Day 17

I've done better. I've done worse. ¯\_(ツ)_/¯

advent-of-code-2021.csproj

@@ -66,6 +66,9 @@
 	  <None Update="inputs\16.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\17.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	</ItemGroup>
 
 </Project>

inputs/17.txt

@@ -0,0 +1 @@
+target area: x=32..65, y=-225..-177

src/17.cs

@@ -0,0 +1,183 @@
+using System.Diagnostics;
+
+namespace aoc2021;
+
+internal class Day17 : Day
+{
+    [DebuggerDisplay("{minX},{minY} -> {maxX},{maxY}")]
+    private struct Rectangle
+    {
+        public int minX = 0;
+        public int minY = 0;
+        public int maxX = 0;
+        public int maxY = 0;
+
+        public bool Contains((int x, int y) pt) => pt.x >= minX && pt.x <= maxX && pt.y >= minY && pt.y <= maxY;
+        public bool Beyond((int x, int y) pt) => IsFar(pt) || IsLow(pt);
+        public bool IsShort((int x, int y) pt) => pt.x < minX;
+        public bool IsFar((int x, int y) pt) => pt.x > maxX;
+        public bool IsHigh((int x, int y) pt) => pt.y > maxY;
+        public bool IsLow((int x, int y) pt) => pt.y < minY;
+    }
+
+    internal override void Go()
+    {
+        var lines = Util.ReadAllLines("inputs/17.txt");
+        var instructions = lines.ElementAt(0).Split(", ");
+
+        Rectangle bounds = new();
+
+        var xs = instructions[0].Split("..");
+        bounds.minX = Convert.ToInt32(xs[0][(xs[0].IndexOf('=') + 1)..]);
+        bounds.maxX = Convert.ToInt32(xs[1]);
+
+        var ys = instructions[1].Split("..");
+        bounds.minY = Convert.ToInt32(ys[0][(ys[0].IndexOf('=') + 1)..]);
+        bounds.maxY = Convert.ToInt32(ys[1]);
+
+        Part1(bounds);
+        Part2(bounds);
+    }
+
+    private static (int x, int y) Step((int x, int y) pt, ref (int x, int y) velocity)
+    {
+        pt.x += velocity.x;
+        pt.y += velocity.y;
+        if (velocity.x > 0)
+        {
+            velocity.x--;
+        }
+        else if (velocity.x < 0)
+        {
+            velocity.x++;
+        }
+        velocity.y--;
+
+        return (pt.x, pt.y);
+    }
+
+    private static void Part1(Rectangle bounds)
+    {
+        using var t = new Timer();
+
+        var successes = GetSuccessfulVelocities(bounds);
+        var max = successes.MaxBy(vel => vel.Value);
+        var numOthers = successes.Count(vel => vel.Value == max.Value);
+
+        Logger.Log($"<+black>> part1: highest Y was at velocity {max.Key} (and {numOthers} other{(numOthers == 1 ? "" : "s")}): <+white>{max.Value}<r>");
+    }
+
+    private static void Part2(Rectangle bounds)
+    {
+        using var t = new Timer();
+
+        var successes = GetSuccessfulVelocities(bounds);
+
+        Logger.Log($"<+black>> part2: #successful velocities: <+white>{successes.Count}<r>");
+    }
+
+    private static Dictionary<(int x, int y), int> GetSuccessfulVelocities(Rectangle bounds)
+    {
+        var (minX, maxX) = GetXVelocityRange(bounds);
+        var (minY, maxY) = GetYVelocityRange(bounds);
+
+        (int x, int y) pt;
+        Dictionary<(int x, int y), int> successes = new();
+        for (int x = minX; x <= maxX; x++)
+        {
+            for (int y = minY; y <= maxY; y++)
+            {
+                pt = (0, 0);
+                var startingVelocity = (x, y);
+                var velocity = startingVelocity;
+                int currHighestY = int.MinValue;
+                while (!bounds.Beyond(pt) && !bounds.Contains(pt))
+                {
+                    pt = Step(pt, ref velocity);
+                    if (pt.y > currHighestY)
+                    {
+                        currHighestY = pt.y;
+                    }
+                }
+
+                if (bounds.Contains(pt))
+                {
+                    successes.Add(startingVelocity, currHighestY);
+                }
+            }
+        }
+
+        return successes;
+    }
+
+    private static (int min, int max) GetXVelocityRange(Rectangle bounds)
+    {
+        var minSuccessXVel = int.MaxValue;
+        for (var guess = bounds.maxX; ; guess--)
+        {
+            var guesspt = (0, 0);
+            var testVel = (x: guess, 0);
+            while (testVel.x > 0)
+            {
+                guesspt = Step(guesspt, ref testVel);
+            }
+
+            if (bounds.IsShort(guesspt))
+            {
+                break;
+            }
+            else if (!bounds.IsFar(guesspt))
+            {
+                if (guess < minSuccessXVel)
+                {
+                    minSuccessXVel = guess;
+                }
+            }
+        }
+
+        return (minSuccessXVel, bounds.maxX);
+    }
+
+    private static (int min, int max) GetYVelocityRange(Rectangle bounds)
+    {
+        int maxSuccessYVel = int.MinValue;
+        int guess = bounds.minY;
+        bool foundYVals = false;
+        while (!foundYVals)
+        {
+            var guesspt = (0, 0);
+            var testVel = (0, guess);
+            bool lastWasAbove = false;
+            while (true)
+            {
+                guesspt = Step(guesspt, ref testVel);
+                if (bounds.IsHigh(guesspt))
+                {
+                    lastWasAbove = true;
+                }
+                else if (!bounds.IsLow(guesspt))
+                {
+                    if (guess > maxSuccessYVel)
+                    {
+                        maxSuccessYVel = guess;
+                    }
+                    break;
+                }
+                else
+                {
+                    // need to find a better way to choose a reasonable guess max...
+                    if (lastWasAbove && guess > bounds.minY * -4)
+                    {
+                        foundYVals = true;
+                    }
+
+                    break;
+                }
+            }
+
+            guess++;
+        }
+
+        return (bounds.minY, maxSuccessYVel);
+    }
+}

src/main.cs

@@ -34,7 +34,8 @@ else
         "13" => new Day13(),
         "14" => new Day14(),
         "15" => new Day15(),
-        _ => new Day16(),
+        "16" => new Day16(),
+        _ => new Day17(),
     };
     day.Go();
 }