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Nicolas DUBIEN
Nicolas DUBIEN

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Advent of PBT 2021 - Day 15

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

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

type RaceParticipants = [number, number, number, number, number];

/**
 * 25 Horses Puzzle
 * >  Let’s say that you have 25 horses, and you want to pick the
 * >  fastest 3 horses out of those 25. In each race, only 5
 * >  horses can run at the same time because there are only 5
 * >  tracks. What is the minimum number of races required to find
 * >  the 3 fastest horses without using a stopwatch?
 *
 * Find the podium!
 * See https://matt-croak.medium.com/google-interview-25-horses-c982d0a9b3af for more details.
 *
 * @param runRace - Run a race with the received participants.
 * Outputs the final ranking. In case of equality the participant
 * with the smallest id wins.
 *
 * @returns
 * Ordered top three.
 */
export function racePodium(
  runRace: (...participants: RaceParticipants) => RaceParticipants
): [number, number, number]
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More details on this subject at: https://matt-croak.medium.com/google-interview-25-horses-c982d0a9b3af

We already wrote some examples based tests for it:

it("should find the right podium for a given race", () => {
  // Arrange
  const speeds = [
    14,  1,  8, 19, 23,
    13, 17, 10,  3,  5,
     2, 21, 22,  9, 11,
    20,  7, 16, 24, 18,
     0, 15, 12,  6,  4
  ];
  const compareParticipants = (pa: number, pb: number) => {
    if (speeds[pa] !== speeds[pb]) return speeds[pb] - speeds[pa];
    else return pa - pb;
  };
  const runRace = (...participants: RaceParticipants): RaceParticipants => {
    return participants.sort(compareParticipants);
  };

  // Act
  const podium = racePodium(runRace);

  // Assert
  expect(podium).toEqual([18, 4, 12]);
});
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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-15-xvst7?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-15-solution-5ako


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.

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