permutations.test.ts

import permutations from '../permutations'

describe('the permutation function', () => {
  it('should handle the empty array case', () => {
    expect(Array.from(permutations([]))).toEqual([])
  })

  it('should handle the a single element', () => {
    expect(Array.from(permutations([['a']]))).toEqual([['a']])
    expect(Array.from(permutations([['a', 'b', 'c']]))).toEqual([['a'], ['b'], ['c']])
  })

  it('should handle multiple elements', () => {
    expect(Array.from(permutations([['a', 'b', 'c'], [1, 2, 3]]))).toEqual([
      ['a', 1],
      ['a', 2],
      ['a', 3],
      ['b', 1],
      ['b', 2],
      ['b', 3],
      ['c', 1],
      ['c', 2],
      ['c', 3]
    ])
  })
})

permutations.ts

export default function* permutations(items: any[][], index: number = 0, ret?: any[]) {
  if (index >= items.length) {
    if (ret) {
      yield ret
    }
  } else {
    const next = ret || []
    for (const item of items[index]) {
      yield* permutations(items, index + 1, next.concat(item))
    }
  }
}