aoc2021/aoc2021/Day20.cs

81 lines
2.8 KiB
C#

namespace aoc2021;
/// <summary>
/// Day 20: <see href="https://adventofcode.com/2021/day/20"/>
/// </summary>
public sealed class Day20 : Day
{
private readonly ImmutableArray<bool> _enhancementAlgorithm;
private readonly Image _initialImage;
public Day20() : base(20, "Trench Map")
{
_enhancementAlgorithm = Input.First().Select(ch => ch == '#').ToImmutableArray();
_initialImage = Parse(Input.Skip(2).ToList());
}
private Image Enhance(Image image, int count) =>
Enumerable.Range(0, count).Aggregate(image, (current, _) => EnhanceOnce(current));
private Image EnhanceOnce(Image image)
{
var bounds = image.Bounds.Grow();
var result = bounds.Points
.Where(pt => _enhancementAlgorithm[image.GetEnhanceInput(pt)])
.ToImmutableHashSet();
return new(bounds, result, image.InfiniteValue
? _enhancementAlgorithm.Last()
: _enhancementAlgorithm.First());
}
private static Image Parse(IEnumerable<string> grid)
{
var image = ImmutableHashSet.CreateBuilder<Point>();
foreach (var (line, y) in grid.Select((a, i) => (a, i)))
{
image.UnionWith(line.Select((ch, i) => (x: i, isLit: ch == '#'))
.Where(p => p.isLit)
.Select(p => new Point(p.x, y)));
}
var bounds = new Rect(image.Min(p => p.X), image.Min(p => p.Y), image.Max(p => p.X), image.Max(p => p.Y));
return new(bounds, image.ToImmutable());
}
public override object Part1() =>
Enhance(_initialImage, 2).PixelCount;
public override object Part2() =>
Enhance(_initialImage, 50).PixelCount;
private record struct Point(int X, int Y);
private record Rect(int MinX, int MinY, int MaxX, int MaxY)
{
public IEnumerable<Point> Points =>
Enumerable.Range(MinY, MaxY - MinY + 1)
.SelectMany(_ => Enumerable.Range(MinX, MaxX - MinX + 1), (y, x) => new Point(x, y));
public bool Contains(Point pt) => pt.X >= MinX && pt.X <= MaxX && pt.Y >= MinY && pt.Y <= MaxY;
public Rect Grow() => new(MinX - 1, MinY - 1, MaxX + 1, MaxY + 1);
}
private record Image(Rect Bounds, ImmutableHashSet<Point> Pixels, bool InfiniteValue = false)
{
private bool this[Point pt] =>
Bounds.Contains(pt) ? Pixels.Contains(pt) : InfiniteValue;
public int PixelCount => Pixels.Count(Bounds.Contains);
public int GetEnhanceInput(Point pt)
{
var (x, y) = pt;
var values =
Enumerable.Range(y - 1, 3)
.SelectMany(_ => Enumerable.Range(x - 1, 3), (yi, xi) => this[new(xi, yi)] ? 1 : 0);
return values.Aggregate(0, (p, n) => (p << 1) | n);
}
}
}