/*******************************************************************************
* Copyright 2011 See AUTHORS file.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
******************************************************************************/
package com.badlogic.gdx.math;
import com.badlogic.gdx.utils.FloatArray;
import com.badlogic.gdx.utils.IntArray;
/** Computes the convex hull of a set of points using the monotone chain convex hull algorithm (aka Andrew's algorithm).
* @author Nathan Sweet */
public class ConvexHull {
private final FloatArray hull = new FloatArray();
private final IntArray quicksortStack = new IntArray();
private float[] sortedPoints;
/** @see #computePolygon(float[], int, int, boolean) */
public FloatArray computePolygon (FloatArray points, boolean sorted) {
return computePolygon(points.items, 0, points.size, sorted);
}
/** @see #computePolygon(float[], int, int, boolean) */
public FloatArray computePolygon (float[] polygon, boolean sorted) {
return computePolygon(polygon, 0, polygon.length, sorted);
}
/** Returns a list of points on the convex hull in counter-clockwise order. Note: the last point in the returned list is the
* same as the first one. */
/** Returns the convex hull polygon for the given point cloud.
* @param points x,y pairs describing points. Duplicate points will result in undefined behavior.
* @param sorted If false, the points will be sorted by the x coordinate then the y coordinate, which is required by the convex
* hull algorithm. If sorting is done the input array is not modified and count additional working memory is needed.
* @return pairs of coordinates that describe the convex hull polygon in counterclockwise order. Note the returned array is
* reused for later calls to the same method. */
public FloatArray computePolygon (float[] points, int offset, int count, boolean sorted) {
int end = offset + count;
if (!sorted) {
if (sortedPoints == null || sortedPoints.length < count) sortedPoints = new float[count];
System.arraycopy(points, offset, sortedPoints, 0, count);
points = sortedPoints;
offset = 0;
sort(points, count);
}
// Lower hull.
FloatArray hull = this.hull;
hull.clear();
for (int i = offset; i < end; i += 2) {
float x = points[i];
float y = points[i + 1];
while (hull.size >= 4 && ccw(x, y) <= 0)
hull.size -= 2;
hull.add(x);
hull.add(y);
}
// Upper hull.
for (int i = end - 4, t = hull.size + 2; i >= offset; i -= 2) {
float x = points[i];
float y = points[i + 1];
while (hull.size >= t && ccw(x, y) <= 0)
hull.size -= 2;
hull.add(x);
hull.add(y);
}
return hull;
}
/** Returns > 0 if the points are a counterclockwise turn, < 0 if clockwise, and 0 if colinear. */
private float ccw (float p3x, float p3y) {
FloatArray hull = this.hull;
int size = hull.size;
float p1x = hull.get(size - 4);
float p1y = hull.get(size - 3);
float p2x = hull.get(size - 2);
float p2y = hull.peek();
return (p2x - p1x) * (p3y - p1y) - (p2y - p1y) * (p3x - p1x);
}
/** Sorts x,y pairs of values by the x value, then the y value.
* @param count Number of indices, must be even. */
private void sort (float[] values, int count) {
int lower = 0;
int upper = count - 1;
IntArray stack = quicksortStack;
stack.add(lower);
stack.add(upper - 1);
while (stack.size > 0) {
upper = stack.pop();
lower = stack.pop();
if (upper <= lower) continue;
int i = quicksortPartition(values, lower, upper);
if (i - lower > upper - i) {
stack.add(lower);
stack.add(i - 2);
}
stack.add(i + 2);
stack.add(upper);
if (upper - i >= i - lower) {
stack.add(lower);
stack.add(i - 2);
}
}
}
private int quicksortPartition (final float[] values, int lower, int upper) {
float x = values[lower];
float y = values[lower + 1];
int up = upper;
int down = lower;
float temp;
while (down < up) {
while (down < up && values[down] <= x)
down = down + 2;
while (values[up] > x || (values[up] == x && values[up + 1] > y))
up = up - 2;
if (down < up) {
temp = values[down];
values[down] = values[up];
values[up] = temp;
temp = values[down + 1];
values[down + 1] = values[up + 1];
values[up + 1] = temp;
}
}
values[lower] = values[up];
values[up] = x;
values[lower + 1] = values[up + 1];
values[up + 1] = y;
return up;
}
}