Add solution for 2015 day 18

2015/README.md

@@ -19,7 +19,7 @@
 | ✓        | 15     | Permutations          | Ruby       |
 | ✓        | 16     | Searching             | Ruby       |
 |          | 17     | Combinations          |            |
-|          | 18     |                       |            |
+| ✓        | 18     | Game of Life          | Ruby       |
 |          | 19     |                       |            |
 |          | 20     |                       |            |
 |          | 21     |                       |            |

2015/day-18/.ruby-version

@@ -0,0 +1 @@
+3.0.0

2015/day-18/inputs/puzzle.txt

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2015/day-18/inputs/test.txt

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+.#.#.#
+...##.
+#....#
+..#...
+#.#..#
+####..

2015/day-18/main.rb

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+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require "pathname"
+require "set"
+
+class Grid
+  NEIGHBOR = {
+    n:  [0, -1],
+    s:  [0, 1],
+    e:  [1, 0],
+    w:  [-1, 0],
+    ne: [1, -1],
+    se: [1, 1],
+    nw: [-1, -1],
+    sw: [-1, 1],
+  }.map(&:freeze).to_h.freeze
+
+  def add_row(row)
+    @area ||= []
+    row = row.map { |x| x == "#" ? 1 : 0 }
+    @width ||= row.size
+    @area.push(row[0..@height])
+  end
+
+  def to_s = @area.map(&:to_s).join("\n")
+  def row_count = @area.size
+  def col_count = @width
+
+  def dup
+    grid = self.class.new
+    grid.instance_variable_set(:@area, @area.map(&:dup).dup)
+    grid.instance_variable_set(:@width, @width.dup)
+    grid.instance_variable_set(:@sticky, @sticky&.dup)
+    grid
+  end
+
+  def []=(x, y, val)
+    return if @sticky&.include?([x, y])
+    @area[y][x] = val
+  end
+
+  def [](x, y)
+    @area[y][x]
+  end
+
+  def make_sticky(x, y, val = 1)
+    @sticky ||= Set.new
+    self[x, y] = val
+    @sticky.add([x, y])
+  end
+
+  def count_on
+    @area.sum { |r| r.sum }
+  end
+
+  def advance(steps)
+    steps.times do |step|
+      change_count = advance_step()
+      return step if change_count.zero?
+    end
+  end
+
+  private
+
+  def advance_step
+    changes = []
+
+    @area.each_with_index do |row, y|
+      row.each_with_index do |_, x|
+        nb_on_count = neighbors_that_are_on(x, y)
+
+        if self[x, y] == 1 && (nb_on_count < 2 || nb_on_count > 3)
+          changes.push([x, y, 0])
+        elsif self[x, y] == 0 && nb_on_count == 3
+          changes.push([x, y, 1])
+        end
+      end
+    end
+
+    changes.each do |(x, y, val)|
+      self[x, y] = val
+    end
+
+    changes.size
+  end
+
+  def neighbors_that_are_on(x, y)
+    NEIGHBOR.keys.sum { |d| neighbor_on?(x, y, d) ? 1 : 0 }
+  end
+
+  def neighbor_on?(x, y, direction)
+    nb_x, nb_y = NEIGHBOR[direction].then { |(nx, ny)| [x + nx, y + ny] }
+    return false if nb_x < 0 || nb_y < 0
+
+    neighbor = @area.dig(nb_y, nb_x)
+    return false unless neighbor
+
+    neighbor == 1
+  end
+end
+
+def parse_file(file)
+  file.each_line.each_with_object(Grid.new) do |line, grid|
+    grid.add_row(line.chomp.chars)
+  end
+end
+
+def main(file_paths)
+  file_paths.each do |file_path|
+    puts "File: #{file_path}"
+
+    grid1 = parse_file(file_path)
+    grid2 = grid1.dup
+
+    grid1.advance(100)
+    solution1 = grid1.count_on
+    puts "  Solution 1: #{solution1}"
+
+    grid2.make_sticky(0, 0)
+    grid2.make_sticky(0, grid2.col_count - 1)
+    grid2.make_sticky(grid2.row_count - 1, 0)
+    grid2.make_sticky(grid2.row_count - 1, grid2.col_count - 1)
+    grid2.advance(100)
+
+    solution2 = grid2.count_on
+    puts "  Solution 2: #{solution2}"
+  end
+end
+
+main(ARGV.map { |x| Pathname(x) })