2021_3.rb

def tally(data, ii)
  Hash.new(0).tap do |h|
    data.each { h[_1[ii]] += 1 }
  end
end

n = data.first.length

# part 1
gamma = (0...n).map do |ii|
  tally = tally(data, ii)
  tally['0'] > tally['1'] ? 0 : 1
end
gamma = gamma.join.to_i(2)
epsilon = gamma ^ ((1 << n) - 1)
p gamma * epsilon

# part 2
oxy, co2 = data.dup, data.dup
(0...n).each do |ii|
  if oxy.length > 1
    tally = tally(oxy, ii)
    keep = tally['0'] > tally['1'] ? '0' : '1'
    oxy.select! { _1[ii] == keep }
  end
  if co2.length > 1
    tally = tally(co2, ii)
    keep = tally['0'] > tally['1'] ? '1' : '0'
    co2.select! { _1[ii] == keep }
  end
end
oxy, co2 = oxy.first.to_i(2), co2.first.to_i(2)
p oxy * co2