From 18311ecbac20ff077bbed95dc36660d9ffd5b997 Mon Sep 17 00:00:00 2001
From: Ben Harris <ben@tilde.team>
Date: Wed, 16 Dec 2020 14:58:07 -0500
Subject: [PATCH] day 16 part 2

---
 aoc2020.test/DayTests.cs |  2 +-
 aoc2020/Day16.cs         | 48 +++++++++++++++++++++++++++++++++++-----
 aoc2020/Extensions.cs    |  6 +++++
 3 files changed, 50 insertions(+), 6 deletions(-)

diff --git a/aoc2020.test/DayTests.cs b/aoc2020.test/DayTests.cs
index 6d2fce6..56e32ad 100644
--- a/aoc2020.test/DayTests.cs
+++ b/aoc2020.test/DayTests.cs
@@ -23,7 +23,7 @@ namespace aoc2020.test
         [DataRow(typeof(Day13), "171", "539746751134958")]
         [DataRow(typeof(Day14), "17481577045893", "4160009892257")]
         [DataRow(typeof(Day15), "257", "8546398")]
-        [DataRow(typeof(Day16), "19093", "")]
+        [DataRow(typeof(Day16), "19093", "5311123569883")]
         public void CheckAllDays(Type dayType, string part1, string part2)
         {
             // create day instance
diff --git a/aoc2020/Day16.cs b/aoc2020/Day16.cs
index c9c2b70..12230a2 100644
--- a/aoc2020/Day16.cs
+++ b/aoc2020/Day16.cs
@@ -9,8 +9,8 @@ namespace aoc2020
     /// </summary>
     public sealed class Day16 : Day
     {
-        private List<List<int>> _tickets;
-        private Dictionary<string, List<Range>> _rules;
+        private readonly Dictionary<string, List<Range>> _rules;
+        private readonly List<List<int>> _tickets;
 
         public Day16() : base(16)
         {
@@ -41,13 +41,51 @@ namespace aoc2020
 
         public override string Part1()
         {
+            var allValues = _tickets.Skip(1).SelectMany(t => t);
+            var allRules = _rules.Values.SelectMany(r => r);
             return
-                $"{_tickets.Skip(1).SelectMany(t => t).Where(t => !_rules.Values.SelectMany(r => r).Any(r => t >= r.Start.Value && t <= r.End.Value)).Sum()}";
+                $"{allValues.Where(t => !allRules.Any(r => r.Contains(t))).Sum()}";
         }
 
         public override string Part2()
         {
-            return "";
+            var ticketFields = _tickets
+                // valid tickets
+                .Where(ticket => ticket
+                    .All(t => _rules.Values
+                        .SelectMany(r => r)
+                        .Any(r => r.Contains(t))))
+                // group by index
+                .SelectMany(inner => inner.Select((item, index) => new {item, index}))
+                .GroupBy(i => i.index, i => i.item)
+                .Select(g => g.ToList())
+                .Select((val, i) => new {Value = val, Index = i})
+                .ToList();
+
+            var matchedRules = _rules
+                // find matching rules and indices
+                .SelectMany(x => ticketFields
+                    .Where(y => y.Value.All(z => x.Value.Any(r => r.Contains(z))))
+                    .Select(y => (x.Key, y.Index))
+                    .ToList())
+                .ToList();
+
+            matchedRules.Sort((a, b) =>
+                matchedRules.Count(x => x.Key == a.Key) - matchedRules.Count(x => x.Key == b.Key));
+
+            while (matchedRules.Any(x => matchedRules.Count(y => y.Key == x.Key) > 1))
+                foreach (var (key, index) in matchedRules.Where(y =>
+                        matchedRules.Count(z => z.Key == y.Key) == 1 &&
+                        matchedRules.Count(z => z.Index == y.Index) > 1))
+                    // filter matches by index
+                    matchedRules = matchedRules
+                        .Where(x => x.Index != index || x.Key == key)
+                        .ToList();
+
+            var departureFields = matchedRules.Where(r => r.Key.StartsWith("departure"));
+
+            return
+                $"{departureFields.Aggregate(1L, (l, match) => l * _tickets.First()[match.Index])}";
         }
     }
-}
+}
\ No newline at end of file
diff --git a/aoc2020/Extensions.cs b/aoc2020/Extensions.cs
index 88c1949..4c75d5e 100644
--- a/aoc2020/Extensions.cs
+++ b/aoc2020/Extensions.cs
@@ -1,3 +1,4 @@
+using System;
 using System.Diagnostics;
 
 namespace aoc2020
@@ -18,5 +19,10 @@ namespace aoc2020
         {
             return 1_000 * stopwatch.ElapsedTicks / (double) Stopwatch.Frequency;
         }
+
+        public static bool Contains(this Range range, int i)
+        {
+            return i >= range.Start.Value && i <= range.End.Value;
+        }
     }
 }
\ No newline at end of file