actually add Day 6 code

2020/Day06.cs

@@ -0,0 +1,92 @@
+using System;
+using static System.Console;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using Xunit;
+
+namespace AdventOfCode {
+
+  public class Day06 {
+
+    static int CountAnswers1(string[] input) {
+      int total = 0;
+      var groupAnswers = new HashSet<char>();
+      foreach (string line in input) {
+        if (line == "") {
+          total += groupAnswers.Count();
+          groupAnswers.Clear();
+          continue;
+        }
+        foreach (char c in line) {
+          groupAnswers.Add(c);
+        }
+      }
+      total += groupAnswers.Count();
+      return total;
+    }
+
+    static int CountAnswers2(string[] input) {
+      int total = 0;
+      var groupAnswers = new HashSet<char>();
+      int groupSize = 0;
+      foreach (string line in input) {
+        if (line == "") {
+          total += groupAnswers.Count();
+          groupAnswers.Clear();
+          groupSize = 0;
+          continue;
+        }
+        groupSize++;
+        if (groupSize == 1) {
+          foreach (char c in line) {
+            groupAnswers.Add(c);
+          }
+        } else {
+          foreach (char c in groupAnswers) {
+            if (!line.Contains(c)) {
+              groupAnswers.Remove(c);
+            }
+          }
+        }
+      }
+      total += groupAnswers.Count();
+      return total;
+    }
+
+    static int Part1() {
+      string[] input = File.ReadAllLines(Util.RootDir + "day06.txt");
+      return CountAnswers1(input);
+    }
+
+    static int Part2() {
+      string[] input = File.ReadAllLines(Util.RootDir + "day06.txt");
+      return CountAnswers2(input);
+    }
+
+    [Fact]
+    public static void Test() {
+      string[] example =
+@"abc
+
+a
+b
+c
+
+ab
+ac
+
+a
+a
+a
+a
+
+b".Split('\n');
+      Assert.Equal(11, CountAnswers1(example));
+      Assert.Equal(6, CountAnswers2(example));
+
+      Assert.Equal(6947, Part1());
+      Assert.Equal(3398, Part2());
+    }
+  }
+}