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| ## --- Day 24: Crossed Wires --- | ||
| You and The Historians arrive at the edge of a [large grove](/2022/day/23) somewhere in the jungle. After the last incident, the Elves installed a small device that monitors the fruit. While The Historians search the grove, one of them asks if you can take a look at the monitoring device; apparently, it's been malfunctioning recently. | ||
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| The device seems to be trying to produce a number through some boolean logic gates. Each gate has two inputs and one output. The gates all operate on values that are either <em>true</em> (<code>1</code>) or <em>false</em> (<code>0</code>). | ||
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| _Visit the website for the full story and [full puzzle](https://adventofcode.com/2024/day/24) description._ | ||
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| The first half of the problem was a familiar logic circuit evaluator, we have done this before. I really liked the second part, although I don't have a super generic solution for it. I generated a graph from my input using Graphviz, then realized that the circuit tries to implement a full adder. | ||
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| A single full adder consists of two half adders: | ||
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|  | ||
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| Which are chained one after the other like this: | ||
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|  | ||
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| I took the images from [build-electronic-circuits.com](https://www.build-electronic-circuits.com/full-adder/). | ||
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| I implemented this logic in my 'fix' method. I start with the output x01 and y01 and try to identify the gates for the individual steps. Where I found an error, I manually checked my input to figure out what went wrong. I had just two different | ||
| kind of errors which can be corrected by the fixer. |
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| namespace AdventOfCode.Y2024.Day24; | ||
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| using System; | ||
| using System.Collections.Generic; | ||
| using System.Linq; | ||
| using System.Text.RegularExpressions; | ||
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| record struct Rule(string in1, string in2, string kind, string output); | ||
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| [ProblemName("Crossed Wires")] | ||
| class Solution : Solver { | ||
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| public object PartOne(string input) { | ||
| var gate = Parse(input); | ||
| var bits = gate.Keys.Where(k => k.StartsWith("z")).OrderByDescending(x => x).ToArray(); | ||
| var res = 0L; | ||
| foreach (var b in bits) { | ||
| res = res * 2 + gate[b](); | ||
| } | ||
| return res; | ||
| } | ||
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| public object PartTwo(string input) { | ||
| var swaps = Fix(ParseRules(input.Split("\n\n")[1])); | ||
| return string.Join(",", swaps.OrderBy(x => x)); | ||
| } | ||
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| // the rules should define a full adder for two 44 bit numbers | ||
| // this fixer is specific to my input. | ||
| IEnumerable<string> Fix(List<Rule> rules) { | ||
| var cin = Output(rules, "x00", "AND", "y00"); | ||
| for (var i = 1; i < 45; i++) { | ||
| var x = $"x{i:D2}"; | ||
| var y = $"y{i:D2}"; | ||
| var z = $"z{i:D2}"; | ||
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| var xor1 = Output(rules, x, "XOR", y); | ||
| var and1 = Output(rules, x, "AND", y); | ||
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| var and2 = Output(rules, cin, "AND", xor1); | ||
| var xor2 = Output(rules, cin, "XOR", xor1); | ||
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| if (xor2 == null && and2 == null) { | ||
| return Swap(rules, xor1, and1); | ||
| } | ||
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| var carry = Output(rules, and1, "OR", and2); | ||
| if (xor2 != z) { | ||
| return Swap(rules,z,xor2); | ||
| } else { | ||
| cin = carry; | ||
| } | ||
| } | ||
| return []; | ||
| } | ||
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| string Output(IEnumerable<Rule> rules, string x, string gate, string y) => | ||
| rules.SingleOrDefault(rule => | ||
| (rule.in1 == x && rule.kind == gate && rule.in2 == y) || | ||
| (rule.in1 == y && rule.kind == gate && rule.in2 == x) | ||
| ).output; | ||
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| IEnumerable<string> Swap(List<Rule> rules, string out1, string out2) { | ||
| rules = rules.Select(rule => | ||
| rule.output == out1 ? rule with {output = out2} : | ||
| rule.output == out2 ? rule with {output = out1} : | ||
| rule | ||
| ).ToList(); | ||
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| return Fix(rules).Concat([out1, out2]); | ||
| } | ||
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| List<Rule> ParseRules(string input) => input | ||
| .Split("\n") | ||
| .Select(line => { | ||
| var parts = line.Split(" "); | ||
| return new Rule(in1: parts[0], in2: parts[2], kind: parts[1], output: parts[4]); | ||
| }) | ||
| .ToList(); | ||
| Dictionary<string, Gate> Parse(string input) { | ||
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| var res = new Dictionary<string, Gate>(); | ||
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| var blocks = input.Split("\n\n"); | ||
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| foreach (var line in blocks[0].Split("\n")) { | ||
| var parts = line.Split(": "); | ||
| res.Add(parts[0], () => int.Parse(parts[1])); | ||
| } | ||
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| foreach (var line in blocks[1].Split("\n")) { | ||
| var parts = Regex.Matches(line, "[a-zA-z0-9]+").Select(m => m.Value).ToArray(); | ||
| Gate gate = (parts[0], parts[1], parts[2]) switch { | ||
| (var in1, "AND", var in2) => () => res[in1]() & res[in2](), | ||
| (var in1, "OR", var in2) => () => res[in1]() | res[in2](), | ||
| (var in1, "XOR", var in2) => () => res[in1]() ^ res[in2](), | ||
| _ => throw new Exception(), | ||
| }; | ||
| res.Add(parts[3], gate); | ||
| } | ||
| return res; | ||
| } | ||
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| delegate int Gate(); | ||
| } |
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| 61495910098126 | ||
| css,cwt,gdd,jmv,pqt,z05,z09,z37 |
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