Zero-prefix numbers

2020/day02/day2.ex

@@ -0,0 +1,65 @@
+defmodule Day2 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp parse(input) do
+    re = ~r/^(?<min>\d+)-(?<max>\d+) (?<letter>[a-z]): (?<password>[a-z]+)$/
+
+    String.split(input, "\n", trim: true)
+    |> Enum.map(fn line ->
+      match = Regex.named_captures(re, line)
+
+      %{
+        policy: %{
+          min: String.to_integer(Map.fetch!(match, "min")),
+          max: String.to_integer(Map.fetch!(match, "max")),
+          letter: Map.fetch!(match, "letter")
+        },
+        password: Map.fetch!(match, "password")
+      }
+    end)
+  end
+
+  defp valid_sled?(policy, password) do
+    count =
+      Enum.reduce(String.codepoints(password), 0, fn char, count ->
+        if char == policy.letter do
+          count + 1
+        else
+          count
+        end
+      end)
+
+    policy.min <= count and count <= policy.max
+  end
+
+  defp valid_toboggan?(policy, password) do
+    chars = String.codepoints(password)
+    l1 = Enum.at(chars, policy.min - 1)
+    l2 = Enum.at(chars, policy.max - 1)
+    (l1 == policy.letter or l2 == policy.letter) and l1 != l2
+  end
+
+  defp xor(a, b) do
+    (a or b) and a != b
+  end
+
+  def part1 do
+    parse(read_input())
+    |> Enum.filter(fn %{policy: policy, password: password} -> valid_sled?(policy, password) end)
+    |> length()
+  end
+
+  def part2 do
+    parse(read_input())
+    |> Enum.filter(fn %{policy: policy, password: password} ->
+      valid_toboggan?(policy, password)
+    end)
+    |> length()
+  end
+end
+
+Day2.part1() |> IO.inspect()
+Day2.part2() |> IO.inspect()