add day_04

day_04/1.rb

@@ -0,0 +1,33 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require_relative "grid"
+
+class Main
+  def run
+    data = []
+    $stdin.each_line do |line|
+      data << line.chomp
+    end
+
+    @sum = 0
+
+    grid = Grid.new(data)
+    (0...grid.rows.size).each do |row_index|
+      (0...grid.columns.size).each do |column_index|
+        next unless grid.cell_at(row_index, column_index).value == "@"
+
+        memo = grid.cell_at(row_index, column_index).neighbours.reject do |cell|
+          grid.cell_at(cell.row, cell.column).value != "@"
+        end
+        if memo.size < 4
+          @sum += 1
+        end
+      end
+    end
+
+    puts @sum
+  end
+end
+
+Main.new.run

day_04/2.rb

@@ -0,0 +1,40 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require_relative "grid"
+
+class Main
+  def run
+    data = []
+    $stdin.each_line do |line|
+      data << line.chomp
+    end
+
+    @sum = 0
+
+    removed_sum = 1
+    grid = Grid.new(data)
+    until removed_sum == 0
+      removed_sum = 0
+      (0...grid.rows.size).each do |row_index|
+        (0...grid.columns.size).each do |column_index|
+          next unless grid.cell_at(row_index, column_index).value == "@"
+
+          memo = grid.cell_at(row_index, column_index).neighbours.reject do |cell|
+            grid.cell_at(cell.row, cell.column).value != "@"
+          end
+          if memo.size < 4
+            data[row_index][column_index] = "x"
+            removed_sum += 1
+          end
+        end
+      end
+      @sum += removed_sum
+      grid = Grid.new(data)
+    end
+
+    puts @sum
+  end
+end
+
+Main.new.run

day_04/cell.rb

@@ -0,0 +1,22 @@
+class Cell
+  attr_reader :row, :column
+  attr_accessor :value
+  attr_accessor :n, :ne, :e, :se, :s, :sw, :w, :nw
+
+  def initialize(row, column)
+    @row, @column = row, column
+  end
+
+  def neighbours
+    list = []
+    list << n if n
+    list << ne if ne
+    list << e if e
+    list << se if se
+    list << s if s
+    list << sw if sw
+    list << w if w
+    list << nw if nw
+    list
+  end
+end

day_04/grid.rb

@@ -0,0 +1,42 @@
+require "forwardable"
+
+require_relative "cell"
+
+class Grid
+  attr_reader :rows, :columns
+  extend Forwardable
+  def_delegators :@rows, :[]
+
+  def initialize(input)
+    @rows = []
+    (0...input.size).each do |row_index|
+      row = []
+      (0...input[row_index].size).each do |column_index|
+        cell = Cell.new(row_index, column_index)
+        cell.value = input[row_index][column_index]
+        row << cell
+      end
+      @rows << row
+    end
+    @columns = @rows.transpose
+
+    # Link neighbours
+    (0...@rows.size).each do |row_index|
+      (0...@columns.size).each do |column_index|
+        cell = @rows[row_index][column_index]
+        cell.n = @rows[row_index - 1][column_index] if row_index - 1 >= 0
+        cell.ne = @rows[row_index - 1][column_index + 1] if row_index - 1 >= 0 && column_index + 1 < @columns.size
+        cell.e = @rows[row_index][column_index + 1] if column_index + 1 < @columns.size
+        cell.se = @rows[row_index + 1][column_index + 1] if row_index + 1 < @rows.size && column_index + 1 < @columns.size
+        cell.s = @rows[row_index + 1][column_index] if row_index + 1 < @rows.size
+        cell.sw = @rows[row_index + 1][column_index - 1] if row_index + 1 < @rows.size && column_index - 1 >= 0
+        cell.w = @rows[row_index][column_index - 1] if column_index - 1 >= 0
+        cell.nw = @rows[row_index - 1][column_index - 1] if row_index - 1 >= 0 && column_index - 1 >= 0
+      end
+    end
+  end
+
+  def cell_at(row, column)
+    @rows[row][column]
+  end
+end