Advent Of Code 2024

This is my code developed for Advent of Code 2024. My primary goals are:

• have fun
• learn something new about Kotlin
• don't spend too much time

I thus favor short and readable code over optimized code. As a rule of thumb, any code which finishes in under 1 minute is "good enough". I also will not try to handle possible corner cases if the provided input file does not contain them.

AoC provides 2 coding challenges for every day in December until Christmas. Every daily challenge also has one file containing the data for the challenges.

Principles

This project setup uses a unit test class per day to provide the solution code. Every class will contain 2 unit tests (one per challenge), and one or more functions which implement the solution. I chose the unit test approach because that allows to code the expected/correct result in a standardized way, and because IntelliJ can directly execute either all or individual tests.

To simplify the test setup, this project uses a Junit5 extension which automates the passing the content of the daily input file to the unit tests. For this to work, the code for every day is stored in a separate test class file in src/test/kotlin, and the input file for that day must be in a file in input. As an example, the code for Day 1 will be in src/test/Kotlin/Day01.kt, and the input for Day 1 will be stored in input/Day01.txt.

To prepare this setup for a day, run ./day which creates both these files for the current day. You can also pass the day number as a parameter to ./day.

Implementation Details

The input test data handling is implemented using a Junit5 extension for test parameters, implemented in InputParameterResolver. To avoid having to annotate test cases, this extension is automatically enabled by using the files resources/META-INF/services/org.junit.jupiter.api.extension.Extension which lists the extension and resources/junit-platform.properties which enables the auto-detection of extensions.

Example / Template

This repository contains the input file input/Day00 and the solution file src/test/kotlin/Day00.kt as an example. This Day00.kt is a good candidate to be copied for the daily challenges.

• input/Day00

1
2
3
4
5

• src/test/kotlin/Day00.kt

import io.kotest.matchers.shouldBe
import org.junit.jupiter.api.Test

/*
 Challenge for day 0: given a list of numbers, return their sum. As an example,
 the answer for the following list is 8:
   3
   1
   4

 --- Part Two ---
 Instead of adding the numbers, multiply them. For the above example input, the answer is 12.
 */

class Day00 {
    private val sample = """
        3
        1
        4
    """.trimIndent().lines()

    @Test
    fun testOne(input: List<String>) {
        // provide explicit lists for testing other cases than the actual test input
        one(sample) shouldBe 8
        one(input) shouldBe 15
    }

    @Test
    fun testTwo(input: List<String>) {
        two(sample) shouldBe 12L
        two(input) shouldBe 120L
    }

    // This should return the sum of the input
    private fun one(input: List<String>): Int = input.map(String::toInt).sum()

    // This should return the product of the input
    private fun two(input: List<String>): Long {
        return input.map(String::toLong).reduce { acc, i -> acc * i }
    }
}