Day 2

lib/day2/day2.ex

@@ -0,0 +1,56 @@
+defmodule Day2 do
+  @example """
+  1-3 a: abcde
+  1-3 b: cdefg
+  2-9 c: ccccccccc
+  """
+
+  def part1(example \\ false) do
+    if(example, do: @example, else: File.read!("lib/day2/input.txt"))
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&parse_policy_and_password/1)
+    |> Enum.count(&password_valid1/1)
+  end
+
+  def part2(example \\ false) do
+    if(example, do: @example, else: File.read!("lib/day2/input.txt"))
+    |> String.trim()
+    |> String.split("\n")
+    |> Enum.map(&parse_policy_and_password/1)
+    |> Enum.count(&password_valid2/1)
+  end
+
+  def parse_policy_and_password(line) do
+    [policy, rest] = String.split(line, ":")
+    rest = String.trim(rest)
+
+    [count, letter] = String.split(policy, " ")
+    [min, max] = String.split(count, "-")
+    min = String.to_integer(min)
+    max = String.to_integer(max)
+
+    {{letter, min, max}, rest}
+  end
+
+  def password_valid1({{letter, min, max}, password}) do
+    letter_count =
+      password
+      |> String.codepoints()
+      |> Enum.count(fn
+        ^letter -> true
+        _ -> false
+      end)
+
+    letter_count >= min && letter_count <= max
+  end
+
+  def password_valid2({{letter, first, second}, password}) do
+    at_first = String.at(password, first - 1)
+    at_second = String.at(password, second - 1)
+    first_matches = letter == at_first
+    second_matches = letter == at_second
+
+    (first_matches || second_matches) && !(first_matches && second_matches)
+  end
+end