Day 3

lib/day3/day3.ex

@@ -0,0 +1,63 @@
+defmodule Day3 do
+  @example """
+  ..##.......
+  #...#...#..
+  .#....#..#.
+  ..#.#...#.#
+  .#...##..#.
+  ..#.##.....
+  .#.#.#....#
+  .#........#
+  #.##...#...
+  #...##....#
+  .#..#...#.#
+  """
+
+  def part1(example \\ false) do
+    count_trees(example, {3, 1})
+  end
+
+  def part2(example \\ false) do
+    one_one = count_trees(example, {1, 1})
+    three_one = count_trees(example, {3, 1})
+    five_one = count_trees(example, {5, 1})
+    seven_one = count_trees(example, {7, 1})
+    one_two = count_trees(example, {1, 2})
+
+    one_one * three_one * five_one * seven_one * one_two
+  end
+
+  def count_trees(example, slope) do
+    example
+    |> parse_input()
+    |> count_trees_for_slope(slope)
+  end
+
+  def parse_input(example) do
+    if(example, do: @example, else: File.read!("lib/day3/input.txt"))
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(fn line ->
+      line
+      |> String.codepoints()
+      |> Enum.map(fn
+        "#" -> 1
+        _ -> 0
+      end)
+      |> Stream.cycle()
+    end)
+  end
+
+  def count_trees_for_slope(lines, slope) do
+    do_count(lines, slope, 0, 0)
+  end
+
+  def do_count([current | rest], {dx, dy} = slope, x, trees) do
+    rest = Enum.drop(rest, dy - 1)
+    do_count(rest, slope, x + dx, trees + Enum.at(current, x))
+  end
+
+  def do_count([], _, _, trees) do
+    trees
+  end
+end

lib/day3/input.txt

@@ -0,0 +1,323 @@
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