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Nicolas DUBIEN
Nicolas DUBIEN

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Advent of PBT 2021 - Day 23

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

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

 * Each year, Santa receives wish lists coming from everywhere.
 * Lists are ordered starting by the most important present to the last
 * important one. Santa does not always offer the most important one.
 * From time to time he wants to wait one more year to check if the
 * present stays in the top.
 * Each year, Santa manually performs a diff of the list with
 * the one of the year before. Based on this diff he makes his choice.
 * You can see the diff manually computed by Santa as an usual git diff
 * between two strings including many lines.
 * Let's take the example:
 *   Year N:
 *     Cars
 *     Trains
 *     Planes
 *   Year N+1:
 *     Cars
 *     Buses
 *     Trains
 *     Boats
 *   Diff of Santa:
 *     === Cars
 *     +++ Buses
 *     === Trains
 *     +++ Boats
 *     --- Planes
 * @param before - String representing the list of presents (before)
 * @param after - String representing the list of presents (after)
declare function wishListsDiffer(before: string, after: string): string;
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We already wrote some examples based tests for it:

it("should compute the appropriate diff", () => {
  // Arrange
  const previousYear = "Cars\nTrains\nPlanes";
  const currentYear = "Cars\nBuses\nTrains\nBoats";

  // Act
  const diff = wishListsDiffer(previousYear, currentYear);

  // Assert
  // prettier-ignore
    "=== Cars\n" +
    "+++ Buses\n" +
    "=== Trains\n" +
    "+++ Boats\n" +
    "--- Planes"
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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:

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.

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