AoC &#Day Red-Nosed Reports (C# and Python)

Today Solution was slighty more complex than the previous day warranting a little more code, but nevertheless was pretty straight forward.

Basic concepts of solution:

In order to be valid, the following conditions for Part 1 must be met:

• We need to decide if the numbers are all increasing, or decreasing from one to the next.

• the gap between the numbers can't be more than 3

Part 1

var reports = File.ReadAllLines("./input1.txt")
    .Select(x => x.Split(" "))
    .ToList();

Part1(reports);

void Part1(List<string[]> input)
{
    var validReports = input
        .Select(report => report.Select(int.Parse).ToList())
        .Count(IsValidReport);

    Console.WriteLine(validReports);
}

bool IsValidReport(List<int> levels)
{
    // We don't care about the numbers, just wether going up or down, not both
    var isIncreasing = IsIncreasing(levels);
    var isDecreasing = IsDecreasing(levels);

    if (!isIncreasing && !isDecreasing) return false;

    // Check that all adjacent levels differ by at least 1 and at most 3
    for (var i = 0; i < levels.Count - 1; i++)
    {
        var diff = Math.Abs(levels[i + 1] - levels[i]);
        if (diff is < 1 or > 3)
        {
            return false;
        }
    }

    return true;
}

bool IsIncreasing(List<int> numbers)
{
    for (var i = 1; i < numbers.Count; i++)
    {
        if (numbers[i] < numbers[i - 1]) return false;
    }

    return true;
}

bool IsDecreasing(List<int> numbers)
{
    for (var i = 1; i < numbers.Count; i++)
    {
        if (numbers[i] > numbers[i - 1]) return false;
    }

    return true;
}

A simple explanation, is that we use LINQ parse the string numbers to integers before passing the report to our helper IsValidReport() method.

This method checks if all the numbers are either increasing or decreasing. This is done by comparing the ids with an ordered list, that will prove all the numbers are all going in one direction.

Once we know they're all in one direction (not the band) we can check if each number is within 3 numbers of each other. Using the Math.Abs() method, which returns the absolute number, helping us account for minus figures.

Note: an absolute number, is the number between the number and zero, i.e -5 and 5 would both return 5;

Part 2

For Part 2 we had an either slightly more complex instructions, but it sounds harder than it is.

The second part introduces the caveat of human error, with the basic concept that you need to know if you can remove any 1 item from the report and it will still be deemed a valid report.

void Part2(List<string[]> input)
{
    var validReports = 0;

    foreach (var ints in input.Select(report => report.Select(int.Parse).ToList()))
    {
        // If valid as-is, count it
        if (IsValidReport(ints))
        {
            validReports++;
            continue;
        }

        // Check if removing any single level makes it valid
        if (ints.Select((t, i) => ints.Where((_, index) => index != i).ToList())
            .Any(IsValidReport))
        {
            validReports++;
        }
    }

    Console.WriteLine(validReports);
}

Here we're maximising the power of LINQ, parsing the string reports, as well as using it to modify the ints array. We modify by looping through the array and removing the current item at index, thus modifying the array of ints.

Once modified, we run the modified report through the validator. This can be done all in one go using the LINQ Select() method, combined with the Any() which will short circuit as soon as the condition is met (i.e as soon as a modified report is valid).

Why only check for a single valid modified report?

This is because it states if any one item can be removed then the report can be deemed valid. It doesn't say a particular item, so as soon as we find a matching item that when removed means the report is valid, we can stop looking at all the others.

We then increment the validReports counter, to keep track of our reports which are valid.

Here's my attempt at converting this to Python:

def main():
    # Read the input file and split each line into a list of strings
    with open("./input1.txt") as file:
        reports = [line.split() for line in file]

    part1(reports)
    part2(reports)


def part1(reports):
    # Count valid reports using a list comprehension and the IsValidReport function

    valid_reports = 0
    for report in reports:
        levels = list(map(int, report))

        if is_valid_report(levels):
            valid_reports += 1

    print(valid_reports)


def part2(reports):
    valid_reports = 0

    for report in reports:
        ints = list(map(int, report))

        # If valid as-is, count it
        if is_valid_report(ints):
            valid_reports += 1
            continue

        # Check if removing any single level makes it valid
        valid_with_dampener = False

        # Iterate over each element in ints
        for i in range(len(ints)):
            # Create a new list without the element at index i
            modified_report = [x for j, x in enumerate(ints) if j != i]

            # Check if the modified report is valid
            if is_valid_report(modified_report):
                valid_with_dampener = True
                break

        if valid_with_dampener:
            valid_reports += 1

    print(valid_reports)


def is_valid_report(levels):
    # Check if the sequence is either all increasing or all decreasing
    is_increasing = is_increasing_sequence(levels)
    is_decreasing = is_decreasing_sequence(levels)

    if not is_increasing and not is_decreasing:
        return False

    # Check that all adjacent levels differ by at least 1 and at most 3
    for i in range(len(levels) - 1):
        diff = abs(levels[i + 1] - levels[i])
        if diff < 1 or diff > 3:
            return False

    return True


def is_increasing_sequence(numbers):
    for i in range(1, len(numbers)):
        if numbers[i] < numbers[i - 1]:
            return False
    return True


def is_decreasing_sequence(numbers):
    for i in range(1, len(numbers)):
        if numbers[i] > numbers[i - 1]:
            return False
    return True


if __name__ == "__main__":
    main()

For a less verbose solution we could replace the Part 2 inner for loop with the modification and checking , like we did with LINQ in the C# solution, like so:

var reports = File.ReadAllLines("./input1.txt")
    .Select(x => x.Split(" "))
    .ToList();

Part1(reports);

void Part1(List<string[]> input)
{
    var validReports = input
        .Select(report => report.Select(int.Parse).ToList())
        .Count(IsValidReport);

    Console.WriteLine(validReports);
}

bool IsValidReport(List<int> levels)
{
    // We don't care about the numbers, just wether going up or down, not both
    var isIncreasing = IsIncreasing(levels);
    var isDecreasing = IsDecreasing(levels);

    if (!isIncreasing && !isDecreasing) return false;

    // Check that all adjacent levels differ by at least 1 and at most 3
    for (var i = 0; i < levels.Count - 1; i++)
    {
        var diff = Math.Abs(levels[i + 1] - levels[i]);
        if (diff is < 1 or > 3)
        {
            return false;
        }
    }

    return true;
}

bool IsIncreasing(List<int> numbers)
{
    for (var i = 1; i < numbers.Count; i++)
    {
        if (numbers[i] < numbers[i - 1]) return false;
    }

    return true;
}

bool IsDecreasing(List<int> numbers)
{
    for (var i = 1; i < numbers.Count; i++)
    {
        if (numbers[i] > numbers[i - 1]) return false;
    }

    return true;
}

But I think you can agree, the verbose method with nicer named variables is much more readable.

Thanks for reading, join me again tomorrow, and as always drop me a follow on twitter

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