Year 2015: Day 04

spec/year_2015/day_04_spec.rb

@@ -0,0 +1,23 @@
+require 'year_2015/day_04'
+
+describe Year2015::Day04 do
+  context 'when Part 1' do
+    it 'gives a final result' do
+      expect(described_class.new('abcdef', true).to_i).to eq(609_043)
+    end
+  end
+
+  context 'when Results' do
+    subject(:input_data) do
+      'iwrupvqb'
+    end
+
+    it 'correctly answers part 1' do
+      expect(described_class.new(input_data, true).to_i).to eq(346_386)
+    end
+
+    it 'correctly answers part 2' do
+      expect(described_class.new(input_data).to_i).to eq(9_958_218)
+    end
+  end
+end

year_2015.md

@@ -48,25 +48,20 @@ The only thing to be wary of is on line 16: without the call to `#dup`, all of S
 
 Passing by value or reference is a really wonky subject, but this [blog post](https://robertheaton.com/2014/07/22/is-ruby-pass-by-reference-or-pass-by-value/) got nice examples that will get you started.
 
-## Day 04: The Ideal Stocking Stuffer
+## Day 04:The Ideal Stocking Stuffer
 
 ```
-Year2015::Day04
-  Part 1
-    finds winning numbers
-    calculates points
+Year2016::Day05
+  when Part 1
     gives a final result
-  Part 2
-    distributes cards
+  when Part 2
     gives a final result
-  Results
+  when Results
     correctly answers part 1
     correctly answers part 2
 ```
 
-Part one is pretty easy. One useful method is the [`#&`](https://ruby-doc.org/core-3.0.1/Array.html#method-i-26) operator that takes two arrays and returns a new one with only the matching contents. The syntax is clearly inspired by [bit masking](https://en.wikipedia.org/wiki/Mask_(computing)), which is a subject you should look into if you've never heard of it.
-
-Part two is a bit more annoying and requires to do a two-pass, but nothing really improbable.
+Not gonna lie, brute-forcing [MD5 hashes](https://en.wikipedia.org/wiki/MD5) is not something interesting.
 
 ## Day 05: Doesn't He Have Intern-Elves For This?
 

year_2015/day_04.rb

@@ -0,0 +1,26 @@
+require 'digest'
+
+class Year2015
+  class Day04
+    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
+
+    def to_i
+      0.upto(100**100) do |i|
+        return i if hash("#{@input}#{i}").start_with?('0' * (@version + 4))
+      end
+    end
+  end
+end