Day 24: Crossed Wires

Author: ahorner

lib/2024/24.rb

@@ -0,0 +1,64 @@
+setup, outputs = INPUT.split("\n\n")
+
+WIRE_MATCHER = /\A(?<wire>.+): (?<value>0|1)\z/
+WIRES = setup.split("\n").each_with_object({}) do |line, wires|
+  match = line.match(WIRE_MATCHER)
+  wires[match[:wire]] = match[:value].to_i(2)
+end
+
+GATE_MATCHER = /\A(?<w1>.+) (?<gate>.+) (?<w2>.+) -> (?<output>.+)\z/
+GATES = outputs.split("\n").each_with_object({}) do |line, gates|
+  match = line.match(GATE_MATCHER)
+  gates[match[:output]] = [match[:w1], match[:w2], match[:gate]]
+end
+
+def value_for(wire, wires, gates)
+  wires[wire] ||= begin
+    w1, w2, gate = gates[wire]
+
+    v1 = value_for(w1, wires, gates)
+    v2 = value_for(w2, wires, gates)
+
+    case gate
+    when "XOR" then v1 ^ v2
+    when "AND" then v1 & v2
+    when "OR" then v1 | v2
+    end
+  end
+end
+
+def value_on(wires, prefix)
+  keys = wires.keys.select { |w| w.start_with?(prefix) }
+  keys.sort.reverse.map { |k| wires[k] }.join.to_i(2)
+end
+
+def value(wires, gates)
+  wires = wires.dup
+
+  gates.keys.each do |output|
+    value_for(output, wires, gates)
+  end
+
+  value_on(wires, "z")
+end
+
+solve!("The final value on the Z wires is:", value(WIRES, GATES))
+
+def used_for?(wire, gates, &block)
+  gates.any? { |_, (w1, w2, g)| (w1 == wire || w2 == wire) && block.call(g) }
+end
+
+HIGHEST_OUTPUT_BIT = GATES.keys.select { |k| k.start_with?("z") }.max
+
+invalid_outputs = GATES.each_with_object([]) do |(output, (w1, w2, gate)), invalid|
+  invalid_operations = [
+    output.start_with?("z") && gate != "XOR" && output != HIGHEST_OUTPUT_BIT,
+    gate == "AND" && w1 != "x00" && w2 != "x00" && used_for?(output, GATES) { |op| op != "OR" },
+    gate == "XOR" && [output, w1, w2].none? { |w| w.start_with?("x", "y", "z") },
+    gate == "XOR" && used_for?(output, GATES) { |op| op == "OR" }
+  ]
+
+  invalid << output if invalid_operations.any?
+end
+
+solve!("The swapped outputs are:", invalid_outputs.sort.join(","))

spec/2024/24_spec.rb

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+require "spec_helper"
+
+RSpec.describe "Day 24: Crossed Wires" do
+  let(:runner) { Runner.new("2024/24") }
+
+  describe "Part One" do
+    let(:solution) { runner.execute!(input, part: 1) }
+
+    it "calculates the number produced on the z wires" do
+      expect(runner.execute!(<<~TXT, part: 1)).to eq(4)
+        x00: 1
+        x01: 1
+        x02: 1
+        y00: 0
+        y01: 1
+        y02: 0
+
+        x00 AND y00 -> z00
+        x01 XOR y01 -> z01
+        x02 OR y02 -> z02
+      TXT
+
+      expect(runner.execute!(<<~TXT, part: 1)).to eq(2024)
+        x00: 1
+        x01: 0
+        x02: 1
+        x03: 1
+        x04: 0
+        y00: 1
+        y01: 1
+        y02: 1
+        y03: 1
+        y04: 1
+
+        ntg XOR fgs -> mjb
+        y02 OR x01 -> tnw
+        kwq OR kpj -> z05
+        x00 OR x03 -> fst
+        tgd XOR rvg -> z01
+        vdt OR tnw -> bfw
+        bfw AND frj -> z10
+        ffh OR nrd -> bqk
+        y00 AND y03 -> djm
+        y03 OR y00 -> psh
+        bqk OR frj -> z08
+        tnw OR fst -> frj
+        gnj AND tgd -> z11
+        bfw XOR mjb -> z00
+        x03 OR x00 -> vdt
+        gnj AND wpb -> z02
+        x04 AND y00 -> kjc
+        djm OR pbm -> qhw
+        nrd AND vdt -> hwm
+        kjc AND fst -> rvg
+        y04 OR y02 -> fgs
+        y01 AND x02 -> pbm
+        ntg OR kjc -> kwq
+        psh XOR fgs -> tgd
+        qhw XOR tgd -> z09
+        pbm OR djm -> kpj
+        x03 XOR y03 -> ffh
+        x00 XOR y04 -> ntg
+        bfw OR bqk -> z06
+        nrd XOR fgs -> wpb
+        frj XOR qhw -> z04
+        bqk OR frj -> z07
+        y03 OR x01 -> nrd
+        hwm AND bqk -> z03
+        tgd XOR rvg -> z12
+        tnw OR pbm -> gnj
+      TXT
+    end
+  end
+
+  describe "Part Two" do
+    # NOTE: The solution only finds invalid outputs within _addition_
+    # calculations, and isn't generic enough to solve for other operations like
+    # the AND example from the exercise.
+  end
+end