Year 2016: Day 18

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 17](year_2016.md)
+* [2016, up to day 18](year_2016.md)
 * [2023, up to day 04](year_2023.md)
 
 # How to use

spec/year_2016/day_18_input

@@ -0,0 +1 @@
+^.^^^.^..^....^^....^^^^.^^.^...^^.^.^^.^^.^^..^.^...^.^..^.^^.^..^.....^^^.^.^^^..^^...^^^...^...^.

spec/year_2016/day_18_spec.rb

@@ -0,0 +1,25 @@
+require 'year_2016/day_18'
+
+describe Year2016::Day18 do
+  context 'when Part 1' do
+    it 'gives a final result' do
+      [['..^^.', 3, 6], ['.^^.^.^^^^', 10, 38]].each do |input, len, result|
+        expect(described_class.new(input, true).to_i(len)).to eq(result)
+      end
+    end
+  end
+
+  context 'when Results' do
+    subject(:input_data) do
+      File.read('spec/year_2016/day_18_input')
+    end
+
+    it 'correctly answers part 1' do
+      expect(described_class.new(input_data, true).to_i(40)).to eq(1926)
+    end
+
+    it 'correctly answers part 2' do
+      expect(described_class.new(input_data).to_i(400_000)).to eq(19_986_699)
+    end
+  end
+end

year_2016.md

@@ -243,3 +243,20 @@ Year2016::Day17
 ```
 
 Nothing we haven't seen yet.
+
+## Day 18: Like a Rogue
+
+```
+Year2016::Day18
+  when Part 1
+    gives a final result
+  when Results
+    correctly answers part 1
+    correctly answers part 2
+```
+
+Very funny exercise. For testing purpose, the first version generated each string and kept it in memory to compare it in testing. Ugh.
+
+The second version took...around 20 seconds for 400K lines, which was sub-optimal.
+
+One thing you should always strive to do is not just *read the rules*, but *understand* them. In this case, the four rules for a trap can be summed up as: "different left and right will produce a trap".

year_2016/day_18.rb

@@ -0,0 +1,24 @@
+class Year2016
+  class Day18
+    def initialize(input_data, input_part_one = false)
+      @version = input_part_one ? 1 : 2
+      @input = input_data.chomp.chars.map do |char|
+        char == '.'
+      end
+    end
+
+    def to_i(len)
+      safe_tiles_cnt = @input.count(true)
+      1.upto(len - 1) do |_idx|
+        @input = @input.map.each_with_index do |_char, idx|
+          left = idx.zero? ? true : @input[idx - 1]
+          right = @input[idx + 1].nil? ? true : @input[idx + 1]
+          left == right
+        end
+
+        safe_tiles_cnt += @input.count(true)
+      end
+      safe_tiles_cnt
+    end
+  end
+end