77 lines
2.3 KiB
PHP
77 lines
2.3 KiB
PHP
<?php
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namespace Graphp\Algorithms\MinimumSpanningTree;
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use Fhaculty\Graph\Edge\Base as Edge;
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use Fhaculty\Graph\Exception\UnexpectedValueException;
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use Fhaculty\Graph\Set\Edges;
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use Fhaculty\Graph\Vertex;
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use SplPriorityQueue;
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class Prim extends Base
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{
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/**
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* @var Vertex
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*/
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private $startVertex;
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public function __construct(Vertex $startVertex)
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{
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$this->startVertex = $startVertex;
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}
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/**
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* @return Edges
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*/
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public function getEdges()
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{
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// Initialize algorithm
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$edgeQueue = new SplPriorityQueue();
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$vertexCurrent = $this->startVertex;
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$markInserted = array();
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$returnEdges = array();
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// iterate n-1 times (per definition, resulting MST MUST have n-1 edges)
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for ($i = 0, $n = \count($this->startVertex->getGraph()->getVertices()) - 1; $i < $n; ++$i) {
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$markInserted[$vertexCurrent->getId()] = true;
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// get unvisited vertex of the edge and add edges from new vertex
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// Add all edges from $currentVertex to priority queue
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$this->addEdgesSorted($vertexCurrent->getEdges(), $edgeQueue);
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do {
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if ($edgeQueue->isEmpty()) {
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throw new UnexpectedValueException('Graph has more than one component');
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}
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// Get next cheapest edge
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$cheapestEdge = $edgeQueue->extract();
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assert($cheapestEdge instanceof Edge);
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// Check if edge is between unmarked and marked edge
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$vertices = $cheapestEdge->getVertices();
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$vertexA = $vertices->getVertexFirst();
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$vertexB = $vertices->getVertexLast();
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} while (!(isset($markInserted[$vertexA->getId()]) XOR isset($markInserted[$vertexB->getId()])));
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// Cheapest Edge found, add edge to returnGraph
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$returnEdges[] = $cheapestEdge;
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// set current vertex for next iteration in order to add its edges to queue
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if (isset($markInserted[$vertexA->getId()])) {
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$vertexCurrent = $vertexB;
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} else {
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$vertexCurrent = $vertexA;
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}
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}
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return new Edges($returnEdges);
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}
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protected function getGraph()
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{
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return $this->startVertex->getGraph();
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}
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}
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