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<?php |
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namespace Michaelc\Voting\STV; |
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use Psr\Log\LoggerInterface as Logger; |
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class VoteHandler |
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{ |
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/** |
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* Election object |
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* |
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* @var \Michaelc\Voting\STV\Election; |
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*/ |
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protected $election; |
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/** |
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* Array of all ballots in election |
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* |
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* @var \MichaelC\Voting\STV\Ballot[] |
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*/ |
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protected $ballots; |
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/** |
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* Quota of votes needed for a candidate to be elected |
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* |
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* @var int |
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*/ |
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protected $quota; |
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/** |
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* Number of candidates elected so far |
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* |
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* @var int |
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*/ |
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protected $electedCandidates; |
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/** |
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* Invalid ballots |
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* |
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* @var \MichaelC\Voting\STV\Ballot[] |
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*/ |
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protected $rejectedBallots; |
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/** |
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* Logger |
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* |
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* @var Psr\Log\LoggerInterface |
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*/ |
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protected $logger; |
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/** |
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* Constructor |
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* |
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* @param Election $election |
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*/ |
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public function __construct(Election $election, Logger $logger) |
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{ |
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$this->logger = $logger; |
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$this->election = $election; |
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$this->ballots = $this->election->getBallots(); |
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$this->rejectedBallots = []; |
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} |
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/** |
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* Run the election |
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* |
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* @return \MichaelC\Voting\STV\Candidate[] Winning candidates |
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*/ |
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public function run() |
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{ |
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$this->logger->notice('Starting to run an election'); |
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$this->rejectInvalidBallots(); |
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$this->quota = $this->getQuota(); |
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$this->firstStep(); |
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$candidates = $this->election->getActiveCandidates(); |
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while ($this->electedCandidates < $this->election->getWinnersCount()) |
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{ |
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if (!$this->checkCandidates($candidates)) |
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{ |
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$this->eliminateCandidates($candidates); |
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} |
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} |
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$this->logger->notice('Election complete'); |
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return $this->election->getElectedCandidates(); |
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} |
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/** |
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* Perform the initial vote allocation |
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* |
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* @return |
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*/ |
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protected function firstStep() |
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{ |
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$this->logger->info('Beginning the first step'); |
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foreach ($this->ballots as $i => $ballot) |
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{ |
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$this->logger->debug("Processing ballot $i in stage 1", |
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['ballot' => $ballot,] |
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); |
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$this->allocateVotes($ballot); |
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} |
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$this->logger->notice('First step complete', |
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['candidatesStatus' => $this->election->getCandidatesStatus()] |
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); |
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return; |
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} |
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/** |
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* Check if any candidates have reached the quota and can be elected |
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* |
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* @param array $candidates Array of active candidates to check |
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* @return bool Whether any candidates were changed to elected |
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*/ |
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protected function checkCandidates(array $candidates): bool |
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{ |
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$this->logger->info('Checking if candidates have passed quota'); |
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foreach ($candidates as $i => $candidate) |
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{ |
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$this->logger->debug('Checking candidate', ['candidate' => $candidate]); |
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if ($candidate->getVotes() >= $this->quota) |
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{ |
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$this->electCandidate($candidate); |
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$elected = true; |
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} |
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} |
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$this->logger->info("Candidate checking complete. Someone was elected: $elected"); |
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return ($elected ?? false); |
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} |
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/** |
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* Allocate the next votes from a Ballot |
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* |
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* @param Ballot $ballot The ballot to allocate the votes from |
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* @param float $multiplier Number to multiply the weight by (surplus) |
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* @param float $divisor The divisor of the weight (Total number of |
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* candidate votes) |
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* @return Ballot The same ballot passed in modified |
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*/ |
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protected function allocateVotes(Ballot $ballot, float $multiplier = 1.0, float $divisor = 1.0): Ballot |
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{ |
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$weight = $ballot->setWeight(($ballot->getWeight() * $multiplier) / $divisor); |
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$candidate = $ballot->getNextChoice(); |
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$this->logger->debug('Allocating votes of ballot', array( |
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'ballot' => $ballot, |
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'weight' => $weight, |
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'candidate' => $candidate |
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)); |
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if ($candidate !== null) |
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{ |
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$this->election->getCandidate($candidate)->addVotes($weight); |
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$ballot->incrementLevelUsed(); |
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$this->logger->debug('Vote added to candidate'); |
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} |
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return $ballot; |
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} |
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/** |
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* Transfer the votes from one candidate to other candidates |
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* |
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* @param float $surplus The number of surplus votes to transfer |
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* @param Candidate $candidate The candidate being elected to transfer |
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* the votes from |
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* @return |
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*/ |
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protected function transferSurplusVotes(float $surplus, Candidate $candidate) |
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{ |
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$totalVotes = $candidate->getVotes(); |
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$candidateId = $candidate->getId(); |
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$this->logger->info('Transfering surplus votes', array( |
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'surplus' => $surplus, |
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'candidate' => $candidate, |
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'totalVotes' => $totalVotes |
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)); |
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foreach ($this->ballots as $i => $ballot) |
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{ |
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if ($ballot->getLastChoice() == $candidateId) |
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{ |
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$this->allocateVotes($ballot, $surplus, $totalVotes); |
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} |
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} |
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return; |
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} |
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/** |
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* Transfer the votes from one eliminated candidate to other candidates |
