Year 2016: Day 05

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 04](year_2016.md)
+* [2016, up to day 05](year_2016.md)
 * [2023, up to day 03](year_2023.md)
 
 # How to use

spec/year_2016/day_05_spec.rb

@@ -0,0 +1,29 @@
+require 'year_2016/day_05'
+
+describe Year2016::Day05 do
+  context 'when Part 1' do
+    it 'gives a final result' do
+      expect(described_class.new('abc', true).code).to eq('18f47a30')
+    end
+  end
+
+  context 'when Part 2' do
+    it 'gives a final result' do
+      expect(described_class.new('abc').code).to eq('05ace8e3')
+    end
+  end
+
+  context 'when Results' do
+    subject(:input_data) do
+      'ojvtpuvg'
+    end
+
+    it 'correctly answers part 1' do
+      expect(described_class.new(input_data, true).code).to eq('4543c154')
+    end
+
+    it 'correctly answers part 2' do
+      expect(described_class.new(input_data).code).to eq('1050cbbd')
+    end
+  end
+end

year_2016.md

@@ -54,3 +54,18 @@ Year2016::Day04
 ```
 
 The second part of this exercise involves implementing a [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher), which is not a hard concept to grasp. It's a bit annoying to work through ASCII values of characters and modulo properly to get the right number.
+
+## Day 05: How About a Nice Game of Chess?
+
+```
+Year2016::Day05
+  when Part 1
+    gives a final result
+  when Part 2
+    gives a final result
+  when Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+Not gonna lie, brute-forcing [MD5 hashes](https://en.wikipedia.org/wiki/MD5) is not something interesting.

year_2016/day_05.rb

@@ -0,0 +1,49 @@
+require 'digest'
+
+class Year2016
+  class Day05
+    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
+      md5.hexdigest
+    end
+
+    # rubocop:disable Metrics/AbcSize, Metrics/MethodLength
+    def build_code
+      @code = '_' * 8
+      added = 0
+      0.upto(100**100) do |i|
+        break if added == 8
+
+        digest = hash("#{@input}#{i}")
+        next unless digest.start_with?('0' * 5)
+
+        if @version == 1
+          @code[added] = digest[5]
+          next added += 1
+        end
+
+        pos = digest[5].ord - 48
+        next if pos.negative? || pos > 7 || @code[pos] != '_'
+
+        code[pos] = digest[6]
+        added += 1
+      end
+      @code
+    end
+    # rubocop:enable Metrics/AbcSize, Metrics/MethodLength
+
+    def code
+      @code ||= build_code
+    end
+  end
+end