Year 2015: Day 10

README.md

@@ -7,7 +7,7 @@ Consistency is hard without proper goals to set a good (let's dare say "atomic")
 I'm also adding notes that may be useful if you're learning Ruby.
 
 Notes for solving:
-* [2015, up to day 09](year_2015.md)
+* [2015, up to day 10](year_2015.md)
 * [2023, up to day 03](year_2023.md)
 
 # How to use

spec/year_2015/day_10_spec.rb

@@ -0,0 +1,29 @@
+require 'year_2015/day_10'
+
+describe Year2015::Day10 do
+  context 'Part 1' do
+    subject do
+      Year2015::Day10.new('1')
+    end
+
+    it 'iterates each look-and-say' do
+      %w(1 11 21 1211 111221 312211).each_with_index do |result, i|
+        expect(subject.iterations(i)).to eq(result)
+      end
+    end
+  end
+
+  context 'Results' do
+    subject do
+      Year2015::Day10.new('1113222113')
+    end
+
+    it 'correctly answers part 1' do
+      expect(subject.to_i(40)).to eq(252594)
+    end
+
+    it 'correctly answers part 2' do
+      expect(subject.to_i(50)).to eq(3579328)
+    end
+  end
+end

year_2015.md

@@ -165,3 +165,18 @@ If you're not fond of graph transversal, the best answer is often to start using
 You can then optimize it, for instance instructing the algorithm to stop searching once it's on a path longer than a previously explored one.
 
 As for finding the "longest path"...just imagine you're not looking for a "shortest" or "longest" path, but a "best" path, and change how that path is selected among others.
+
+## Day 10
+
+```
+Year2015::Day10
+  Part 1
+    iterates each look-and-say
+  Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+This exercise is based on John H. Conway's [Look-and-say sequences](https://en.wikipedia.org/wiki/Look-and-say_sequence), you probably know him for the [Game of Life](https://en.wikipedia.org/wiki/Conway%27s_Game_of_Life), but the mathematician provided us with lot of science.
+
+Nothing really hard in this exercise, except complexity quickly running high, it becomes important to use the best algorithm to generate the next sequence.

year_2015/day_10.rb

@@ -0,0 +1,30 @@
+class Year2015
+  class Day10
+    def iterations(desired_iteration = -1)
+      look_and_say until @inner_iterations.length > desired_iteration
+      @inner_iterations[desired_iteration]
+    end
+
+    def look_and_say
+      starter = @inner_iterations.last
+      next_iteration = starter.each_char.with_object([starter[0], 0]) do |char, memo|
+        if char == memo[-2]
+          memo[-1] += 1
+          next memo
+        end
+        memo.push(char, 1)
+        memo
+      end.each_slice(2).map(&:reverse).flatten.map(&:to_s).join
+
+      @inner_iterations.push(next_iteration)
+    end
+
+    def initialize(input_data)
+      @inner_iterations = [input_data]
+    end
+
+    def to_i(desired_iteration = -1)
+      iterations(desired_iteration).length
+    end
+  end
+end