
  const current: T[] = []

  const walk = (): void => {
    if (current.length === target) {
      out.push([...current])
      return
    }

    for (let i = 0; i < arr.length; i += 1) {
      if (used[i]) {
        continue
      }
      used[i] = true
      current.push(arr[i] as T)
      walk()
      current.pop()
      used[i] = false
    }
  }

  walk()
  return out
}

export function permutation(arr, r) {
  // parity verified: Ruby 3.3
  //      discuss at: https://locutus.io/ruby/Array/permutation/
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Returns all r-length permutations without repetition (default r = array length).
  //       example 1: permutation([1, 2, 3], 2)
  //       returns 1: [[1, 2], [1, 3], [2, 1], [2, 3], [3, 1], [3, 2]]
  //       example 2: permutation(['a', 'b'])
  //       returns 2: [['a', 'b'], ['b', 'a']]
  //       example 3: permutation([1, 2, 3], 0)
  //       returns 3: [[]]

  if (!Array.isArray(arr)) {
    return []
  }

  const target = r === undefined ? arr.length : Math.trunc(Number(r))
  if (!Number.isFinite(target) || target < 0 || target > arr.length) {
    return []
  }
  if (target === 0) {
    return [[]]
  }

  const out = []
  const used = new Array(arr.length).fill(false)
  const current = []

  const walk = () => {
    if (current.length === target) {
      out.push([...current])
      return
    }

    for (let i = 0; i < arr.length; i += 1) {
      if (used[i]) {
        continue
      }
      used[i] = true
      current.push(arr[i])
      walk()
      current.pop()
      used[i] = false
    }
  }

  walk()
  return out
}

Improve this function

Locutus is a community effort following The McDonald's Theory: we ship first iterations, hoping others will improve them. If you see something that could be better, we'd love your contribution.

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We have 39 Ruby functions so far - help us add more

Got a rainy Sunday afternoon and a taste for a porting puzzle?

• Get inspiration from the Ruby core documentation.
• Click "New file" in the appropriate folder on GitHub. This will fork the project to your account, directly add the file to it, and send a Pull Request to us.

We will then review it. If it's useful to the project and in line with our contributing guidelines your work will become part of Locutus and you'll be automatically credited in the authors section accordingly.

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