Year 2016: Day 16

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 15](year_2016.md)
+* [2016, up to day 16](year_2016.md)
 * [2023, up to day 04](year_2023.md)
 
 # How to use

spec/year_2016/day_16_spec.rb

@@ -0,0 +1,23 @@
+require 'year_2016/day_16'
+
+describe Year2016::Day16 do
+  context 'when Part 1' do
+    it 'gives a final result' do
+      expect(described_class.new('10000', true).to_s(20)).to eq('01100')
+    end
+  end
+
+  context 'when Results' do
+    subject(:input_data) do
+      '11100010111110100'
+    end
+
+    it 'correctly answers part 1' do
+      expect(described_class.new(input_data, true).to_s(272)).to eq('10100011010101011')
+    end
+
+    it 'correctly answers part 2' do
+      expect(described_class.new(input_data).to_s(35_651_584)).to eq('01010001101011001')
+    end
+  end
+end

year_2016.md

@@ -215,3 +215,16 @@ Year2016::Day15
 ```
 
 This one is funnier as a maths problem than an engineering problem. An [elegant solution](https://github.com/rHermes/adventofcode/blob/master/2016/15/y2016_d15_p01.py) worked in a very few lines.
+
+## Day 16: Dragon Checksum
+
+```
+Year2016::Day16
+  when Part 1
+    gives a final result
+  when Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+I liked this one, a lot.

year_2016/day_16.rb

@@ -0,0 +1,28 @@
+class Year2016
+  class Day16
+    def initialize(input_data, input_part_one = false)
+      @version = input_part_one ? 1 : 2
+      @input = input_data.chomp.chars.map do |char|
+        char == '1'
+      end
+    end
+
+    def expand_string(new_str)
+      [new_str, [false], new_str.reverse.map(&:!)].inject(&:concat)
+    end
+
+    def checksum(string)
+      string.each_slice(2).map do |lhs, rhs|
+        lhs == rhs
+      end
+    end
+
+    def to_s(disk_size)
+      str = @input
+      str = expand_string(str) until str.length >= disk_size
+      str = str[0..disk_size - 1] if str.length >= disk_size
+      str = checksum(str) until str.length.odd?
+      str.map{|elt| elt ? '1' : '0' }.join
+    end
+  end
+end