DEV Community 👩‍💻👨‍💻

Nicolas DUBIEN
Nicolas DUBIEN

Posted on • Updated on

Advent of PBT 2021 - Day 15 - Solution

Our algorithm was: racePodium.
Go to the subject itself for more details

CodeSandbox with a possible set of properties you may have come with: https://codesandbox.io/s/advent-of-pbt-day-15-solution-e328q?file=/src/index.spec.ts&previewwindow=tests


Property 1: should predict the right podium

for any candidates
it should predict the right podium

Written with fast-check:

it("should predict the right podium", () => {
  fc.assert(
    fc.property(
      fc.array(fc.nat(), { minLength: 25, maxLength: 25 }),
      (speeds) => {
        // Arrange
        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
        const rankedParticipants = [...Array(25)]
          .map((_, i) => i)
          .sort(compareParticipants);
        const expectedPodium = rankedParticipants.slice(0, 3);
        expect(podium).toEqual(expectedPodium);
      }
    )
  );
});
Enter fullscreen mode Exit fullscreen mode

While this property ensures that we always compute the right podium it does not make sure that we do it in an efficient way as it never checks if we make unneeded races.


Property 2: should insert all the selected tabs before the move position

for any candidates
it should at most do 7 races

Written with fast-check:

it("should never do more than 7 races", () => {
  fc.assert(
    fc.property(
      fc.array(fc.nat(), { minLength: 25, maxLength: 25 }),
      (speeds) => {
        // Arrange
        const compareParticipants = (pa: number, pb: number) => {
          if (speeds[pa] !== speeds[pb]) return speeds[pb] - speeds[pa];
          else return pa - pb;
        };
        const runRace = jest.fn(
          (...participants: RaceParticipants): RaceParticipants => {
            return participants.sort(compareParticipants);
          }
        );

        // Act
        racePodium(runRace);

        // Assert
        expect(runRace.mock.calls.length).toBeLessThanOrEqual(7);
      }
    )
  );
});
Enter fullscreen mode Exit fullscreen mode

One of the key characteristics of this problem is that we have a known upper bound for the number of races. Requesting for more than 7 races means that we planned our races in a sub-optimal way leading for extra races to be executed.


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)

We want your help! Become a Tag Moderator.
Fill out this survey and help us moderate our community by becoming a tag moderator here at DEV.