Class: Equalshares::Rules::MethodOfEqualShares

Inherits:
Base
  • Object
show all
Defined in:
lib/equalshares/rules/method_of_equal_shares.rb

Overview

The Method of Equal Shares for approval ballots — the equalshares.net rule, including its completion methods (none/utilitarian/add1/...) and comparison step. Faithful port of equalShares() in js/methodOfEqualSharesWorker.js.

Cost and budget are treated as floats at this level (statistics, comparison, utilitarian/greedy committees, the everything-affordable check); only the inner fixed-budget MES loop switches to exact rationals for accuracy "fractions".

Constant Summary collapse

ADD1_COMPLETIONS = 
%w[add1 add1e add1u add1eu].freeze
UTILITARIAN_COMPLETIONS = 
%w[utilitarian add1u].freeze

Instance Method Summary collapse

Methods inherited from Base

call, #initialize

Constructor Details

This class inherits a constructor from Equalshares::Rules::Base

Instance Method Details

#call ⇒ Object 



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# File 'lib/equalshares/rules/method_of_equal_shares.rb', line 16

def call
  start = now
  voter_ids = election.voter_ids
  project_ids = election.project_ids
  approvers = election.approvers

  # Strings for the MES loop (it wraps them per accuracy); floats for the
  # completion/comparison/statistics bookkeeping (as the JS does).
  cost_source = project_ids.to_h { |c| [c, instance.projects[c]["cost"]] }
  cost = election.float_costs
  b_float = election.float_budget

  mes = run_mes(cost_source, instance.budget)
  winners = mes.fetch(:winners)
  report = mes.fetch(:report)
  notes = {
    endowment: report[:endowment],
    money_behind_candidate: report[:money_behind_candidate],
    effective_vote_count: report[:effective_vote_count]
  }

  # utilitarian completion if needed
  if UTILITARIAN_COMPLETIONS.include?(params.completion)
    completion = Completion.utilitarian(voter_ids, project_ids, cost, approvers, b_float, winners)
    winners = completion.fetch(:winners)
    notes[:added_by_utilitarian_completion] = completion.fetch(:added)
  end

  # comparison step
  greedy = Completion.utilitarian(voter_ids, project_ids, cost, approvers, b_float, []).fetch(:winners)
  unless params.comparison == "none"
    cmp = Comparison.comparison_step(voter_ids, approvers, greedy, winners, params)
    unless cmp[:stick_to_mes]
      winners = greedy
      notes[:comparison] =
        "The committee chosen by the greedy algorithm is preferred by #{cmp[:prefers_greedy]} voters, " \
        "while the committee chosen by the method of equal shares is preferred by #{cmp[:prefers_mes]} voters."
    end
  end

  notes[:greedy_stats] = election.statistics(greedy)
  result(winners, start, notes)
end