AoC 2024 Day 01: Historian Hysteria

Day 1 - Advent of Code 2024: Historian Hysteria. Eric Wastl. adventofcode.com . Accessed Jun 25, 2025.

Data

There’s just one problem: by holding the two lists up side by side, it quickly becomes clear that the lists aren’t very similar. Maybe you can help The Historians reconcile their lists?

For example:

To parse the input:

using System.Text.RegularExpressions;

namespace AoC2024;

public static partial class HistorianHysteria
{
    public static LocationIds ParseLocationIds(string filePath)
    {
        List<int> left = [];
        List<int> right = [];

        using StreamReader inputReader = new(filePath);
        string? line;
        while ((line = inputReader.ReadLine()) != null)
        {
            Match match = InputLineRegex().Match(line);
            if (!match.Success)
                throw new ArgumentException($"{line} is not well formatted");

            left.Add(int.Parse(match.Groups["left"].Value));
            right.Add(int.Parse(match.Groups["right"].Value));
        }

        return new(left, right);
    }

    public readonly record struct LocationIds(List<int> Left, List<int> Right);

    [GeneratedRegex(@"(?<left>\d+)\s+(?<right>\d+)")]
    private static partial Regex InputLineRegex();
}

Creating a static class is basically the same as creating a class that contains only static members and a private constructor. A private constructor prevents the class from being instantiated. The advantage of using a static class is that the compiler can check to make sure that no instance members are accidentally added. The compiler guarantees that instances of this class can’t be created. Static classes are sealed and therefore can’t be inherited. They can’t inherit from any class or interface except Object. Static classes can’t contain an instance constructor. However, they can contain a static constructor. Non-static classes should also define a static constructor if the class contains static members that require non-trivial initialization.

Using source generation provides the most efficient regex code. RegexOptions.Compiled improves on bare new Regex(), e.g., “match the input character at the current position against ‘a’ or ‘c’” vs. “match the input character at the current position against the set specified in this set description”. However, RegexOptions.Compiled is costly to construct and uses reflection. GeneratedRegex sidesteps these limitations while still being efficient.

Part One

Maybe the lists are only off by a small amount! To find out, pair up the numbers and measure how far apart they are. Pair up the smallest number in the left list with the smallest number in the right list, then the second-smallest left number with the second-smallest right number, and so on.

Within each pair, figure out how far apart the two numbers are; you’ll need to add up all of those distances. For example, if you pair up a 3 from the left list with a 7 from the right list, the distance apart is 4; if you pair up a 9 with a 3, the distance apart is 6.

To find the total distance between the left list and the right list, add up the distances between all of the pairs you found. What is the total distance between your lists?

namespace AoC2024;

public static partial class HistorianHysteria
{
    public static int PartOne(LocationIds locationIds)
    {
        var (left, right) = locationIds;
        left.Sort();
        right.Sort();
        return left.Zip(right, (x1, x2) => int.Abs(x1 - x2)).Sum();
    }
}

Given the positional syntax in readonly record struct LocationIds(List<int> Left, List<int> Right), the compiler creates a Deconstruct method with an out parameters for each positional parameter. However, Deconstruct ignores properties that are defined using standard property syntax.

Part Two

This time, you’ll need to figure out exactly how often each number from the left list appears in the right list. Calculate a total similarity score by adding up each number in the left list after multiplying it by the number of times that number appears in the right list.

For example, the first number in the left list is 3. It appears in the right list 3 times, so the similarity score increases by \(3 \times 3 = 9\). The third number in the left list is 2. It does not appear in the right list once, so the similarity score does not increase (\(2 \times 0 = 0\)). For the example list, the similarity score at the end of this process is \(9 + 4 + 0 + 0 + 9 + 9 = 31\).

Once again, consider your left and right lists. What is their similarity score?

namespace AoC2024;

public static partial class HistorianHysteria
{
    public static int PartTwo(LocationIds locationIds)
    {
        var (left, right) = locationIds;

        // Objective: Use a functional approach. Avoid mutating values.
        var rightLookupTable = right
            .GroupBy(id => id)
            .ToDictionary(group => group.Key, group => group.Count());
        return (
            from id in left
            select id * rightLookupTable.GetValueOrDefault(id, 0)
        ).Sum();
    }
}

References

Static Classes and Static Class Members - C# | Microsoft Learn. learn.microsoft.com . Accessed Aug 28, 2025.
.NET regular expression source generators - .NET | Microsoft Learn. learn.microsoft.com . Accessed Aug 28, 2025.
Records - C# reference | Microsoft Learn. learn.microsoft.com . Accessed Aug 28, 2025.