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package de.lmu.ifi.dbs.elki.index.tree.metrical.mtreevariants.mktrees.mkcop;
/*
This file is part of ELKI:
Environment for Developing KDD-Applications Supported by Index-Structures
Copyright (C) 2011
Ludwig-Maximilians-Universität München
Lehr- und Forschungseinheit für Datenbanksysteme
ELKI Development Team
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU Affero General Public License for more details.
You should have received a copy of the GNU Affero General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
/**
* Holds the lower and upper hull for some values.
*
* @author Elke Achtert
*/
public class ConvexHull {
/**
* The lower hull.
*/
private int[] lowerHull;
/**
* The upper hull.
*/
private int[] upperHull;
/**
* Number of points in lower hull.
*/
private int l;
/**
* Number of points in upper hull.
*/
private int u;
/**
* Creates a new convex hull for the specified distances.
*
* @param x the x-values of the points for which the lower and upper hull
* should be computed
* @param y the y-values of the points for which the lower and upper hull
* should be computed
*/
public ConvexHull(double[] x, double[] y) {
if(x.length != y.length) {
throw new IllegalArgumentException("x and y have different lengths!");
}
lowerHull = new int[x.length];
upperHull = new int[x.length];
determineLowerAndUpperHull(x, y);
}
/**
* Returns the lower hull.
*
* @return the lower hull
*/
public int[] getLowerHull() {
return lowerHull;
}
/**
* Returns the upper hull.
*
* @return the upper hull
*/
public int[] getUpperHull() {
return upperHull;
}
/**
* Returns the number of points in lower hull
*
* @return the number of points in lower hull
*/
public int getNumberOfPointsInLowerHull() {
return l;
}
/**
* Returns the number of points in upper hull
*
* @return the number of points in upper hull
*/
public int getNumberOfPointsInUpperHull() {
return u;
}
/**
* Computes the lower and upper hull of the specified distances.
*
* @param x the x-values of the points for which the lower and upper hull
* should be computed
* @param y the y-values of the points for which the lower and upper hull
* should be computed
*/
private void determineLowerAndUpperHull(double[] x, double[] y) {
StringBuffer msg = new StringBuffer();
// first point is always in lowerHull and upperHull
lowerHull[0] = 0;
l = 1;
upperHull[0] = 0;
u = 1;
// Determine the convex hulls (using point stack)
for(int i = 1; i < x.length; i++) {
// lower hull
lowerHull[l] = i;
while(l >= 2 && (y[lowerHull[l]] - y[lowerHull[l - 1]]) / (x[lowerHull[l]] - x[lowerHull[l - 1]]) <= (y[lowerHull[l - 1]] - y[lowerHull[l - 2]]) / (x[lowerHull[l - 1]] - x[lowerHull[l - 2]])) {
// right curved
lowerHull[l - 1] = lowerHull[l];
this.l--;
}
this.l++;
// upper hull
upperHull[u] = i;
while(u >= 2 && (y[upperHull[u]] - y[upperHull[u - 1]]) / (x[upperHull[u]] - x[upperHull[u - 1]]) >= (y[upperHull[u - 1]] - y[upperHull[u - 2]]) / (x[upperHull[u - 1]] - x[upperHull[u - 2]])) {
// left curved
upperHull[u - 1] = upperHull[u];
u--;
}
u++;
}
msg.append("lower and upper hull\n");
for(int i = 0; i < i; i++) {
msg.append(" uhull ").append(i).append("=").append(upperHull[i]).append(" y=").append(y[upperHull[i]]).append("\n");
msg.append(" lhull ").append(i).append("=").append(lowerHull[i]).append(" y=").append(y[lowerHull[i]]).append("\n");
}
}
}
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