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* |
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* @param Candidate $candidate Candidate being eliminated to transfer |
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* the votes from |
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* @return |
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*/ |
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protected function transferEliminatedVotes(Candidate $candidate) |
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{ |
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$candidateId = $candidate->getId(); |
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$this->logger->info('Transfering votes from eliminated candidate', array( |
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'votes' => $candidate->getVotes(), |
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'candidate' => $candidate |
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)); |
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foreach ($this->ballots as $i => $ballot) |
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{ |
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if ($ballot->getLastChoice() == $candidateId) |
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{ |
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$this->allocateVotes($ballot); |
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} |
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} |
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return; |
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} |
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/** |
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* Elect a candidate after they've passed the threshold |
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* |
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* @param \Michaelc\Voting\STV\Candidate $candidate |
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* @return null |
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*/ |
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protected function electCandidate(Candidate $candidate) |
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{ |
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if ($candidate->getVotes() < $this->quota) |
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{ |
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throw new VotingException("We shouldn't be electing someone who hasn't met the quota"); |
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} |
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$this->logger->notice('Electing a candidate', array( |
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'candidate' => $candidate |
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)); |
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$candidate->setState(Candidate::ELECTED); |
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$this->electedCandidates++; |
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if ($this->electedCandidates < $this->election->getWinnersCount()) |
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{ |
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$surplus = $candidate->getVotes() - $this->quota; |
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$this->transferSurplusVotes($surplus, $candidate); |
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} |
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return; |
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} |
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/** |
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* Eliminate the candidate with the lowest number of votes |
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* and reallocated their votes |
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* |
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* TODO: Eliminate all lowest candidates after step one, then |
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* randomly choose. |
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* |
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* @param \Michaelc\Voting\STV\Candidate[] $candidates |
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* Array of active candidates |
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* @return int Number of candidates eliminated |
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*/ |
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protected function eliminateCandidates(array $candidates): int |
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{ |
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$minimumCandidates = $this->getLowestCandidates($candidates); |
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foreach ($minimumCandidates as $minimumCandidate) |
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{ |
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$this->logger->notice('Eliminating a candidate', array( |
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'candidate' => $candidate |
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)); |
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$this->transferEliminatedVotes($minimumCandidate); |
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$minimumCandidate->setState(Candidate::DEFEATED); |
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} |
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return count($minimumCandidates); |
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} |
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/** |
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* Get candidates with the lowest number of votes |
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* |
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* @param \Michaelc\Voting\STV\Candidate[] $candidates |
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* Array of active candidates |
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* @return \Michaelc\Voting\STV\Candidate[] |
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* Candidates with lowest score |
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*/ |
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public function getLowestCandidates(array $candidates): array |
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{ |
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$minimum = 0; |
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$minimumCandidates = []; |
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foreach ($candidates as $i => $candidate) |
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{ |
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if ($candidate->getVotes() > $minimum) |
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{ |
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$minimum = $candidate->getVotes(); |
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unset($minimumCandidates); |
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$minimumCandidates[] = $candidate; |
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} |
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elseif ($candidate->getVotes() == $minimum) |
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{ |
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$minimumCandidates[] = $candidate; |
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} |
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} |
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$this->logger->info('Calculated lowest candidates', array( |
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'minimumCandidates' => $minimumCandidates |
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)); |
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return $minimumCandidates; |
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} |
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/** |
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* Reject any invalid ballots |
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* |
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* @return int Number of rejected ballots |
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*/ |
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protected function rejectInvalidBallots(): int |
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{ |
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foreach ($this->ballots as $i => $ballot) |
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{ |
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if (!$this->checkBallotValidity($ballot)) |
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{ |
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$this->rejectedBallots[] = clone $ballot; |
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unset($this->ballots[$i]); |
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} |
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} |
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$count = count($this->rejectedBallots); |
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$this->logger->notice('Found $count rejected ballots ', array( |
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'ballots' => $this->rejectedBallots, |
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)); |
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return count($this->rejectedBallots); |
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} |
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/** |
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* Check if ballot is valid |
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* |
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* @param Ballot $Ballot Ballot to test |
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* @return bool True if valid, false if invalid |
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*/ |
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public function checkBallotValidity(Ballot $ballot): bool |
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{ |
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if (count($ballot->getRanking()) > $this->election->getCandidateCount()) |
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{ |
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$this->logger->debug('Invalid ballot - number of candidates', array( |
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'ballot' => $ballot |
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)); |
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return false; |
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} |
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else |
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{ |
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$candidateIds = $this->election->getCandidateIds(); |
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foreach ($ballot->getRanking() as $i => $candidate) |
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{ |
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if (!in_array($candidate, $candidateIds)) |
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{ |
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$this->logger->debug('Invalid ballot - invalid candidate', array( |
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'ballot' => $ballot, |
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'candidate' => $candidate, |
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'candidates' => $candidateIds |
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)); |
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return false; |
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} |
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} |
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} |
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$this->logger->debug('No invalid ballots found'); |
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return true; |
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} |
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/** |
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* Get the quota to win |
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* |
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* @return int |
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*/ |
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public function getQuota(): int |
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{ |
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$quota = floor( |
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|
|
($this->election->getNumBallots() / |
396
|
|
|
($this->election->getWinnersCount() + 1) |
397
|
|
|
) |
398
|
|
|
+ 1); |
399
|
|
|
|
400
|
|
|
$this->logger->info("Quota set: $quota"); |
401
|
|
|
|
402
|
|
|
return $quota; |
403
|
|
|
} |
404
|
|
|
} |
405
|
|
|
|
Our type inference engine has found an assignment to a property that is incompatible with the declared type of that property.
Either this assignment is in error or the assigned type should be added to the documentation/type hint for that property..