day(14): Restroom Redoubt 🎄

src/AdventOfCode/Day14.cs

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+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Text;
+using AdventOfCode.Utilities;
+
+namespace AdventOfCode
+{
+    /// <summary>
+    /// Solver for Day 14
+    /// </summary>
+    public class Day14
+    {
+        public int Part1(string[] input, int width, int height)
+        {
+            ICollection<Robot> bots = Parse(input);
+
+            for (int i = 0; i < 100; i++)
+            {
+                MoveBots(bots, width, height);
+            }
+
+            int q1 = 0;
+            int q2 = 0;
+            int q3 = 0;
+            int q4 = 0;
+            int midX = width / 2;
+            int midY = height / 2;
+
+            foreach (Robot bot in bots)
+            {
+                if (bot.Position.X < midX)
+                {
+                    if (bot.Position.Y < midY)
+                    {
+                        q1++;
+                    }
+                    else if (bot.Position.Y > midY)
+                    {
+                        q2++;
+                    }
+                }
+                else if (bot.Position.X > midX)
+                {
+                    if (bot.Position.Y < midY)
+                    {
+                        q3++;
+                    }
+                    else if (bot.Position.Y > midY)
+                    {
+                        q4++;
+                    }
+                }
+            }
+
+            return q1 * q2 * q3 * q4;
+        }
+
+        public int Part2(string[] input, int width, int height)
+        {
+            ICollection<Robot> bots = Parse(input);
+
+            int i = 0;
+
+            while (true)
+            {
+                i++;
+
+                MoveBots(bots, width, height);
+
+                var positions = bots.Select(b => b.Position).ToHashSet();
+
+                // there's a 32x34 'frame' around the image with the top corner at 40,47 which I found empirically the first time :D
+                if (positions.Count(p => p.X == 40) == 34 && positions.Count(p => p.Y == 47) == 32)
+                {
+                    Print(i, positions, width, height);
+                    return i;
+                }
+            }
+        }
+
+        /// <summary>
+        /// Parse the input
+        /// </summary>
+        /// <param name="input">Input</param>
+        /// <returns>Parsed robots</returns>
+        private static ICollection<Robot> Parse(string[] input) => input.Select(line => line.Numbers<int>())
+                                                                        .Select(n => new Robot
+                                                                         {
+                                                                             Position = (n[0], n[1]),
+                                                                             Vector = (n[2], n[3])
+                                                                         })
+                                                                        .ToArray();
+
+        /// <summary>
+        /// Move all the bots - this mutates the given collection in-place
+        /// </summary>
+        /// <param name="bots">Bots</param>
+        /// <param name="width">Grid width</param>
+        /// <param name="height">Grid height</param>
+        private static void MoveBots(ICollection<Robot> bots, int width, int height)
+        {
+            foreach (Robot bot in bots)
+            {
+                bot.Position = ((bot.Position.X + bot.Vector.X + width) % width,
+                                (bot.Position.Y + bot.Vector.Y + height) % height);
+            }
+        }
+
+        /// <summary>
+        /// Print the grid and indicate where all the bots are
+        /// </summary>
+        /// <param name="i">Number of moves to get to this point</param>
+        /// <param name="bots">Bot positions</param>
+        /// <param name="width">Grid width</param>
+        /// <param name="height">Grid height</param>
+        private static void Print(int i, ISet<Point2D> bots, int width, int height)
+        {
+            if (!Debugger.IsAttached)
+            {
+                return;
+            }
+
+            StringBuilder s = new StringBuilder(width * height + height * Environment.NewLine.Length);
+
+            for (int y = 0; y < height; y++)
+            {
+                for (int x = 0; x < width; x++)
+                {
+                    s.Append(bots.Contains((x, y)) ? '#' : '.');
+                }
+
+                s.AppendLine();
+            }
+
+            Debug.WriteLine($"Step {i}");
+            Debug.WriteLine(s.ToString());
+        }
+
+        private class Robot
+        {
+            public Point2D Position { get; set; }
+            public Point2D Vector { get; init; }
+        } 
+    }
+}

tests/AdventOfCode.Tests/Day14Tests.cs

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+using System.IO;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace AdventOfCode.Tests
+{
+    public class Day14Tests
+    {
+        private readonly ITestOutputHelper output;
+        private readonly Day14 solver;
+
+        public Day14Tests(ITestOutputHelper output)
+        {
+            this.output = output;
+            this.solver = new Day14();
+        }
+
+        private static string[] GetRealInput()
+        {
+            string[] input = File.ReadAllLines("inputs/day14.txt");
+            return input;
+        }
+
+        private static string[] GetSampleInput()
+        {
+            return new string[]
+            {
+                "p=0,4 v=3,-3",
+                "p=6,3 v=-1,-3",
+                "p=10,3 v=-1,2",
+                "p=2,0 v=2,-1",
+                "p=0,0 v=1,3",
+                "p=3,0 v=-2,-2",
+                "p=7,6 v=-1,-3",
+                "p=3,0 v=-1,-2",
+                "p=9,3 v=2,3",
+                "p=7,3 v=-1,2",
+                "p=2,4 v=2,-3",
+                "p=9,5 v=-3,-3",
+            };
+        }
+
+        [Fact]
+        public void Part1_SampleInput_ProducesCorrectResponse()
+        {
+            var expected = 12;
+
+            var result = solver.Part1(GetSampleInput(), 11, 7);
+
+            Assert.Equal(expected, result);
+        }
+
+        [Fact]
+        public void Part1_RealInput_ProducesCorrectResponse()
+        {
+            var expected = 211692000;
+
+            var result = solver.Part1(GetRealInput(), 101, 103);
+            output.WriteLine($"Day 14 - Part 1 - {result}");
+
+            Assert.Equal(expected, result);
+        }
+
+        [Fact]
+        public void Part2_RealInput_ProducesCorrectResponse()
+        {
+            var expected = 6587;
+
+            var result = solver.Part2(GetRealInput(), 101, 103);
+            output.WriteLine($"Day 14 - Part 2 - {result}");
+
+            Assert.Equal(expected, result);
+        }
+    }
+}