org.bbottema.javareflection.util.graph.Dijkstra Maven / Gradle / Ivy

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package org.bbottema.javareflection.util.graph;

import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map.Entry;
import java.util.Set;

@SuppressWarnings("unused")
final class Dijkstra {
	
	private Dijkstra() {
	}
	
	@SuppressWarnings("WeakerAccess")
	public static  void findShortestPathToAllOtherNodes(Node startingPoint) {
		startingPoint.setCost(0);
		
		Set> settledNodes = new HashSet<>();
		Set> unsettledNodes = new HashSet<>();
		unsettledNodes.add(startingPoint);
		
		while (unsettledNodes.size() != 0) {
			Node currentNode = getLowestDistanceNode(unsettledNodes);
			unsettledNodes.remove(currentNode);
			for (Entry, Integer> adjacencyPair : currentNode.getToNodes().entrySet()) {
				Node adjacentNode = adjacencyPair.getKey();
				
				if (!settledNodes.contains(adjacentNode)) {
					Integer edgeWeight = adjacencyPair.getValue();
					calculateMinimumDistance(adjacentNode, edgeWeight, currentNode);
					unsettledNodes.add(adjacentNode);
				}
			}
			settledNodes.add(currentNode);
		}
	}
	
	private static  void calculateMinimumDistance(Node evaluationNode, Integer edgeWeigh, Node sourceNode) {
		if (sourceNode.getCost() + edgeWeigh < evaluationNode.getCost()) {
			evaluationNode.setCost(sourceNode.getCost() + edgeWeigh);
			LinkedList> shortestPath = new LinkedList<>(sourceNode.getLeastExpensivePath());
			shortestPath.add(sourceNode);
			evaluationNode.setLeastExpensivePath(shortestPath);
		}
	}
	
	private static  Node getLowestDistanceNode(Set> unsettledNodes) {
		Node lowestDistanceNode = null;
		int lowestDistance = Integer.MAX_VALUE;
		for (Node node : unsettledNodes) {
			if (node.getCost() < lowestDistance) {
				lowestDistance = node.getCost();
				lowestDistanceNode = node;
			}
		}
		return lowestDistanceNode;
	}
}