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<?php declare(strict_types=1); |
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namespace Stratadox\Pathfinder; |
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use function count; |
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use const INF; |
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use Stratadox\Pathfinder\Reconstruction\PathRetracer; |
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final class BellmanFordPathfinder implements MultiPathfinder |
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{ |
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private $network; |
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private $path; |
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private function __construct(Network $network) |
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{ |
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$this->network = $network; |
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$this->path = new PathRetracer(); |
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} |
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public static function operatingIn(Network $graph): MultiPathfinder |
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{ |
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return new self($graph); |
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} |
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public function from(string $start): array |
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{ |
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return $this->shortestPaths($this->startingAt($start), $start); |
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} |
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private function shortestPaths(array $lastSteps, string $start): array |
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{ |
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$paths = []; |
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foreach ($lastSteps as $goal => $lastStep) { |
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if ($goal !== $start) { |
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$paths[$goal] = $this->path->retrace($start, $goal, $lastSteps); |
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} |
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} |
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return $paths; |
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} |
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/** @throws NoPathAvailable */ |
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private function startingAt(string $start): array |
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{ |
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if (!$this->network->has($start)) { |
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throw NonExistingStart::tried($start); |
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} |
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$distanceTo = [$start => 0]; |
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$lastStepBefore = []; |
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/** @var string[] $nodes */ |
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$nodes = $this->network->all()->items(); |
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for ($i = count($nodes); $i > 0; --$i) { |
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foreach ($nodes as $from) { |
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foreach ($this->network->neighboursOf($from) as $to) { |
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$proposal = ($distanceTo[$from] ?? INF) + |
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$this->network->movementCostBetween($from, $to); |
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if ($proposal < ($distanceTo[$to] ?? INF)) { |
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$distanceTo[$to] = $proposal; |
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$lastStepBefore[$to] = $from; |
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} |
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} |
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} |
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} |
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foreach ($nodes as $from) { |
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foreach ($this->network->neighboursOf($from) as $to) { |
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$proposal = ($distanceTo[$from] ?? INF) + |
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$this->network->movementCostBetween($from, $to); |
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if ($proposal < ($distanceTo[$to] ?? INF)) { |
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throw NegativeCycleDetected::amongTheNodes($from, $to); |
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} |
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} |
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} |
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return $lastStepBefore; |
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} |
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} |
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Stratadox /
Pathfinder
