Class: Beatpath::VoteMatrix

Inherits:
Matrix
  • Object
show all
Defined in:
lib/beatpath/vote_matrix.rb

Instance Attribute Summary collapse

Class Method Summary collapse

Instance Method Summary collapse

Instance Attribute Details

#vote_candidates ⇒ Object

Returns the value of attribute vote_candidates.



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# File 'lib/beatpath/vote_matrix.rb', line 8

def vote_candidates
  @vote_candidates
end

Class Method Details

.pairwise_from_ballots(ballots, candidates = nil) ⇒ Object 



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# File 'lib/beatpath/vote_matrix.rb', line 16

def self.pairwise_from_ballots(ballots, candidates = nil)
  if candidates.nil?
    candidates = SortedSet.new

    # compute distinct candidates
    ballots.each do |b|
      candidates += b
    end

    candidates = candidates.to_a
  end

  d = zero_with_candidates(candidates)

  # compute pairwise rankings from all ballots
  ballots.each do |b|
    # for each ballot, compute which candidates were not voted on
    unvoted_candidates = candidates - b

    # for each candidate on the given ballot
    b.each_with_index do |c1, idx|
      # compute our internal mapping of the candidate
      c1_idx = candidates.find_index(c1)

      # for each candidate, c2, that follows this candidate in order of preference,
      # mark c1 as pairwise better than c2
      b[(idx + 1)..-1].each do |c2|
        d[c1_idx, candidates.find_index(c2)] += 1
      end

      # candidates not mentioned on this ballot are always strictly worse
      # than the candidates on the ballot (c1)
      unvoted_candidates.each do |c2|
        d[c1_idx, candidates.find_index(c2)] += 1
      end
    end
  end

  d
end

.zero_with_candidates(candidates = []) ⇒ Object 



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# File 'lib/beatpath/vote_matrix.rb', line 10

def self.zero_with_candidates(candidates = [])
  zeroed = zero(candidates.size)
  zeroed.vote_candidates = candidates
  zeroed
end

Instance Method Details

#floyd_warshall ⇒ Object

en.wikipedia.org/wiki/Schulze_method Input: self, the number of voters who prefer candidate i to candidate j. Output: p, the strength of the strongest path from candidate i to candidate j.



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# File 'lib/beatpath/vote_matrix.rb', line 60

def floyd_warshall
  p = VoteMatrix.zero_with_candidates(vote_candidates)

  # for i from 1 to C
  #    for j from 1 to C
  #       if (i ≠ j) then
  #          if (d[i,j] > d[j,i]) then
  #             p[i,j] := d[i,j]
  #          else
  #             p[i,j] := 0
  row_count.times do |i|
    row_count.times do |j|
      next if i == j

      if self[i, j] > self[j, i]
        p[i, j] = self[i, j]
      else
        p[i, j] = 0
      end
    end
  end

  # for i from 1 to C
  #    for j from 1 to C
  #       if (i ≠ j) then
  #          for k from 1 to C
  #             if (i ≠ k and j ≠ k) then
  #                p[j,k] := max ( p[j,k], min ( p[j,i], p[i,k] )
  row_count.times do |i|
    row_count.times do |j|
      next if i == j

      row_count.times do |k|
        next if i == k || j == k

        p[j, k] = [p[j, k], [p[j, i], p[i, k]].min].max
      end
    end
  end

  p
end

#winners ⇒ Object 



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# File 'lib/beatpath/vote_matrix.rb', line 103

def winners
  results = VoteMatrix.zero_with_candidates(vote_candidates)

  # compute which candidate has the strongest path
  row_count.times do |i|
    row_count.times do |j|
      next if i == j

      # candidate i is strictly better than j if the strongest path goes through i
      results[i, j] += 1 if self[i, j] > self[j, i]
    end
  end

  # and finally compute the final ordering of the candidates
  results = Hash[results.row_vectors.each_with_index.map do |r, i|
    [vote_candidates[i], r.inject(:+)]
  end]

  results.sort_by { |_, v| v }.map(&:first).reverse
end