Day 4

lib/day4/day4.ex

@@ -0,0 +1,55 @@
+defmodule Day4 do
+  def part1 do
+    158_126..624_574
+    |> Enum.reduce(0, fn n, acc ->
+      if test1(n) do
+        IO.inspect(n)
+        acc + 1
+      else
+        acc
+      end
+    end)
+  end
+
+  def part2 do
+    158_126..624_574
+    |> Enum.reduce(0, fn n, acc ->
+      if test2(n) do
+        IO.inspect(n)
+        acc + 1
+      else
+        acc
+      end
+    end)
+  end
+
+  def test1(n) do
+    cl = Integer.to_charlist(n)
+    chunks = chunk_by_digits(cl)
+
+    chunks == Enum.sort(chunks) && Enum.any?(chunks, &(length(&1) >= 2))
+  end
+
+  def test2(n) do
+    cl = Integer.to_charlist(n)
+    chunks = chunk_by_digits(cl)
+
+    chunks == Enum.sort(chunks) && Enum.any?(chunks, &(length(&1) == 2))
+  end
+
+  def chunk_by_digits(cl, current_chunk \\ [], chunks \\ [])
+
+  def chunk_by_digits([a, b | rest], current_chunk, chunks) do
+    rest = [b | rest]
+
+    if a == b do
+      chunk_by_digits(rest, [a | current_chunk], chunks)
+    else
+      chunk_by_digits(rest, [], [Enum.reverse([a | current_chunk]) | chunks])
+    end
+  end
+
+  def chunk_by_digits([a | []], current_chunk, chunks) do
+    Enum.reverse([Enum.reverse([a | current_chunk]) | chunks])
+  end
+end