Year 2016: Day 14

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 13](year_2016.md)
+* [2016, up to day 14](year_2016.md)
 * [2023, up to day 04](year_2023.md)
 
 # How to use

spec/year_2016/day_14_spec.rb

@@ -0,0 +1,29 @@
+require 'year_2016/day_14'
+
+describe Year2016::Day14 do
+  context 'when Part 1' do
+    it 'gives a final result' do
+      expect(described_class.new('abc', true).to_i).to eq(22_728)
+    end
+  end
+
+  context 'when Part 2' do
+    it 'gives a final result', skip: 'Test is too slow for CI' do
+      expect(described_class.new('abc').to_i).to eq(22_551)
+    end
+  end
+
+  context 'when Results' do
+    subject(:input_data) do
+      'yjdafjpo'
+    end
+
+    it 'correctly answers part 1' do
+      expect(described_class.new(input_data, true).to_i).to eq(25_427)
+    end
+
+    it 'correctly answers part 2', skip: 'Test is too slow for CI' do
+      expect(described_class.new(input_data).to_i).to eq(22_045)
+    end
+  end
+end

year_2016.md

@@ -187,3 +187,28 @@ Year2016::Day13
 ```
 
 Thinking in terms of X and Y can be a source of pain, but this one was straight enough.
+
+## Day 14: One-Time Pad
+
+```
+Year2016::Day14
+  when Part 1
+    gives a final result
+  when Part 2
+    gives a final result
+  when Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+This one is even more annoying than [the last one](year_2016.md#day-05-how-about-a-nice-game-of-chess).
+
+There is also a funny edge-case where your last key may look something like this:
+```
+abc22034 => Triple
+abc22045 => Triple
+...
+abc-smth => Quintuple for abc22045
+...
+abc-smth => Quintuple for abc22034
+```

year_2016/day_14.rb

@@ -0,0 +1,56 @@
+require 'digest'
+
+class Year2016
+  class Day14
+    def initialize(input_data, input_part_one = false)
+      @version = input_part_one ? 1 : 2
+      @input = input_data.chomp
+    end
+
+    def md5
+      @md5 ||= Digest::MD5.new
+    end
+
+    def hash(str)
+      md5.reset
+      md5 << str
+      return md5.hexdigest if @version == 1
+
+      digest = md5.hexdigest
+      1.upto(2016) do
+        md5.reset
+        md5 << digest
+        digest = md5.hexdigest
+      end
+      digest
+    end
+
+    def generate_hashes(starter, length)
+      0.upto(length).map{|cnt| hash("#{@input}#{starter + cnt}") }
+    end
+
+    # rubocop:disable Metrics/AbcSize, Metrics/MethodLength
+    def to_i
+      cnt = -1
+      hash_queue = generate_hashes(0, 1000)
+      keys = []
+      loop do
+        cnt += 1
+        hash_queue.concat(generate_hashes(cnt, 1000)) if hash_queue.empty?
+        digest = hash_queue.shift
+        next unless digest =~ /(.)\1\1/
+
+        letter = $1
+        hash_queue.concat(generate_hashes(cnt + 1 + hash_queue.length, 1000 - hash_queue.length))
+        0.upto(999) do |idx|
+          next unless hash_queue[idx].include?(letter * 5)
+          return cnt if keys.length == 63
+
+          keys.push(digest)
+          break
+        end
+      end
+    end
+    # rubocop:enable Metrics/AbcSize, Metrics/MethodLength
+  end
+end