feat: add Jarvis-March algorithm

Author: saahil-mahato

src/main/java/com/thealgorithms/geometry/JarvisMarch.java

@@ -0,0 +1,136 @@
+package com.thealgorithms.geometry;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Objects;
+
+/**
+ * This class implements the Jarvis March algorithm (also known as the Gift Wrapping algorithm)
+ * for computing the convex hull of a set of points in a 2D plane.
+ * The convex hull is the smallest convex polygon that can enclose all given points.
+ */
+public final class JarvisMarch {
+
+    private JarvisMarch() {
+        // Private constructor to prevent instantiation
+    }
+
+    /**
+     * Represents a point in 2D space with x and y coordinates.
+     */
+    static class Point {
+        private double x;
+        private double y;
+
+        /**
+         * Constructs a Point with specified x and y coordinates.
+         *
+         * @param x the x-coordinate of the point
+         * @param y the y-coordinate of the point
+         */
+        public Point(double x, double y) {
+            this.x = x;
+            this.y = y;
+        }
+
+        public double getX() {
+            return x;
+        }
+
+        public double getY() {
+            return y;
+        }
+
+        @Override
+        public boolean equals(Object obj) {
+            if (this == obj) return true; // Check if both references point to the same object
+            if (!(obj instanceof Point)) return false; // Check if obj is an instance of Point
+            Point other = (Point) obj;
+            // Compare x and y coordinates for equality
+            return Double.compare(x, other.x) == 0 && Double.compare(y, other.y) == 0;
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(x, y); // Generate hash code based on x and y coordinates
+        }
+    }
+
+    /**
+     * Computes the convex hull of a given list of points using the Jarvis March algorithm.
+     *
+     * @param points a list of Points for which to compute the convex hull
+     * @return a list of Points representing the vertices of the convex hull in counter-clockwise order
+     */
+    public static List<Point> jarvisMarch(List<Point> points) {
+        List<Point> hull = new ArrayList<>();
+
+        // If there are less than 3 points, a convex hull cannot be formed
+        if (points.size() < 3) return hull;
+
+        // Find the leftmost point (with the smallest x-coordinate)
+        Point leftmost = points.get(0);
+        for (Point p : points) {
+            if (p.getX() < leftmost.getX()) {
+                leftmost = p; // Update leftmost point if a new leftmost point is found
+            }
+        }
+
+        Point current = leftmost; // Start from the leftmost point
+
+        do {
+            hull.add(current); // Add current point to the hull
+
+            Point nextTarget = points.get(0); // Initialize next target as first point in list
+
+            for (Point candidate : points) {
+                if (candidate.equals(current)) continue; // Skip current point
+
+                // Check if candidate makes a left turn or is collinear and farther than nextTarget
+                if ((nextTarget.equals(current) || isLeftTurn(current, nextTarget, candidate)) || (isCollinear(current, nextTarget, candidate) && distance(current, candidate) > distance(current, nextTarget))) {
+                    nextTarget = candidate; // Update next target if conditions are met
+                }
+            }
+
+            current = nextTarget; // Move to the next target point
+
+        } while (!current.equals(leftmost)); // Continue until we loop back to the starting point
+
+        return hull; // Return the computed convex hull
+    }
+
+    /**
+     * Determines whether moving from point A to point B to point C makes a left turn.
+     *
+     * @param a the starting point
+     * @param b the second point
+     * @param c the third point
+     * @return true if it makes a left turn, false otherwise
+     */
+    private static boolean isLeftTurn(Point a, Point b, Point c) {
+        return (b.getX() - a.getX()) * (c.getY() - a.getY()) - (b.getY() - a.getY()) * (c.getX() - a.getX()) > 0;
+    }
+
+    /**
+     * Checks whether three points A, B, and C are collinear.
+     *
+     * @param a the first point
+     * @param b the second point
+     * @param c the third point
+     * @return true if points are collinear, false otherwise
+     */
+    private static boolean isCollinear(Point a, Point b, Point c) {
+        return (b.getX() - a.getX()) * (c.getY() - a.getY()) == (b.getY() - a.getY()) * (c.getX() - a.getX());
+    }
+
+    /**
+     * Calculates the Euclidean distance between two points A and B.
+     *
+     * @param a the first point
+     * @param b the second point
+     * @return the distance between points A and B
+     */
+    private static double distance(Point a, Point b) {
+        return Math.sqrt(Math.pow(b.getX() - a.getX(), 2) + Math.pow(b.getY() - a.getY(), 2));
+    }
+}

src/test/java/com/thealgorithms/geometry/JarvisMarchTest.java

@@ -0,0 +1,71 @@
+package com.thealgorithms.geometry;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+/**
+ * Unit tests for the {@link JarvisMarch} class, which implements the Jarvis March algorithm
+ * for computing the convex hull of a set of points.
+ */
+class JarvisMarchTest {
+
+    /**
+     * Tests the equals method of the Point class with an object of a different type.
+     * It verifies that a Point instance is not equal to a non-Point object.
+     */
+    @Test
+    void testEqualsMethodWithDifferentType() {
+        JarvisMarch.Point pointA = new JarvisMarch.Point(1, 1);
+        String notAPoint = "I am not a Point";
+
+        // Assert that pointA is not equal to a String object
+        assertNotEquals(pointA, notAPoint);
+    }
+
+    /**
+     * Provides test cases for the convex hull computation.
+     * Each case consists of a list of input points and the expected convex hull result.
+     *
+     * @return a stream of arguments containing input points and expected hull points
+     */
+    private static Stream<Arguments> providePointsForConvexHull() {
+        return Stream.of(
+            // Test case 1: Simple triangle
+            Arguments.of(Arrays.asList(new JarvisMarch.Point(0, 0), new JarvisMarch.Point(1, 1), new JarvisMarch.Point(1, 0)), Arrays.asList(new JarvisMarch.Point(0, 0), new JarvisMarch.Point(1, 1), new JarvisMarch.Point(1, 0))),
+            // Test case 2: Points with one point inside the hull
+            Arguments.of(Arrays.asList(new JarvisMarch.Point(1, 1), new JarvisMarch.Point(0, 0), new JarvisMarch.Point(2, 2), new JarvisMarch.Point(3, 1), new JarvisMarch.Point(2, 0)),
+                Arrays.asList(new JarvisMarch.Point(0, 0), new JarvisMarch.Point(2, 2), new JarvisMarch.Point(3, 1), new JarvisMarch.Point(2, 0))),
+            // Test case 3: Single point (no hull)
+            Arguments.of(Arrays.asList(new JarvisMarch.Point(0, 0)), Arrays.asList()));
+    }
+
+    /**
+     * Parameterized test for the jarvisMarch method.
+     * It checks if the actual convex hull computed matches the expected hull
+     * for various sets of input points.
+     *
+     * @param inputPoints a list of points to compute the convex hull from
+     * @param expectedHull a list of expected points forming the convex hull
+     */
+    @ParameterizedTest
+    @MethodSource("providePointsForConvexHull")
+    void testConvexHull(List<JarvisMarch.Point> inputPoints, List<JarvisMarch.Point> expectedHull) {
+        List<JarvisMarch.Point> actualHull = JarvisMarch.jarvisMarch(inputPoints);
+
+        // Assert that the size of actual hull matches the expected hull size
+        assertEquals(expectedHull.size(), actualHull.size());
+
+        // Assert that each point in the expected hull matches the corresponding point in the actual hull
+        for (int i = 0; i < expectedHull.size(); i++) {
+            assertEquals(expectedHull.get(i), actualHull.get(i));
+        }
+    }
+}