Ruby Discord Halloween 2024 Challenge

vampire.rb

VAMPIRES = ARGV.each_with_index.filter_map do|input, idx|
  number = Integer input
  length = input.length
  next if length.odd?
  
  fangs = input.chars.permutation(length).filter_map do|digits|
    digits2 = digits.pop(length/2)
    next if digits.last == '0' and digits2.last == '0' # This isn’t counted
    a = Integer digits .join, 10
    b = Integer digits2.join, 10
    # Pairs will more often not multiply out than be a duplicate, so I check fang pairs before dedup.
    next if a * b != number
    # Duplication rate is at least 50% due to multiplication’s commutative property,
    # so I use ordered tuples for their (slightly?) faster read speed.
    a > b ? [b, a] : [a, b]
  end
  # I expect most of the input be non-vampires, so I dedicate them a fast-track
  # (vs. utilizing `nil`ability of {Array#uniq!})
  next if fangs.empty? # not vampiric at all
  
  fangs.uniq!
  [
    ~fangs.size, # main sorting key  (descending)
    idx,         # stable sorting key (ascending)
    input        # original value – won’t actually participate in sorting, but is here hitching the ride
  ]
end
VAMPIRES.sort!
VAMPIRES.each { puts _3 }