From 38ae43c77d5e30d5cdda59d181c68434cf5b298d Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Thu, 10 Dec 2020 22:49:03 -0800
Subject: [PATCH] Day 11 Part 2

---
 2020/day11/day11.ex | 166 +++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 165 insertions(+), 1 deletion(-)

diff --git a/2020/day11/day11.ex b/2020/day11/day11.ex
index 27cb65b..72a3b25 100644
--- a/2020/day11/day11.ex
+++ b/2020/day11/day11.ex
@@ -25,7 +25,7 @@ defmodule Day11 do
   end
 
   defp content(grid, cell) do
-    Map.get(grid, cell, :floor)
+    Map.get(grid, cell, :void)
   end
 
   defp neighbors(grid, {r, c}) do
@@ -88,6 +88,170 @@ defmodule Day11 do
     |> run()
     |> count_seated()
   end
+
+  defp tick2(grid, dims) do
+    occupied = Enum.filter(grid, fn {_, state} -> state == :occupied end)
+
+    neighbors =
+      Enum.reduce(occupied, %{}, fn {cell, _}, counts ->
+        visible_cells(grid, cell)
+        |> Enum.reduce(counts, fn cell, counts ->
+          count = Map.get(counts, cell, 0)
+          Map.put(counts, cell, count + 1)
+        end)
+      end)
+
+    # show_counts(neighbors, dims)
+
+    Enum.into(grid, %{}, fn {cell, state} ->
+      new_state =
+        case state do
+          :floor ->
+            :floor
+
+          :empty ->
+            if Map.get(neighbors, cell, 0) == 0 do
+              :occupied
+            else
+              :empty
+            end
+
+          :occupied ->
+            if Map.get(neighbors, cell, 0) >= 5 do
+              :empty
+            else
+              :occupied
+            end
+        end
+
+      {cell, new_state}
+    end)
+  end
+
+  defp visible_cells(grid, cell) do
+    [
+      {-1, -1},
+      {-1, 0},
+      {-1, +1},
+      {0, -1},
+      {0, +1},
+      {+1, -1},
+      {+1, 0},
+      {+1, +1}
+    ]
+    |> Enum.flat_map(&ray_segment(grid, cell, &1))
+  end
+
+  defp ray_segment(grid, {rr, cc}, {dr, dc}) do
+    # Skip current cell
+    first = {rr + dr, cc + dc}
+
+    Stream.iterate(first, fn {r, c} -> {r + dr, c + dc} end)
+    |> Enum.reduce_while([], fn cell, cells ->
+      case content(grid, cell) do
+        :occupied -> {:halt, [cell | cells]}
+        :empty -> {:halt, [cell | cells]}
+        :floor -> {:cont, [cell | cells]}
+        # nothing to see here
+        :void -> {:halt, cells}
+      end
+    end)
+  end
+
+  defp run2(grid, dims = {h, w}) do
+    # IO.puts("-------------------------")
+    # show(grid, {h, w})
+
+    new_grid = tick2(grid, dims)
+
+    if new_grid == grid do
+      grid
+    else
+      run2(new_grid, dims)
+    end
+  end
+
+  defp show(grid, {h, w}) do
+    for r <- 0..(h - 1) do
+      for c <- 0..(w - 1) do
+        IO.write(content_to_char(content(grid, {r, c})))
+      end
+
+      IO.write("\n")
+    end
+  end
+
+  defp show_counts(grid, {h, w}) do
+    for r <- 0..(h - 1) do
+      for c <- 0..(w - 1) do
+        IO.write(Map.get(grid, {r, c}, 0))
+      end
+
+      IO.write("\n")
+    end
+  end
+
+  defp content_to_char(content) do
+    case content do
+      :floor -> "."
+      :empty -> "L"
+      :occupied -> "#"
+    end
+  end
+
+  defp test_input do
+    """
+    L.LL.LL.LL
+    LLLLLLL.LL
+    L.L.L..L..
+    LLLL.LL.LL
+    L.LL.LL.LL
+    L.LLLLL.LL
+    ..L.L.....
+    LLLLLLLLLL
+    L.LLLLLL.L
+    L.LLLLL.LL
+    """
+  end
+
+  def part2 do
+    grid = read_input() |> parse_layout()
+    h = Enum.map(grid, fn {{r, _c}, _} -> r end) |> Enum.max()
+    w = Enum.map(grid, fn {{_r, c}, _} -> c end) |> Enum.max()
+
+    run2(grid, {h + 1, w + 1})
+    |> count_seated()
+  end
+
+  def debug_state do
+    """
+    #.##.##.##
+    #######.##
+    #.#.#..#..
+    ####.##.##
+    #.##.##.##
+    #.#####.##
+    ..#.#.....
+    ##########
+    #.######.#
+    #.#####.##
+    """
+  end
+
+  def debug do
+    grid = debug_state() |> parse_layout()
+    show(grid, {10, 10})
+
+    Map.keys(grid)
+    |> Enum.filter(fn cell ->
+      Enum.member?(visible_cells(grid, cell), {0, 6})
+    end)
+    |> Enum.into(%{}, fn cell -> {cell, content(grid, cell)} end)
+    |> IO.inspect()
+
+    tick(grid) |> show({10, 10})
+  end
 end
 
 Day11.part1() |> IO.inspect()
+Day11.part2() |> IO.inspect()