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Update GrahamScan.java (#5310)

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* Update GrahamScan.java

improved the Javadoc comments, clarified some methods in the Point class, and corrected some text.

* Minor adjustment to GrahamScan.java

* revised GrahamScan.java

* Update-2 GrahamScan.java

* clang format GrahamScan.java

* reverted GrahamScan.java

* minor updates.java

* minor updates

* Spc.java

* clang format

---------

Co-authored-by: Alex Klymenko <alexanderklmn@gmail.com>
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Bayram Turgut 2024-08-09 15:03:54 +03:00 committed by GitHub
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1 changed files with 54 additions and 73 deletions
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src/main/java/com/thealgorithms/geometry/GrahamScan.java
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@ -1,56 +1,56 @@
package com.thealgorithms.geometry;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Stack;
/*
* A Java program that computes the convex hull using the Graham Scan algorithm
* In the best case, time complexity is O(n), while in the worst case, it is O(nlog(n)).
* O(n) space complexity
/**
* A Java program that computes the convex hull using the Graham Scan algorithm.
* The time complexity is O(n) in the best case and O(n log(n)) in the worst case.
* The space complexity is O(n).
* This algorithm is applicable only to integral coordinates.
*
* This algorithm is only applicable to integral coordinates.
*
* Reference:
* References:
* https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/geometry/graham_scan_algorithm.cpp
* https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/geometry/graham_scan_functions.hpp
* https://algs4.cs.princeton.edu/99hull/GrahamScan.java.html
*/
public class GrahamScan {
private final Stack<Point> hull = new Stack<>();
public GrahamScan(Point[] points) {
/*
* pre-process the points by sorting them with respect to the bottom-most point, then we'll
* push the first point in the array to be our first extreme point.
*/
// Pre-process points: sort by y-coordinate, then by polar order with respect to the first point
Arrays.sort(points);
Arrays.sort(points, 1, points.length, points[0].polarOrder());
hull.push(points[0]);
// find index of first point not equal to a[0] (indexPoint1) and the first point that's not
// collinear with either (indexPoint2).
int indexPoint1;
for (indexPoint1 = 1; indexPoint1 < points.length; indexPoint1++) {
if (!points[0].equals(points[indexPoint1])) {
// Find the first point not equal to points[0] (firstNonEqualIndex)
// and the first point not collinear firstNonCollinearIndex with the previous points
int firstNonEqualIndex;
for (firstNonEqualIndex = 1; firstNonEqualIndex < points.length; firstNonEqualIndex++) {
if (!points[0].equals(points[firstNonEqualIndex])) {
break;
}
}
if (indexPoint1 == points.length) {
if (firstNonEqualIndex == points.length) {
return;
}
int indexPoint2;
for (indexPoint2 = indexPoint1 + 1; indexPoint2 < points.length; indexPoint2++) {
if (Point.orientation(points[0], points[indexPoint1], points[indexPoint2]) != 0) {
int firstNonCollinearIndex;
for (firstNonCollinearIndex = firstNonEqualIndex + 1; firstNonCollinearIndex < points.length; firstNonCollinearIndex++) {
if (Point.orientation(points[0], points[firstNonEqualIndex], points[firstNonCollinearIndex]) != 0) {
break;
}
}
hull.push(points[indexPoint2 - 1]);
// Now we simply add the point to the stack based on the orientation.
for (int i = indexPoint2; i < points.length; i++) {
hull.push(points[firstNonCollinearIndex - 1]);
// Process the remaining points and update the hull
for (int i = firstNonCollinearIndex; i < points.length; i++) {
Point top = hull.pop();
while (Point.orientation(hull.peek(), top, points[i]) <= 0) {
top = hull.pop();
@ -61,14 +61,10 @@ public class GrahamScan {
}
/**
* @return A stack of points representing the convex hull.
* @return An iterable collection of points representing the convex hull.
*/
public Iterable<Point> hull() {
Stack<Point> s = new Stack<>();
for (Point p : hull) {
s.push(p);
}
return s;
return new ArrayList<>(hull);
}
public record Point(int x, int y) implements Comparable<Point> {
@ -98,47 +94,41 @@ public class GrahamScan {
}
/**
* Finds the orientation of ordered triplet.
* Determines the orientation of the triplet (a, b, c).
*
* @param a Co-ordinates of point a <int, int>
* @param b Co-ordinates of point a <int, int>
* @param c Co-ordinates of point a <int, int>
* @return { -1, 0, +1 } if a -→ b -→ c is a { clockwise, collinear; counterclockwise }
* turn.
* @param a The first point
* @param b The second point
* @param c The third point
* @return -1 if (a, b, c) is clockwise, 0 if collinear, +1 if counterclockwise
*/
public static int orientation(Point a, Point b, Point c) {
int val = (b.x - a.x) * (c.y - a.y) - (b.y - a.y) * (c.x - a.x);
if (val == 0) {
return 0;
}
return (val > 0) ? +1 : -1;
return Integer.compare(val, 0);
}
/**
* @param p2 Co-ordinate of point to compare to.
* This function will compare the points and will return a positive integer if the
* point is greater than the argument point and a negative integer if the point is
* less than the argument point.
*/
public int compareTo(Point p2) {
int res = Integer.compare(this.y, p2.y);
if (res == 0) {
res = Integer.compare(this.x, p2.x);
}
return res;
}
/**
* A helper function that will let us sort points by their polar order
* This function will compare the angle between 2 polar Co-ordinates
* Compares this point with another point.
*
* @return the comparator
* @param p2 The point to compare to
* @return A positive integer if this point is greater, a negative integer if less, or 0 if equal
*/
@Override
public int compareTo(Point p2) {
int cmpY = Integer.compare(this.y, p2.y);
return cmpY != 0 ? cmpY : Integer.compare(this.x, p2.x);
}
/**
* Returns a comparator to sort points by their polar order relative to this point.
*
* @return A polar order comparator
*/
public Comparator<Point> polarOrder() {
return new PolarOrder();
}
private final class PolarOrder implements Comparator<Point> {
@Override
public int compare(Point p1, Point p2) {
int dx1 = p1.x - x;
int dy1 = p1.y - y;
@ -146,32 +136,23 @@ public class GrahamScan {
int dy2 = p2.y - y;
if (dy1 >= 0 && dy2 < 0) {
return -1; // q1 above; q2 below
return -1; // p1 above p2
} else if (dy2 >= 0 && dy1 < 0) {
return +1; // q1 below; q2 above
} else if (dy1 == 0 && dy2 == 0) { // 3-collinear and horizontal
if (dx1 >= 0 && dx2 < 0) {
return -1;
} else if (dx2 >= 0 && dx1 < 0) {
return +1;
} else {
return 0;
}
return 1; // p1 below p2
} else if (dy1 == 0 && dy2 == 0) { // Collinear and horizontal
return Integer.compare(dx2, dx1);
} else {
return -orientation(Point.this, p1, p2); // both above or below
return -orientation(Point.this, p1, p2); // Compare orientation
}
}
}
/**
* Override of the toString method, necessary to compute the difference
* between the expected result and the derived result
*
* @return a string representation of any given 2D point in the format (x, y)
* @return A string representation of this point in the format (x, y)
*/
@Override
public String toString() {
return "(" + x + ", " + y + ")";
return String.format("(%d, %d)", x, y);
}
}
}