Day seventeen

17/input

@@ -0,0 +1,5 @@
+Register A: 17323786
+Register B: 0
+Register C: 0
+
+Program: 2,4,1,1,7,5,1,5,4,1,5,5,0,3,3,0

17/main.rb

@@ -0,0 +1,80 @@
+#!/usr/bin/env ruby
+
+input = (ARGV.first.nil? ? DATA : ARGF)
+  .readlines(chomp: true)
+split = input.index("")
+
+registers = input[...split]
+  .map { _1.split(": ").last.to_i }
+  .zip(%i[A B C]).map(&:reverse)
+  .to_h
+program = input[split+1].split(" ").last.split(",").map(&:to_i)
+
+def execute(registers, program)
+  stdout, ic = Array.new, 0
+
+  combo = ->(operand) { operand < 4 ? operand : registers.values[operand - 4] }
+
+  while ic < program.size
+    opcode, operand = program[ic], program[ic + 1]
+    ic += 2
+
+    case opcode
+    when 0 then registers[:A] = registers[:A] / 2 ** combo.(operand)
+    when 1 then registers[:B] = registers[:B] ^ operand
+    when 2 then registers[:B] = combo.(operand) % 8
+    when 3 then registers[:A] == 0 || ic = operand
+    when 4 then registers[:B] = registers[:B] ^ registers[:C]
+    when 5 then stdout << combo.(operand) % 8
+    when 6 then registers[:B] = registers[:A] / 2 ** combo.(operand)
+    when 7 then registers[:C] = registers[:A] / 2 ** combo.(operand)
+    end
+  end
+
+  stdout
+end
+
+part_1 = execute(registers.dup, program).join(",")
+p part_1
+
+def reverse_engineer(program, slot, candidate)
+  return candidate if slot < 0
+
+  candidate <<= 3 unless slot == program.size - 1
+
+  (0..7)
+    .map { [candidate + _1, execute({A: candidate + _1, B: 0, C: 0}, program)] }
+    .select { _1.last == program[slot..] }
+    .flat_map { reverse_engineer(program, slot - 1, _1.first) }
+end
+
+p reverse_engineer(program, program.size - 1, 0).sort.first
+
+# part_2 = 0
+# (program.size - 1).downto(0) do |slot|
+#   candidates = []
+#   (0..8).each do |i|
+#     candidate = part_2 + i
+#     _, _, output = execute({A: candidate, B: 0, C: 0}, program)
+#     new_program = output.split(",").map(&:to_i)
+#
+#     if new_program == program[slot..]
+#       candidates << candidate
+#       unless [301,2566855701950, 20534845615581, 20534845615573, 20534845615583].include?(candidate)
+#         part_2 = candidate
+#       end
+#     end
+#   end
+#   p candidates
+#   if slot == 0
+#     p candidates.first
+#   end
+#   part_2 <<= 3
+# end
+
+__END__
+Register A: 117440
+Register B: 0
+Register C: 0
+
+Program: 0,3,5,4,3,0