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PlantUML is a component that allows to quickly write diagrams from text.
There is a newer version: 1.2025.0
// THIS FILE HAS BEEN GENERATED BY A PREPROCESSOR.
package net.sourceforge.plantuml.quantization;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

/**
 * Implements median cut quantization.
 *
 * 
 * The algorithm works as follows:
 *
 * 

 * Begin with one cluster containing all the original colors.
 * Find the cluster containing the greatest spread along a single color
 * component (red, green or blue).
 * Find the median of that color component among colors in the cluster.
 * Split the cluster into two halves, using that median as a threshold.
 * Repeat this process until the desired number of clusters is reached.
 * 
 */
public final class MedianCutQuantizer implements ColorQuantizer {
	public static final MedianCutQuantizer INSTANCE = new MedianCutQuantizer();

	private MedianCutQuantizer() {
	}

	@Override
	public Set quantize(Multiset originalColors, int maxColorCount) {
		TreeSet clusters = new TreeSet<>(new ClusterSpreadComparator());
		clusters.add(new Cluster(originalColors));

		while (clusters.size() < maxColorCount) {
			Cluster clusterWithLargestSpread = clusters.pollFirst();
			clusters.addAll(clusterWithLargestSpread.split());
		}

		Set clusterCentroids = new HashSet<>();
		for (Cluster cluster : clusters) {
			clusterCentroids.add(QColor.getCentroid(cluster.colors));
		}
		return clusterCentroids;
	}

	private static final class Cluster {
		final Multiset colors;
		double largestSpread;
		int componentWithLargestSpread;

		Cluster(Multiset colors) {
			this.colors = colors;
			this.largestSpread = -1;
			for (int component = 0; component < 3; ++component) {
				double componentSpread = getComponentSpread(component);
				if (componentSpread > largestSpread) {
					largestSpread = componentSpread;
					componentWithLargestSpread = component;
				}
			}
		}

		double getComponentSpread(int component) {
			double min = Double.POSITIVE_INFINITY;
			double max = Double.NEGATIVE_INFINITY;
			for (QColor color : colors) {
				min = Math.min(min, color.getComponent(component));
				max = Math.max(max, color.getComponent(component));
			}
			return max - min;
		}

		Collection split() {
			List orderedColors = new ArrayList<>(colors);
			Collections.sort(orderedColors, new ColorComponentComparator(componentWithLargestSpread));
			int medianIndex = orderedColors.size() / 2;
			return Arrays.asList(new Cluster(new HashMultiset<>(orderedColors.subList(0, medianIndex))),
					new Cluster(new HashMultiset<>(orderedColors.subList(medianIndex, orderedColors.size()))));
		}
	}

	/**
	 * Orders clusters according to their maximum spread, in descending order.
	 */
	static final class ClusterSpreadComparator implements Comparator {
		@Override
		public int compare(Cluster a, Cluster b) {
			double spreadDifference = b.largestSpread - a.largestSpread;
			if (spreadDifference == 0) {
				return ArbitraryComparator.INSTANCE.compare(a, b);
			}
			return (int) Math.signum(spreadDifference);
		}
	}

	/**
	 * Orders colors according to the value of one particular component, in
	 * ascending order.
	 */
	static final class ColorComponentComparator implements Comparator {
		final int component;

		ColorComponentComparator(int component) {
			this.component = component;
		}

		@Override
		public int compare(QColor a, QColor b) {
			double componentDifference = a.getComponent(component) - b.getComponent(component);
			if (componentDifference == 0) {
				return ArbitraryComparator.INSTANCE.compare(a, b);
			}
			return (int) Math.signum(componentDifference);
		}
	}
}