diff --git a/AOC.Test/Test2015.cs b/AOC.Test/Test2015.cs
index ceac1cc..1c0b16e 100644
--- a/AOC.Test/Test2015.cs
+++ b/AOC.Test/Test2015.cs
@@ -24,6 +24,7 @@ public class Test2015
     [DataRow(typeof(Day16), "103", "405")]
     [DataRow(typeof(Day17), "1304", "18")]
     [DataRow(typeof(Day18), "1061", "1006")]
+    [DataRow(typeof(Day19), "576", "207")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {
         Common.CheckDay(dayType, part1, part2);
@@ -42,6 +43,7 @@ public class Test2015
     [DataRow(typeof(Day10), "237746", "3369156")]
     [DataRow(typeof(Day13), "330", "286")]
     [DataRow(typeof(Day15), "62842880", "57600000")]
+    [DataRow(typeof(Day19), "4", "2")]
     public void CheckTestInputs(Type dayType, string part1, string part2)
     {
         Common.CheckDay(dayType, part1, part2, true);
diff --git a/AOC2015/AOC2015.csproj b/AOC2015/AOC2015.csproj
index 77ab940..50a1bbe 100644
--- a/AOC2015/AOC2015.csproj
+++ b/AOC2015/AOC2015.csproj
@@ -27,6 +27,7 @@
     </ItemGroup>
 
     <ItemGroup>
+      <PackageReference Include="morelinq" Version="3.4.2" />
       <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
     </ItemGroup>
 
diff --git a/AOC2015/Day19.cs b/AOC2015/Day19.cs
index 21a1f1b..ddd2c6b 100644
--- a/AOC2015/Day19.cs
+++ b/AOC2015/Day19.cs
@@ -1,3 +1,5 @@
+using MoreLinq;
+
 namespace AOC2015;
 
 /// <summary>
@@ -5,11 +7,42 @@ namespace AOC2015;
 /// </summary>
 public sealed class Day19() : Day(2015, 19, "Medicine for Rudolph")
 {
+    private IEnumerable<string[]>? _rules;
+    private string? _compound;
+
     public override void ProcessInput()
     {
+        var i = Input.Split("").ToList();
+        _rules = i[0].Select(r => r.Split(" => "));
+        _compound = i[1].Single();
     }
 
-    public override object Part1() => "";
+    private int CountSubstring(string needle)
+    {
+        var count = 0;
+        for (var i = _compound!.IndexOf(needle, StringComparison.InvariantCulture);
+             i >= 0;
+             ++count, i = _compound.IndexOf(needle, i + 1, StringComparison.InvariantCulture))
+        {
+        }
 
-    public override object Part2() => "";
-}
+        return count;
+    }
+
+    public override object Part1()
+    {
+        HashSet<string> compounds = new();
+        foreach (var rule in _rules!)
+            foreach (var match in Regex.EnumerateMatches(_compound, rule[0]))
+                compounds.Add(_compound!.Remove(match.Index, rule[0].Length).Insert(match.Index, rule[1]));
+
+        return compounds.Count;
+    }
+
+    public override object Part2() =>
+        _compound!.Count(char.IsUpper)
+        - CountSubstring("Rn")
+        - CountSubstring("Ar")
+        - 2 * CountSubstring("Y")
+        - 1;
+}
\ No newline at end of file
diff --git a/AOC2015/input2015/test19.in b/AOC2015/input2015/test19.in
new file mode 100644
index 0000000..af30faa
--- /dev/null
+++ b/AOC2015/input2015/test19.in
@@ -0,0 +1,5 @@
+H => HO
+H => OH
+O => HH
+
+HOH