NAME

Vote::Count::IRV

VERSION 2.01

IRV

Implements Instant Runoff Voting for Vote::Count.

SYNOPSIS

  use Vote::Count::Method;
  use Vote::Count::ReadBallots 'read_ballots';

  my $Election = Vote::Count::->new(
    BallotSet => read_ballots('%path_to_my_ballots'),
    TieBreakMethod => 'grandjunction');

  my $result = $Election->RunIRV();
  my $winner = $result->{'winner'};

  say $Election->logv(); # Print the full Log.

Method Summary

Instant Runoff Voting Looks for a Majority Winner. If one isn't present the choice with the lowest Top Count is removed.

Instant Runoff Voting is easy to count by hand and meets the Later Harm and Condorcet Loser Criteria. It, unfortunately, fails a large number of consistency criteria; the order of candidate dropping matters and small changes to the votes of non-winning choices that result in changes to the dropping order can change the outcome.

Instant Runoff Voting is also known as Alternative Vote and as the Hare Method.

Tie Handling

There is no standard accepted method for IRV tie resolution, Eliminate All is a common one and the default.

Returns a tie when all of the remaining choices are in a tie.

An optional value to RunIRV is to specify tiebreaker, see Vote::Count::TieBreaker.

RunIRV

  $Election->RunIRV();
  $Election->RunIRV( $active )
  $Election->RunIRV( $active, 'approval' )

Runs IRV on the provided Ballot Set. Takes an optional parameter of $active which is a hashref for which the keys are the currently active choices.

Returns results in a hashref which will be the results of Vote::Count::TopCount->EvaluateTopCountMajority, if there is no winner hash will instead be:

  tie => [true or false],
  tied => [ array of tied choices ],
  winner => a false value

Supports the Vote::Count logt, logv, and logd methods for providing details of the method.

Bottom Two Runoff IRV

Bottom Two Runoff IRV is the simplest modification to IRV which meets the Condorcet Winner Criteria. Instead of eliminating the low choice, the lowest two choices enter a virtual runoff, eliminating the loser. This is the easiest possible Hand Count Condorcet method, there will always be fewer pairings than choices. As a Condorcet method it fails Later No Harm.

BTR IRV will only eliminate a member of the Smith Set when both members of the runoff are in it, so it can never eliminate the final member of the Smith Set, and is thus Smith compliant.

RunBTRIRV

  my $result = $Election->RunBTRIRV();
  my $result = $Election->RunBTRIRV( 'ranking2' => 'Approval');

Choices are ordered by TopCount, ties for position are decided by Precedence. It is mandatory that either the TieBreakMethod is Precedence or TieBreakerFallBackPrecedence is True. The optional ranking2 option will use a second method before Precedence, see UnTieList in Vote::Count::TieBreaker.

The returned values and logging are the same as for RunIRV.

BUG TRACKER

https://github.com/brainbuz/Vote-Count/issues

AUTHOR

John Karr (BRAINBUZ) brainbuz@cpan.org

CONTRIBUTORS

Copyright 2019-2021 by John Karr (BRAINBUZ) brainbuz@cpan.org.

LICENSE

This module is released under the GNU Public License Version 3. See license file for details. For more information on this license visit http://fsf.org.

SUPPORT

This software is provided as is, per the terms of the GNU Public License. Professional support and customisation services are available from the author.