Solve 2018-25

Author: code-shoily

lib/2018/day_25.ex

@@ -0,0 +1,81 @@
+defmodule AdventOfCode.Y2018.Day25 do
+  @moduledoc """
+  --- Day 25: Four-Dimensional Adventure ---
+  Problem Link: https://adventofcode.com/2018/day/25
+  Difficulty: m
+  Tags: graph connectivity constellation
+  """
+  alias AdventOfCode.Helpers.{InputReader, Transformers}
+  alias Yog.Connectivity
+
+  def input, do: InputReader.read_from_file(2018, 25)
+
+  def run(input \\ input()) do
+    points = parse(input)
+
+    {run_1(points), run_2(points)}
+  end
+
+  defp run_1(points) do
+    # Points form "constellations" if their Manhattan distance is <= 3.
+    # This is exactly finding connected components in a graph where
+    # nodes are points and edges connect points with distance <= 3.
+    graph = build_graph(points)
+
+    Connectivity.connected_components(graph)
+    |> length()
+  end
+
+  defp run_2(_points) do
+    "Finished the 4D adventure! 🥳"
+  end
+
+  defp build_graph(points_with_index) do
+    # Each points has an index for stable node ID.
+    points =
+      points_with_index
+      |> Enum.with_index()
+
+    # Create the undirected graph with nodes.
+    initial_graph =
+      Enum.reduce(points, Yog.undirected(), fn {p, idx}, acc ->
+        Yog.add_node(acc, idx, p)
+      end)
+
+    # Consider all pairs of points and add edges if they are connected.
+    # O(N^2) checks. N ~ 1250 => ~780k checks.
+    # We only check each pair once.
+    add_edges(points, initial_graph)
+  end
+
+  defp add_edges([], graph), do: graph
+
+  defp add_edges([{p1, i1} | rest], graph) do
+    new_graph =
+      Enum.reduce(rest, graph, fn {p2, i2}, acc ->
+        if manhattan_dist(p1, p2) <= 3 do
+          # Graph is undirected, so order doesn't matter.
+          # add_edge! ensures we don't handle error tuples inside the loop.
+          Yog.add_edge!(acc, from: i1, to: i2, with: 1)
+        else
+          acc
+        end
+      end)
+
+    add_edges(rest, new_graph)
+  end
+
+  defp manhattan_dist({x1, y1, z1, w1}, {x2, y2, z2, w2}) do
+    abs(x1 - x2) + abs(y1 - y2) + abs(z1 - z2) + abs(w1 - w2)
+  end
+
+  def parse(data \\ input()) do
+    for line <- Transformers.lines(data) do
+      line
+      |> String.split(",")
+      |> Enum.map(&String.trim/1)
+      |> Enum.map(&String.to_integer/1)
+      |> List.to_tuple()
+    end
+  end
+end