DEV Community

Nicolas DUBIEN
Nicolas DUBIEN

Posted on • Updated on

Advent of PBT 2021 - Day 22

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

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

/**
 * Santa' elves often want to know in advance what will be the plan
 * of Santa for the upcoming Christmas. As a consequence, they try
 * to spy on Santa by looking on top of a wall separating them from
 * him when Santa starts receiving letters.
 *
 * They usually try to mount on each others shoulders to have the exact
 * same height as the whole. But even if they tried many years in a raw
 * they have only succeeding with 1, 2 or 3 eleves.
 *
 * Could you help them in their mission?
 * In other words: could you return one, two or three elves (by index)
 * such as:
 *   height(elves[i]) = height(wall)
 * OR
 *   height(elves[i]) + height(elves[j]) = height(wall)
 * OR
 *   height(elves[i]) + height(elves[j]) + height(elves[k]) = height(wall)
 *
 * @param elvesHeight - Strictly positive integers representing
 *                      the heights of our elves
 * @param wallHeight - The height of the wall
 *
 * @returns
 * The one, two or three selected elves if there is a solution,
 * undefined otherwise.
 */
declare function spyOnSanta(
  elvesHeight: number[],
  wallHeight: number
): number[] | undefined;
Enter fullscreen mode Exit fullscreen mode

We already wrote some examples based tests for it:

it("should find combinations including one elf", () => {
  // Arrange
  const elves = [1, 3, 6, 11, 13];

  // Act
  const selectedElves = spyOnSanta(elves, 11);
  // 11 = 11

  // Assert
  expect(selectedElves).toEqual([3]);
});

it("should find combinations including two elves", () => {
  // Arrange
  const elves = [1, 3, 6, 11, 13];

  // Act
  const selectedElves = spyOnSanta(elves, 4);
  // 4 = 1 + 3

  // Assert
  expect(selectedElves).toEqual([0, 1]);
});

it("should find combinations including three elves", () => {
  // Arrange
  const elves = [1, 3, 6, 11, 13];

  // Act
  const selectedElves = spyOnSanta(elves, 10);
  // 10 = 1 + 3 + 6

  // Assert
  expect(selectedElves).toEqual([0, 1, 2]);
});

it("should not find combinations including four elves", () => {
  // Arrange
  const elves = [1, 1, 3, 6, 11, 13];

  // Act
  const selectedElves = spyOnSanta(elves, 33);
  // 33 = 3 + 6 + 11 + 13

  // Assert
  expect(selectedElves).toBe(undefined);
});

it("should be able to deal with elves having the same height", () => {
  // Arrange
  const elves = [1, 1, 5, 10, 15];

  // Act
  const selectedElves = spyOnSanta(elves, 12);
  // 12 = 1 + 1 + 10

  // Assert
  expect(selectedElves).toEqual([0, 1, 3]);
});
Enter fullscreen mode Exit fullscreen mode

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-22-7zgcs?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:


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.

Top comments (0)