Solve day04 part 1

day04/day04.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class Day04
+  def solve(lines)
+    p1 = 0
+
+    lines.each do |line|
+      _card, winning, mine = line.split(/:|\|/)
+      win_count = nums(winning).intersection(nums(mine)).size
+
+      p1 += 2**(win_count - 1) unless win_count.zero?
+    end
+
+    [p1, -2]
+  end
+
+  private
+
+  def nums(str)
+    str.scan(/(\d+)/).flatten.map(&:to_i)
+  end
+end

day04/main.rb

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+# frozen_string_literal: true
+
+require_relative '../utils/run_day'
+require_relative 'day04'
+
+RunDay.run(Day04.new)

spec/day04/day04_spec.rb

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+# frozen_string_literal: true
+
+require_relative '../../day04/day04'
+
+RSpec.describe Day04 do
+  subject(:solve) { described_class.new.solve(lines) }
+
+  let(:lines) do
+    [
+      'Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53',
+      'Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19',
+      'Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1',
+      'Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83',
+      'Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36',
+      'Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11'
+    ]
+  end
+
+  describe '#solve' do
+    context 'when part 1' do
+      it 'returns the right result' do
+        p1, _p2 = solve
+
+        expect(p1).to eq(13)
+      end
+    end
+
+    context 'when part 2' do
+      it 'returns the right result' do
+        _p1, p2 = solve
+
+        expect(p2).to eq(-2)
+      end
+    end
+  end
+end