Day 11! Not my best effort

Took me much longer than I'd like to admit to get part 1 working. Really
strugled with my `neighbours` method because I kept trying to write it
with loops and failing to properly account for edges. Things went much
better/quicker after I added `Vector`.

2020/11/input.txt

@@ -0,0 +1,95 @@
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2020/11/run.rb

@@ -0,0 +1,9 @@
+require_relative "seating"
+
+input = File.readlines("input.txt")
+
+seating = Seating.new(input)
+puts "Stable: #{seating.stable}"
+
+seating = Seating.new(input)
+puts "Stable (ignore floor): #{seating.stable(true)}"

2020/11/seating.rb

@@ -0,0 +1,78 @@
+require "matrix"
+
+class Seating
+  FLOOR = "."
+  EMPTY = "L"
+  OCCUPIED = "#"
+
+  def initialize(seats)
+    @rows = seats.map { |row| row.strip.chars }
+  end
+
+  def stable(ignore_floor = false)
+    loop do
+      next_rows = step(ignore_floor)
+      break if next_rows == @rows
+      @rows = next_rows
+    end
+
+    @rows.sum { |row| row.count { |s| s == OCCUPIED } }
+  end
+
+  private
+
+  def step(ignore_floor)
+    required = ignore_floor ? 5 : 4
+    new_rows = []
+
+    @rows.each_with_index do |row, y|
+      new_rows << []
+      row.each_with_index do |spot, x|
+        case spot
+        when EMPTY
+          free = neighbours(x, y, ignore_floor).all? { |n| n == EMPTY || n == FLOOR }
+          new_rows[y][x] = free ? OCCUPIED : EMPTY
+        when OCCUPIED
+          occupied = neighbours(x, y, ignore_floor).count { |n| n == OCCUPIED }
+          new_rows[y][x] = (occupied >= required) ? EMPTY : OCCUPIED
+        when FLOOR
+          new_rows[y][x] = FLOOR
+        end
+      end
+    end
+
+    new_rows
+  end
+
+  def neighbours(x, y, ignore_floor)
+    pos = Vector[x, y]
+    [
+      Vector[-1, -1], Vector[-1, 0], Vector[-1, 1],
+      Vector[0, -1], Vector[0, 1],
+      Vector[1, -1], Vector[1, 0], Vector[1, 1]
+    ].map { |vector|
+      ignore_floor ? line(pos + vector, vector) : space(pos + vector)
+    }.compact
+  end
+
+  def line(pos, vector)
+    loop do
+      spot = space(pos)
+      break spot unless spot == FLOOR
+      pos += vector
+    end
+  end
+
+  def space(pos)
+    x, y = pos.to_a
+    @rows[y][x] unless x < 0 || x >= width || y < 0 || y >= height
+  end
+
+  def width
+    @rows[0].length
+  end
+
+  def height
+    @rows.length
+  end
+end

2020/11/test.rb

@@ -0,0 +1,36 @@
+require "minitest/autorun"
+require_relative "seating"
+
+class SeatingTest < Minitest::Test
+  def test_stable
+    seating = Seating.new([
+      "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"
+    ])
+    assert_equal 37, seating.stable
+  end
+
+  def test_alt_stable
+    seating = Seating.new([
+      "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"
+    ])
+    assert_equal 26, seating.stable(true)
+  end
+end