Day 15

examples/15.txt

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+rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7

lib/days/15.rb

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+class Day15
+
+  def part1(input)
+    input.split(',').map { |s| compute_hash(s) }.sum
+  end
+
+  def compute_hash(str)
+    str.chars.reduce(0) do |value, char|
+      code = char.ord
+      value += code
+      value *= 17
+      value %= 256
+      value
+    end
+  end
+
+  Lens = Struct.new(:label, :focal_length)
+
+  def part2(input)
+    boxes = {}
+
+    input.split(',').each do |step|
+      if step.include?('=')
+        label, value = step.split('=')
+        box_num = compute_hash(label)
+
+        boxes[box_num] ||= []
+        new_lens = Lens.new(label, value.to_i)
+
+        index = boxes[box_num].index { |b| b.label == label }
+        if index
+          boxes[box_num][index] = new_lens
+        else
+          boxes[box_num] << new_lens
+        end
+      else
+        label = step[0...-1]
+        box_num = compute_hash(label)
+
+        if boxes[box_num]
+          index = boxes[box_num].index { |b| b.label == label }
+          boxes[box_num].delete_at(index) if index
+        end
+      end
+    end
+
+    boxes.sum do |box_num, box|
+      box.each_with_index.sum do |lens, index|
+        (box_num + 1) * (index + 1) * lens.focal_length
+      end
+    end
+  end
+
+end

test/15_test.rb

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+require 'minitest/autorun'
+require 'minitest/pride'
+require_relative '../app'
+
+class TestDay15 < Minitest::Test
+  def setup
+    @data = File.read(File.join(APP_ROOT, 'examples', '15.txt')).rstrip
+    @day = Day15.new
+  end
+
+  def test_part1
+    assert_equal @day.part1(@data), 1320
+  end
+
+  def test_part2
+    assert_equal @day.part2(@data), 145
+  end
+end