From 34d4dea789e77dadf11b9dbab27b0a68545e4df2 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Wed, 16 Dec 2020 21:49:43 -0800
Subject: [PATCH] Day 17 Part 1

---
 2020/day17/day17.ex | 165 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 165 insertions(+)
 create mode 100644 2020/day17/day17.ex

diff --git a/2020/day17/day17.ex b/2020/day17/day17.ex
new file mode 100644
index 0000000..b23f7f0
--- /dev/null
+++ b/2020/day17/day17.ex
@@ -0,0 +1,165 @@
+defmodule Day17 do
+  defp read_input do
+    Path.expand('input', Path.dirname(__ENV__.file))
+    |> File.read!()
+  end
+
+  defp test_input do
+    """
+    .#.
+    ..#
+    ###
+    """
+  end
+
+  defp parse_space(text) do
+    String.split(text, "\n", trim: true)
+    |> Enum.with_index()
+    |> Enum.reduce(%{}, fn {line, y}, map ->
+      String.codepoints(line)
+      |> Enum.with_index()
+      |> Enum.reduce(map, fn {char, x}, map ->
+        Map.put(map, {x, y, 0}, char_to_content(char))
+      end)
+    end)
+  end
+
+  defp char_to_content(char) do
+    case char do
+      "." -> :inactive
+      "#" -> :active
+    end
+  end
+
+  defp state_char(char) do
+    case char do
+      :inactive -> "."
+      :active -> "#"
+    end
+  end
+
+  defp expand_range(lo..hi, val) do
+    cond do
+      val < lo -> val..hi
+      val > hi -> lo..val
+      true -> lo..hi
+    end
+  end
+
+  defp get_bounds(space) do
+    Enum.reduce(space, %{x: 0..0, y: 0..0, z: 0..0}, fn {{x, y, z}, state}, bounds ->
+      case state do
+        :inactive ->
+          bounds
+
+        :active ->
+          %{
+            x: expand_range(bounds.x, x),
+            y: expand_range(bounds.y, y),
+            z: expand_range(bounds.z, z)
+          }
+      end
+    end)
+  end
+
+  defp show(space) do
+    bounds = get_bounds(space)
+
+    for z <- bounds.z do
+      IO.puts("z = #{z}")
+
+      for y <- bounds.y do
+        for x <- bounds.x do
+          state = Map.get(space, {x, y, z}, :inactive)
+          IO.write(state_char(state))
+        end
+
+        IO.write("\n")
+      end
+
+      IO.write("\n\n")
+    end
+
+    space
+  end
+
+  defp populate(space) do
+    Enum.reduce(space, %{}, fn {cell, state}, counts ->
+      if state == :inactive do
+        counts
+      else
+        neighbors(cell)
+        |> Enum.reduce(counts, fn neighbor, counts ->
+          Map.put(counts, neighbor, 1 + Map.get(counts, neighbor, 0))
+        end)
+      end
+    end)
+  end
+
+  defp tick(space) do
+    populate(space)
+    |> Enum.reduce(%{}, fn {cell, count}, new_space ->
+      new_state =
+        case Map.get(space, cell, :inactive) do
+          :active ->
+            if count == 2 or count == 3 do
+              :active
+            else
+              :inactive
+            end
+
+          :inactive ->
+            if count == 3 do
+              :active
+            else
+              :inactive
+            end
+        end
+
+      case new_state do
+        :active -> Map.put(new_space, cell, :active)
+        :inactive -> new_space
+      end
+    end)
+  end
+
+  defp neighbors(cell = {x, y, z}) do
+    for dx <- -1..1,
+        dy <- -1..1,
+        dz <- -1..1 do
+      {x + dx, y + dy, z + dz}
+    end
+    |> Enum.reject(&(&1 == cell))
+  end
+
+  defp run(space, 0) do
+    space
+  end
+
+  defp run(space, cycles) do
+    IO.puts("cycles = #{cycles}")
+    show(space)
+
+    tick(space)
+    |> run(cycles - 1)
+  end
+
+  def part1 do
+    read_input()
+    |> parse_space()
+    |> run(6)
+    |> show()
+    |> Enum.count(fn {_cell, state} -> state == :active end)
+  end
+
+  def debug do
+    test_input()
+    |> parse_space()
+    |> tick()
+
+    # neighbors(%{}, {0, 0, 0})
+  end
+end
+
+Day17.part1() |> IO.inspect()
+# Day17.debug() |> IO.inspect()