Day 6

lib/day6/day6.ex

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+defmodule Day6 do
+  
+  @doc """
+  Parses a node from a string.
+
+  ## Examples
+
+      iex> Day6.parse_node("1, 6")
+      {1, 6}
+
+  """
+  def parse_node(s) do
+    [x, y] = String.split(s, ", ")
+    {
+      x |> String.to_integer(),
+      y |> String.to_integer()
+    }
+  end
+
+  @doc """
+  Distance between two coordinates.
+
+  ## Examples
+
+      iex> Day6.distance({0, 0}, {3, 7})
+      10
+  """
+  def distance({x1, y1}, {x2, y2}) do
+    abs(x2 - x1) + abs(y2 - y1)
+  end
+
+  @doc """
+
+  ## Examples
+
+      iex> Day6.nearest_node({0, 0}, [{-1, -1}, {1, 2}, {0, 1}, {3, 4}])
+      {0, 1}
+      iex> Day6.nearest_node({0, 0}, [{0, -1}, {1, 0}])
+      nil
+  """
+  def nearest_node(point, all) do
+    nearest = 
+      all
+      |> Enum.map(fn other -> {other, distance(point, other)} end)
+
+    {min_pt, min_dist} = nearest |> Enum.min_by(fn {_pt, dist} -> dist end)
+
+    case Enum.count(nearest, fn {_pt, dist} -> dist == min_dist end) do
+      1 -> min_pt
+      _ -> nil
+    end
+  end
+
+  def largest_region(nodes, grid_size \\ 1000) do
+    {areas, infinite_area_nodes} =
+      0..grid_size
+      |> Enum.reduce({%{}, []}, fn y, acc ->
+        0..grid_size
+        |> Enum.reduce(acc, fn x, {areas, infinite_area_nodes} ->
+          pt = {x, y}
+          case nearest_node(pt, nodes) do
+            nil ->
+              {areas, infinite_area_nodes}
+            nearest ->
+              if x == 0 or y == 0 or x == grid_size or y == grid_size do
+                {
+                  areas,
+                  [nearest | infinite_area_nodes]
+                }
+              else
+                {
+                  Map.update(areas, nearest, 1, &(&1 + 1)),
+                  infinite_area_nodes
+                }
+              end
+          end
+        end)
+      end)
+
+    areas
+    |> Enum.reject(fn {node, _area} -> node in infinite_area_nodes end)
+    |> Enum.max_by(fn {_node, area} -> area end)
+  end
+
+  @doc """
+  Sums the distance between the point and each of the nodes.
+
+  ## Examples
+
+      iex> Day6.total_distances({0, 0}, [{0, 2}, {-2, 0}, {3, 5}])
+      12
+  """
+  def total_distances(pt, nodes) do
+    nodes
+    |> Enum.map(fn node -> distance(pt, node) end)
+    |> Enum.sum()
+  end
+
+  def region_near_most(nodes, grid_size \\ 1000, max_dist \\ 10000) do
+    0..grid_size
+    |> Enum.reduce([], fn y, acc ->
+      0..grid_size
+      |> Enum.reduce(acc, fn x, acc ->
+        pt = {x, y}
+        cond do
+          total_distances(pt, nodes) < max_dist ->
+            [pt | acc]
+          true ->
+            acc
+        end
+      end)
+    end)
+  end
+
+  def parse_input() do
+    File.read!("lib/day6/input.txt")
+    |> String.split("\n", trim: true)
+    |> Enum.map(&parse_node/1)
+  end
+
+  def part1() do
+    parse_input()
+    |> largest_region()
+  end
+
+  def part2() do
+    parse_input()
+    |> region_near_most()
+    |> Enum.count()
+  end
+
+end

lib/day6/input.txt

@@ -0,0 +1,50 @@
+227, 133
+140, 168
+99, 112
+318, 95
+219, 266
+134, 144
+306, 301
+189, 188
+58, 334
+337, 117
+255, 73
+245, 144
+102, 257
+255, 353
+303, 216
+141, 167
+40, 321
+201, 50
+60, 188
+132, 74
+125, 199
+176, 307
+204, 218
+338, 323
+276, 278
+292, 229
+109, 228
+85, 305
+86, 343
+97, 254
+182, 151
+110, 292
+285, 124
+43, 223
+153, 188
+285, 136
+334, 203
+84, 243
+92, 185
+330, 223
+259, 275
+106, 199
+183, 205
+188, 212
+231, 150
+158, 95
+174, 212
+279, 97
+172, 131
+247, 320

test/day6_test.exs

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+defmodule Day6Test do
+  use ExUnit.Case
+  doctest Day6
+end