Find championship meetings between two teams, neither of which has won a prior championship

championships_between_non_champions.rb

# Assuming data is a tab-delimited list with columns 
# date, winner, loser

def champs(data)
  winners = {}
  losers = {}

  data.split(/\n/).sort.each do |line| 
    date, winner, loser = line.split(/\t/)
    winners[winner] = date unless winners.key?(winner)
    losers[loser] = date unless winners.key?(loser)
  end

  winners = winners.invert
  losers = losers.invert
  winners.delete_if { |k,v| not losers.key? k }
  losers.delete_if { |k,v| not winners.key? k }

  winners.keys.sort.reverse.each do |date|
    puts "#{date}: #{winners[date]} def. #{losers[date]}"
  end
end