Advent of PBT 2021 - Day 2

Advent of PBT 2021 — Learn how to use property based testing and fast-check through examples

Our algorithm today is: decomposeIntoPrimes.
It comes with the following documentation and prototype:

/**
 * Decompose the received integer number into prime factors.
 * The numbers may come in any order in the output.
 *
 * @param n - Integer value to be decomposed into prime factors,
 * must be in range 2 (included) to 2**31-1 (included)
 *
 * @returns
 * The prime factors to build n.
 */
declare function decomposeIntoPrimes(n: number): number[];

The definition of a prime number according to Wikipedia is the following:

A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers.

In other words, the numbers 2, 3, 5, 7 are prime numbers whereas 4, 6, 8 or 9 are not.

In order to clarify our expectations, we already wrote some examples based tests for it:

it("should decompose a prime number into itself", () => {
  expect(sorted(decomposeIntoPrimes(5))).toEqual([5]);
});

it("should decompose a number product of two primes", () => {
  expect(sorted(decomposeIntoPrimes(10))).toEqual([2, 5]);
});

it("should decompose a number product of three primes", () => {
  expect(sorted(decomposeIntoPrimes(30))).toEqual([2, 3, 5]);
});

it("should decompose a number product of many times the same prime", () => {
  expect(sorted(decomposeIntoPrimes(8))).toEqual([2, 2, 2]);
});

How would you cover it with Property Based Tests?

In order to ease your task we provide you with an already setup CodeSandbox, with examples based tests already written and a possible implementation of the algorithm: https://codesandbox.io/s/advent-of-pbt-day-2-pjpqx?file=/src/index.spec.ts&previewwindow=tests

You wanna see the solution? Here is the set of properties I came with to cover today's algorithm: https://dev.to/dubzzz/advent-of-pbt-2021-day-2-solution-367b

Back to "Advent of PBT 2021" to see topics covered during the other days and their solutions.

More about this serie on @ndubien or with the hashtag #AdventOfPBT.