Day 18

aoc2023/Code/Day18.cs

@@ -1,14 +1,77 @@
-namespace aoc2023.Code;
+namespace aoc2023.Code;
 
 internal class Day18 : BaseDay
 {
-    protected override object Part1()
-    {
-        return 0;
-    }
+    record DigPlan(char D, int S, string Color);
+
+    record XY(long X, long Y);
 
-    protected override object Part2()
+    class Lagoon
     {
-        return 0;
+        readonly List<DigPlan> _plan;
+        readonly List<XY> _edges;
+
+        public Lagoon(IEnumerable<string[]> digPlan)
+        {
+            _plan = [];
+            _edges = [];
+
+            foreach (var plan in digPlan)
+            {
+                _plan.Add(new DigPlan(plan[0][0], int.Parse(plan[1]), plan[2]));
+            }
+        }
+
+        internal Lagoon Trench(bool fix)
+        {
+            var edge = new XY(0, 0);
+            _edges.Add(edge);
+
+            foreach (var plan in _plan)
+            {
+                var length = plan.S;
+                var direction = plan.D;
+
+                if (fix)
+                {
+                    length = int.Parse(plan.Color[2..7], System.Globalization.NumberStyles.HexNumber);
+                    direction = "RDLU"[plan.Color[^2] - '0'];
+                }
+
+                edge = direction switch
+                {
+                    'R' => new(edge.X + length, edge.Y),
+                    'L' => new(edge.X - length, edge.Y),
+                    'U' => new(edge.X, edge.Y - length),
+                    'D' => new(edge.X, edge.Y + length),
+                    _ => throw new NotImplementedException()
+                };
+
+                _edges.Add(edge);
+            }
+
+            return this;
+        }
+
+        internal long Shoelace()
+        {
+            var s = 0L;
+            var p = 0L;
+
+            for (int i = 0; i < _edges.Count - 1; i++)
+            {
+                var a = _edges[i];
+                var b = _edges[i + 1];
+
+                s += (a.X * b.Y - b.X * a.Y);
+                p += Math.Abs(a.X - b.X + a.Y - b.Y);
+            }
+
+            return (s + p) / 2 + 1;
+        }
     }
+
+    protected override object Part1() => new Lagoon(ReadAllLinesSplit(" ", true)).Trench(false).Shoelace();
+
+    protected override object Part2() => new Lagoon(ReadAllLinesSplit(" ", true)).Trench(true).Shoelace();
 }