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| ## --- Day 6: Guard Gallivant --- | ||
| The Historians use their fancy [device](4) again, this time to whisk you all away to the North Pole prototype suit manufacturing lab... in the year [1518](/2018/day/5)! It turns out that having direct access to history is very convenient for a group of historians. | ||
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| You still have to be careful of time paradoxes, and so it will be important to avoid anyone from 1518 while The Historians search for the Chief. Unfortunately, a single <em>guard</em> is patrolling this part of the lab. | ||
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| Maybe you can work out where the guard will go ahead of time so that The Historians can search safely? | ||
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| Read the [full puzzle](https://adventofcode.com/2024/day/6). |
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| namespace AdventOfCode.Y2024.Day06; | ||
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| using System; | ||
| using System.Collections.Generic; | ||
| using System.Collections.Immutable; | ||
| using System.Linq; | ||
| using System.Numerics; | ||
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| using Map = System.Collections.Generic.Dictionary<System.Numerics.Complex, char>; | ||
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| [ProblemName("Guard Gallivant")] | ||
| class Solution : Solver { | ||
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| Complex Up = Complex.ImaginaryOne; | ||
| Complex TurnRight = -Complex.ImaginaryOne; | ||
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| public object PartOne(string input) { | ||
| var (map, start) = Parse(input); | ||
| return Walk(map, start).positions.Count(); | ||
| } | ||
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| public object PartTwo(string input) { | ||
| var (map, start) = Parse(input); | ||
| var positions = Walk(map, start).positions; | ||
| var loops = 0; | ||
| // simply try a blocker in each locations visited by the guard and count the loops | ||
| foreach (var block in positions.Where(pos => map[pos] == '.')) { | ||
| map[block] = '#'; | ||
| if (Walk(map, start).isLoop) { | ||
| loops++; | ||
| } | ||
| map[block] = '.'; | ||
| } | ||
| return loops; | ||
| } | ||
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| // returns the positions visited when starting from 'pos', isLoop is set if the | ||
| // guard enters a cycle. | ||
| (IEnumerable<Complex> positions, bool isLoop) Walk(Map map, Complex pos) { | ||
| var seen = new HashSet<(Complex pos, Complex dir)>(); | ||
| var dir = Up; | ||
| while (map.ContainsKey(pos) && !seen.Contains((pos, dir))) { | ||
| seen.Add((pos, dir)); | ||
| if (map.GetValueOrDefault(pos + dir) == '#') { | ||
| dir *= TurnRight; | ||
| } else { | ||
| pos += dir; | ||
| } | ||
| } | ||
| return ( | ||
| positions: seen.Select(s => s.pos).Distinct(), | ||
| isLoop: seen.Contains((pos, dir)) | ||
| ); | ||
| } | ||
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| // store the grid in a dictionary, to make bounds checks and navigation simple | ||
| // start represents the starting postion of the guard | ||
| (Map map, Complex start) Parse(string input) { | ||
| var lines = input.Split("\n"); | ||
| var map = ( | ||
| from y in Enumerable.Range(0, lines.Length) | ||
| from x in Enumerable.Range(0, lines[0].Length) | ||
| select new KeyValuePair<Complex, char>(-Up * y + x, lines[y][x]) | ||
| ).ToDictionary(); | ||
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| var start = map.First(x => x.Value == '^').Key; | ||
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| return (map, start); | ||
| } | ||
| } |
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| 1304 |
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