tidy up day3 variable names

aoc2020/Day3.cs

@@ -5,19 +5,21 @@ namespace aoc2020
     public sealed class Day3 : Day
     {
         private readonly string[] _grid;
+        private readonly int _width;
 
         public Day3()
         {
             _grid = Input.ToArray();
+            _width = _grid[0].Length;
         }
 
         public override int DayNumber => 3;
 
-        private int CountSlope(int di, int dx)
+        private long CountSlope(int dx, int dy)
         {
-            var hits = 0;
-            for (int i = 0, x = 0; i < _grid.Length; i += di, x = (x + dx) % _grid.First().Length)
-                if (_grid[i][x] == '#')
+            long hits = 0;
+            for (int x = 0, y = 0; y < _grid.Length; y += dy, x = (x + dx) % _width)
+                if (_grid[y][x] == '#')
                     hits++;
 
             return hits;
@@ -25,16 +27,24 @@ namespace aoc2020
 
         public override string Part1()
         {
-            return $"{CountSlope(1, 3)}";
+            return $"{CountSlope(3, 1)}";
         }
 
         public override string Part2()
         {
-            return $@"{CountSlope(1, 1)
-                       * CountSlope(1, 3)
-                       * CountSlope(1, 5)
-                       * CountSlope(1, 7)
-                       * CountSlope(2, 1)}";
+            var slopes = new[]
+            {
+                (1, 1),
+                (3, 1),
+                (5, 1),
+                (7, 1),
+                (1, 2)
+            };
+
+            return slopes
+                .Select(s => CountSlope(s.Item1, s.Item2))
+                .Aggregate((acc, i) => acc * i)
+                .ToString();
         }
     }
-}
+}