day 7 part 2 solution (yay recursion)

2020/Day07.cs

@@ -64,6 +64,16 @@ namespace AdventOfCode {
       }
     }
 
+    static int CountBags(List<Contains> rules, string target) {
+      int total = 1;
+      foreach (Contains rule in rules) {
+        if (rule.Outer == target) {
+          total += rule.Count * CountBags(rules, rule.Inner);
+        }
+      }
+      return total;
+    }
+
     static int Part1() {
       List<Contains> contains = ParseRules(File.ReadAllLines(Util.RootDir + "day07.txt"));
       return ComputePart1(contains);
@@ -71,7 +81,7 @@ namespace AdventOfCode {
 
     static int Part2() {
       List<Contains> contains = ParseRules(File.ReadAllLines(Util.RootDir + "day07.txt"));
-      return -1;
+      return CountBags(contains, "shiny gold") - 1;
     }
 
     [Fact]
@@ -88,9 +98,19 @@ faded blue bags contain no other bags.
 dotted black bags contain no other bags.".Split('\n');
 
       Assert.Equal(4, ComputePart1(ParseRules(example)));
-
       Assert.Equal(213, Part1());
-      Assert.Equal(-1, Part2());
+
+      string[] example2 =
+@"shiny gold bags contain 2 dark red bags.
+dark red bags contain 2 dark orange bags.
+dark orange bags contain 2 dark yellow bags.
+dark yellow bags contain 2 dark green bags.
+dark green bags contain 2 dark blue bags.
+dark blue bags contain 2 dark violet bags.
+dark violet bags contain no other bags.".Split('\n');
+      Assert.Equal(127, CountBags(ParseRules(example2), "shiny gold"));
+
+      Assert.Equal(38426, Part2());
     }
   }
 }