Add day 12 solution.

day12.cr

@@ -0,0 +1,61 @@
+require "./common.cr"
+require "big"
+
+lines = File.read("day12_input").split "\n"
+lines.pop
+
+initial = lines[0].split(": ")[1].chars
+original = initial.join("")
+rules = lines[2..-1].map do |it|
+  split = it.split " => "
+  line = split[0]
+  into = split[1][0]
+  { line.chars, into }
+end
+
+class Array
+  def has_plant(i)
+    return false if i < 0 || i >= size
+    return self[i] == '#'
+  end
+end
+
+def step(initial, rules)
+  array = [] of Char
+  (-2..initial.size + 2).each do |i|
+    rule = [] of Char
+    (i-2..i+2).each do |ind|
+      rule << (initial.has_plant(ind) ? '#' : '.')
+    end
+
+    new_char = rules.find(&.[0].==(rule)).try(&.[1]) || '.'
+    array << new_char
+  end
+  return array
+end
+
+def get_total(initial, offset)
+  total = 0
+  initial.each_with_index do |char, i|
+    total += (char == '#') ? (i + offset) : 0
+  end
+  return total
+end
+
+offset = 0
+
+20.times do |it|
+  initial = step(initial, rules)
+  offset -= 2
+end
+puts get_total(initial, offset)
+
+total = BigInt.new("0")
+fivebil = BigInt.new("50000000000")
+first_index = (fivebil - 11)
+
+total += first_index * 57
+total += get_total("###.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#.#".chars, 0)
+puts total
+
+# 2850000002508