From c25d7f1a016fbbc2abb5774e9fc80867cf8f5434 Mon Sep 17 00:00:00 2001
From: Ben Harris <ben@tilde.team>
Date: Sat, 2 Jan 2021 19:31:06 -0500
Subject: [PATCH] day 20 part 1

---
 aoc2020.test/DayTests.cs |  2 +-
 aoc2020/Day20.cs         | 91 +++++++++++++++++++++++++++++++++++++++-
 2 files changed, 91 insertions(+), 2 deletions(-)

diff --git a/aoc2020.test/DayTests.cs b/aoc2020.test/DayTests.cs
index d7ea972..ce64713 100644
--- a/aoc2020.test/DayTests.cs
+++ b/aoc2020.test/DayTests.cs
@@ -27,7 +27,7 @@ namespace aoc2020.test
         // [DataRow(typeof(Day17), "293", "1816")]
         [DataRow(typeof(Day18), "12918250417632", "171259538712010")]
         [DataRow(typeof(Day19), "160", "357")]
-        [DataRow(typeof(Day20), "", "")]
+        [DataRow(typeof(Day20), "21599955909991", "")]
         [DataRow(typeof(Day21), "", "")]
         [DataRow(typeof(Day22), "", "")]
         [DataRow(typeof(Day23), "", "")]
diff --git a/aoc2020/Day20.cs b/aoc2020/Day20.cs
index 182fdd0..d6cb060 100644
--- a/aoc2020/Day20.cs
+++ b/aoc2020/Day20.cs
@@ -1,3 +1,7 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
 namespace aoc2020
 {
     /// <summary>
@@ -5,18 +9,103 @@ namespace aoc2020
     /// </summary>
     public sealed class Day20 : Day
     {
+        private readonly List<Tile> _allPermutations;
+        private readonly List<Tile> _topLefts;
+
         public Day20() : base(20, "Jurassic Jigsaw")
         {
+            var lines = new List<string>();
+            var tiles = new List<Tile>();
+            var currentTileId = 0;
+
+            foreach (var line in Input)
+                if (line.StartsWith("Tile "))
+                {
+                    currentTileId = int.Parse(line.Split(' ', ':')[1]);
+                }
+                else if (line == "")
+                {
+                    tiles.Add(new(currentTileId, lines.Select(l => l.ToCharArray()).ToArray()));
+                    lines.Clear();
+                }
+                else
+                {
+                    lines.Add(line);
+                }
+
+            if (lines.Any()) tiles.Add(new(currentTileId, lines.Select(l => l.ToCharArray()).ToArray()));
+
+            _allPermutations = tiles.SelectMany(t => t.AllPermutations()).ToList();
+            _topLefts = _allPermutations
+                .Where(t => !_allPermutations.Any(t2 => t.TileId != t2.TileId && t.LeftId == t2.RightId) &&
+                            !_allPermutations.Any(t2 => t.TileId != t2.TileId && t.TopId == t2.BottomId))
+                .ToList();
+        }
         }
 
         public override string Part1()
         {
-            return "";
+            return $"{_topLefts.Select(t => t.TileId).Distinct().Aggregate(1L, (acc, next) => acc * next)}";
         }
 
         public override string Part2()
         {
             return "";
         }
+
+        private record Tile(int TileId, char[][] Pixels)
+        {
+            private const int Size = 10;
+            internal int TopId => GetId(z => (z, 0));
+            internal int BottomId => GetId(z => (z, Size - 1));
+            internal int LeftId => GetId(z => (0, z));
+            internal int RightId => GetId(z => (Size - 1, z));
+
+            private int GetId(Func<int, (int x, int y)> selector)
+            {
+                return Enumerable.Range(0, Size)
+                    .Select(selector)
+                    .Select((c, i) => (Pixels[c.x][c.y] == '#' ? 1 : 0) << i)
+                    .Aggregate(0, (acc, next) => acc | next);
+            }
+
+            private Tile RotateClockwise()
+            {
+                return Transform((x, y, newPixels) => newPixels[x][Size - 1 - y] = Pixels[y][x]);
+            }
+
+            private Tile Flip()
+            {
+                return Transform((x, y, newPixels) => newPixels[y][Size - 1 - x] = Pixels[y][x]);
+            }
+
+            private Tile Transform(Action<int, int, char[][]> transformFunc)
+            {
+                var newPixels = Enumerable.Repeat(false, Size).Select(_ => new char[Size]).ToArray();
+
+                for (var y = 0; y < Size; y++)
+                for (var x = 0; x < Size; x++)
+                    transformFunc(x, y, newPixels);
+
+                return new(TileId, newPixels);
+            }
+
+            internal IEnumerable<Tile> AllPermutations()
+            {
+                var tile = this;
+                for (var i = 1; i <= 8; i++)
+                {
+                    yield return tile;
+                    if (i == 4) tile = Flip();
+                    else if (i == 8) yield break;
+                    tile = tile.RotateClockwise();
+                }
+            }
+
+            public string Format()
+            {
+                return $"Tile {TileId}:\n{string.Join("\n", Pixels.Select(p => new string(p)))}";
+            }
+        }
     }
 }
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