diff --git a/README.md b/README.md
index 7578901..d22c9cd 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@ I'm also adding notes that may be useful if you're learning Ruby.
 
 Notes for solving:
 * [2015, complete](year_2015.md)
-* [2016, up to day 11](year_2016.md)
+* [2016, up to day 12](year_2016.md)
 * [2023, up to day 04](year_2023.md)
 
 # How to use
diff --git a/spec/year_2016/day_12_input b/spec/year_2016/day_12_input
new file mode 100644
index 0000000..0eeb395
--- /dev/null
+++ b/spec/year_2016/day_12_input
@@ -0,0 +1,23 @@
+cpy 1 a
+cpy 1 b
+cpy 26 d
+jnz c 2
+jnz 1 5
+cpy 7 c
+inc d
+dec c
+jnz c -2
+cpy a c
+inc a
+dec b
+jnz b -2
+cpy c b
+dec d
+jnz d -6
+cpy 14 c
+cpy 14 d
+inc a
+dec d
+jnz d -2
+dec c
+jnz c -5
diff --git a/spec/year_2016/day_12_sample_one b/spec/year_2016/day_12_sample_one
new file mode 100644
index 0000000..01a004b
--- /dev/null
+++ b/spec/year_2016/day_12_sample_one
@@ -0,0 +1,6 @@
+cpy 41 a
+inc a
+inc a
+dec a
+jnz a 2
+dec a
diff --git a/spec/year_2016/day_12_spec.rb b/spec/year_2016/day_12_spec.rb
new file mode 100644
index 0000000..64580b4
--- /dev/null
+++ b/spec/year_2016/day_12_spec.rb
@@ -0,0 +1,28 @@
+require 'year_2016/day_12'
+
+describe Year2016::Day12 do
+  context 'when Part 1' do
+    subject(:sample_one) do
+      described_class.new(File.read('spec/year_2016/day_12_sample_one')).program
+    end
+
+    it 'gives a final result' do
+      sample_one.execute
+      expect(sample_one.registers['a']).to eq(42)
+    end
+  end
+
+  context 'when Results' do
+    subject(:input_data) do
+      File.read('spec/year_2016/day_12_input')
+    end
+
+    it 'correctly answers part 1' do
+      expect(described_class.new(input_data).part_one).to eq(318_007)
+    end
+
+    it 'correctly answers part 2' do
+      expect(described_class.new(input_data).part_two).to eq(9_227_661)
+    end
+  end
+end
diff --git a/year_2016.md b/year_2016.md
index 9328739..9ac252f 100644
--- a/year_2016.md
+++ b/year_2016.md
@@ -161,3 +161,16 @@ Year2016::Day11
 It seems this exercise got [some people to give up](https://markheath.net/post/aoc-2016-day11), and quite frankly, I felt like it for a while too. In the end, brute-forcing wasn't efficient, but trying a proper implementation of [A* (A-Star)](https://en.wikipedia.org/wiki/A*_search_algorithm) will solve it in a good time.
 
 Now, as for part two... I hope you got twelve minutes to run tests.
+
+## Day 12: Leonardo's Monorail
+
+```
+Year2016::Day12
+  when Part 1
+    gives a final result
+  when Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+After yesterday's mess, building a [virtual machine](https://en.wikipedia.org/wiki/Virtual_machine) is a delight. Nothing specific, refer to [Year 2015, Day 23](year_2015.md#day-23-opening-the-turing-lock) for more info.
diff --git a/year_2016/day_12.rb b/year_2016/day_12.rb
new file mode 100644
index 0000000..18545e4
--- /dev/null
+++ b/year_2016/day_12.rb
@@ -0,0 +1,70 @@
+class Year2016
+  class Day12
+    attr_accessor :program
+
+    class Program
+      attr_accessor :instructions, :pc, :registers
+
+      def initialize(input_instructions)
+        @instructions = input_instructions.map do |instruction|
+          [instruction, compile(instruction)]
+        end
+      end
+
+      def inc(register)
+        @pc += 1
+        @registers[register] += 1
+      end
+
+      def dec(register)
+        @pc += 1
+        @registers[register] -= 1
+      end
+
+      def cpy(from, to)
+        @pc += 1
+        @registers[to] = registers[from] || from.to_i
+      end
+
+      def jnz(value, jmp)
+        return @pc += 1 if (@registers[value] || value.to_i).zero?
+
+        @pc += jmp.to_i
+      end
+
+      def compile(instruction)
+        case instruction
+        when /^cpy (-?\d+|[abcd]) ([abcd])$/
+          [:cpy, $1, $2]
+        when /^inc ([abcd])$/
+          [:inc, $1]
+        when /^dec ([abcd])$/
+          [:dec, $1]
+        when /^jnz (-?\d+|[abcd]) (-?\d+)$/
+          [:jnz, $1, $2]
+        end
+      end
+
+      def execute(init_registers = { 'a' => 0, 'b' => 0, 'c' => 0, 'd' => 0 })
+        @pc = 0
+        @registers = init_registers
+
+        send(*instructions[@pc].last) while instructions[@pc]
+      end
+    end
+
+    def initialize(input_data)
+      @program = Program.new(input_data.chomp.split("\n"))
+    end
+
+    def part_one
+      @program.execute
+      @program.registers['a']
+    end
+
+    def part_two
+      @program.execute('a' => 0, 'b' => 0, 'c' => 1, 'd' => 0)
+      @program.registers['a']
+    end
+  end
+end