AoC# 2024 - Day 21: Keypad Conundrum

It’s Advent of Code time 🎄

Advent of Code is an Advent calendar of small programming puzzles for a variety of skill levels that can be solved in any programming language you like.

Day 21 is a conundrum alright…

Part 1 ❌

This looked tricky from the outset. Daunted, I started breaking down the problem into chunks and an approach quickly appeared - I figured storing the keypads as Dictionary<char, Point>¹ meant quick look-ups:

private readonly Dictionary<char, Point> _numericKeypad = new()
{
    {'7', new Point(0, 0)}, {'8', new Point(1, 0)}, {'9', new Point(2, 0)},
    {'4', new Point(0, 1)}, {'5', new Point(1, 1)}, {'6', new Point(2, 1)},
    {'1', new Point(0, 2)}, {'2', new Point(1, 2)}, {'3', new Point(2, 2)},
    /* gap at       0, 3 */ {'0', new Point(1, 3)}, {'A', new Point(2, 3)},
};

private readonly Dictionary<char, Point> _directionalKeypad = new()
{
    /* gap at       0, 0 */ {'^', new Point(1, 0)}, {'A', new Point(2, 0)},
    {'<', new Point(0, 1)}, {'v', new Point(1, 1)}, {'>', new Point(2, 1)},
};

Then it was a matter of working out the X,Y difference between the char that is the current/last pressed button and the next.

Writing that function it became clear the logic was the same for both keypads, so I refactored it to accept the keypad, then the caller passes in the numeric keypad followed by the directional one twice, like so:

public string Part1()
{
    var result = 0;

    foreach (var code in _codes)
    {
        var keypadSequence = GetKeyPadSequence(code, _numericKeypad);

        keypadSequence = GetKeyPadSequence(keypadSequence, _directionalKeypad);

        keypadSequence = GetKeyPadSequence(keypadSequence, _directionalKeypad);

        Console.WriteLine($"{code}: {keypadSequence}");  // for debugging, keep reading...

        result += keypadSequence.Length * int.Parse(code.Replace("A", ""));
    }

    return result.ToString();
}

Which works for the example input but gives the wrong answer for the actual puzzle input 😕

I’m sure the issue is with how GetKeyPadSequence() is generating the sequences and I have checks to ensure the gaps are avoided, but after tinkering on and off I’ve run out of day 🙁

There’s still time to pick up at least one of the remaining 8 stars to get a new Personal Best this year… 🤞

Part 2 ❌

Can’t get to Part 2 without Part 1

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1. Point() is a simple POCO coordinate class from my AdventOfCode.Common library, which is available as a NuGet package ↩