From dbbe75307292971de46962cfd362b00d3e39c44c Mon Sep 17 00:00:00 2001
From: "Ruidy (aider)" <ruidy.nemausat@gmail.com>
Date: Mon, 2 Dec 2024 07:43:59 +0100
Subject: [PATCH] feat: Add Day 2 solution for reactor report safety analysis

---
 lib/advent_code2024/solutions/day02/day2.ex | 66 +++++++++++++++++++++
 1 file changed, 66 insertions(+)
 create mode 100644 lib/advent_code2024/solutions/day02/day2.ex

diff --git a/lib/advent_code2024/solutions/day02/day2.ex b/lib/advent_code2024/solutions/day02/day2.ex
new file mode 100644
index 0000000..8de64f1
--- /dev/null
+++ b/lib/advent_code2024/solutions/day02/day2.ex
@@ -0,0 +1,66 @@
+defmodule AdventCode2024.Solutions.Day2 do
+  @moduledoc """
+  Solution for Day 2: Red-Nosed Reports
+  """
+
+  @doc """
+  Analyzes reactor reports to count how many are safe according to the rules.
+  Returns {:ok, count} for valid input or {:error, reason} for invalid input.
+  """
+  def solve(input) when is_binary(input) and input != "" do
+    lines = String.split(input, "\n", trim: true)
+
+    with {:ok, reports} <- parse_reports(lines) do
+      safe_count = Enum.count(reports, &safe_report?/1)
+      {:ok, safe_count}
+    end
+  end
+
+  def solve(""), do: {:error, :no_valid_reports}
+
+  defp parse_reports(lines) do
+    reports = Enum.map(lines, &parse_line/1)
+
+    if Enum.all?(reports, &is_list/1) do
+      {:ok, reports}
+    else
+      {:error, :invalid_format}
+    end
+  end
+
+  defp parse_line(line) do
+    line
+    |> String.split(" ", trim: true)
+    |> Enum.map(fn num ->
+      case Integer.parse(num) do
+        {n, ""} -> n
+        _ -> nil
+      end
+    end)
+    |> then(fn nums ->
+      if Enum.any?(nums, &is_nil/1), do: nil, else: nums
+    end)
+  end
+
+  defp safe_report?(levels) do
+    differences =
+      Enum.chunk_every(levels, 2, 1, :discard)
+      |> Enum.map(fn [a, b] -> b - a end)
+
+    case differences do
+      [] ->
+        false
+
+      diffs ->
+        all_increasing?(diffs) or all_decreasing?(diffs)
+    end
+  end
+
+  defp all_increasing?(diffs) do
+    Enum.all?(diffs, fn diff -> diff > 0 and diff <= 3 end)
+  end
+
+  defp all_decreasing?(diffs) do
+    Enum.all?(diffs, fn diff -> diff < 0 and diff >= -3 end)
+  end
+end