gistfile1.txt

❯ ruby irv.rb
{:rain=>7, :women=>5, :islands=>4, :trees=>6, :salmon=>9, :parks=>5}
Eliminating: [:islands]
{:rain=>8, :women=>5, :parks=>6, :trees=>6, :salmon=>11}
Eliminating: [:women]
{:rain=>9, :parks=>6, :trees=>7, :salmon=>14}
Eliminating: [:parks]
{:rain=>10, :trees=>10, :salmon=>16}
Eliminating: [:rain, :trees]
Winner: salmon

irv.rb

require "set"

class Election
  def initialize(ballots)
    @eliminated = Set.new
    @ballots = ballots
  end

  def run
    while winner.nil?
      p count
      eliminate!
    end

    puts "Winner: #{winner}"
  end

  private

  def winner
    count.find {|_,v| v > (0.5 * @ballots.size) }&.first
  end

  def eliminate!
    min = count.map(&:last).min
    last_place = count.select {|_,v| v == min }.map(&:first)
    puts "Eliminating: #{last_place.inspect}"
    @eliminated = @eliminated.merge(last_place)
  end

  def count
    @ballots.map {|ballot| ballot - @eliminated.to_a }.map(&:first).each.with_object(Hash.new(0)) {|ballot, h| h[ballot] += 1 }
  end

end

candidates = %i[ salmon islands rain trees parks women ]
ballots = DATA.read.split("\n").map {|row| row.split("\t").map(&:to_i).zip(candidates).sort.map(&:last) }
Election.new(ballots).run

__END__
3	5	1	2	6	4
5	4	2	6	3	1
5	3	1	2	4	6
5	1	6	4	2	3
2	3	5	1	4	6
6	3	2	1	5	4
1	2	6	4	3	5
1	3	6	4	2	5
2	6	4	1	5	3
5	3	6	4	1	2
1	2	4	5	3	6
2	6	4	5	3	1
2	1	6	3	4	5
4	3	1	2	5	6
6	5	1	2	4	3
1	4	2	5	6	3
1	5	6	2	3	4
3	5	1	4	2	6
1	3	6	5	4	2
4	5	2	1	6	3
1	4	6	2	5	3
2	4	5	3	1	6
5	3	4	6	1	2
2	4	6	5	3	1
1	2	6	4	3	5
6	3	4	2	1	5
5	4	6	1	3	2
5	3	1	6	2	4
2	1	5	3	4	6
3	4	6	2	5	1
2	3	5	4	6	1
1	3	6	4	2	5
2	3	4	1	6	5
4	2	1	3	6	5
3	2	6	4	1	5
4	1	2	5	3	6