Added Jarvis Algorithm to compute convex hull

geometry/jarvis_algorithm.cpp

@@ -0,0 +1,179 @@
+/**
+ * @file
+ * @brief Implementation of [Jarvis’s](https://en.wikipedia.org/wiki/Gift_wrapping_algorithm) algorithm.
+ *
+ * @details
+ * Given a set of points in the plane. the convex hull of the set
+ * is the smallest convex polygon that contains all the points of it.
+ *
+ * ### Algorithm
+ * The idea of Jarvis’s Algorithm is simple, we start from the leftmost point
+ * (or point with minimum x coordinate value) and we
+ * keep wrapping points in counterclockwise direction.
+ *
+ * The idea is to use orientation() here. Next point is selected as the
+ * point that beats all other points at counterclockwise orientation, i.e.,
+ * next point is q if for any other point r,
+ * we have “orientation(p, q, r) = counterclockwise”.
+ *
+ * For Example,
+ * If points = {{0, 3}, {2, 2}, {1, 1}, {2, 1},
+                      {3, 0}, {0, 0}, {3, 3}};
+ *
+ * then the convex hull is
+ * (0, 3), (0, 0), (3, 0), (3, 3)
+ *
+ * @author [Rishabh Agarwal](https://github.com/rishabh-997)
+ */
+
+#include <vector>
+#include <cassert>
+#include <iostream>
+
+/**
+ *  @namespace geometry
+ *  @brief Geometry algorithms
+ */
+namespace geometry {
+    /**
+     * @namespace jarvis
+     * @brief Functions for [Jarvis’s](https://en.wikipedia.org/wiki/Gift_wrapping_algorithm) algorithm
+     */
+    namespace jarvis {
+        /**
+         * Structure defining the x and y co-ordinates of the given
+         * point in space
+         */
+        struct Point {
+            int x, y;
+        };
+
+        /**
+         * Class which can be called from main and is globally available
+         * throughout the code
+         */
+        class Convexhull {
+            std::vector<Point> points;
+            int size;
+
+        public:
+            /**
+             * Constructor of given class
+             *
+             * @param pointList list of all points in the space
+             * @param n number of points in space
+             */
+            explicit Convexhull(const std::vector<Point> &pointList) {
+                points = pointList;
+                size = points.size();
+            }
+
+            /**
+             * Creates convex hull of a set of n points.
+             * There must be 3 points at least for the convex hull to exist
+             *
+             * @returns an vector array containing points in space
+             * which enclose all given points thus forming a hull
+             */
+            std::vector<Point> getConvexHull() const {
+                // Initialize Result
+                std::vector<Point> hull;
+
+                // Find the leftmost point
+                int leftmost_point = 0;
+                for (int i = 1; i < size; i++) {
+                    if (points[i].x < points[leftmost_point].x) {
+                        leftmost_point = i;
+                    }
+                }
+                // Start from leftmost point, keep moving counterclockwise
+                // until reach the start point again.  This loop runs O(h)
+                // times where h is number of points in result or output.
+                int p = leftmost_point, q = 0;
+                do {
+                    // Add current point to result
+                    hull.push_back(points[p]);
+
+                    // Search for a point 'q' such that orientation(p, x, q)
+                    // is counterclockwise for all points 'x'. The idea
+                    // is to keep track of last visited most counter clock-
+                    // wise point in q. If any point 'i' is more counter clock-
+                    // wise than q, then update q.
+                    q = (p + 1) % size;
+                    for (int i = 0; i < size; i++) {
+                        // If i is more counterclockwise than current q, then
+                        // update q
+                        if (orientation(points[p], points[i], points[q]) == 2) {
+                            q = i;
+                        }
+                    }
+
+                    // Now q is the most counterclockwise with respect to p
+                    // Set p as q for next iteration, so that q is added to
+                    // result 'hull'
+                    p = q;
+
+                } while (p != leftmost_point);        // While we don't come to first point
+
+                return hull;
+            }
+
+            /**
+             * This function returns the geometric orientation for the three points
+             * in a space, ie, whether they are linear ir clockwise or
+             * anti-clockwise
+             * @param p first point selected
+             * @param q adjacent point for q
+             * @param r adjacent point for q
+             *
+             * @returns 0 -> Linear
+             * @returns 1 -> Clock Wise
+             * @returns 2 -> Anti Clock Wise
+             */
+            static int orientation(const Point &p, const Point &q, const Point &r) {
+                int val = (q.y - p.y) * (r.x - q.x) - (q.x - p.x) * (r.y - q.y);
+
+                if (val == 0) {
+                    return 0;
+                }
+                return (val > 0) ? 1 : 2;
+            }
+
+        };
+
+    } // namespace jarvis
+} // namespace geometry
+
+/**
+ * Test function
+ * @returns void
+ */
+static void test() {
+    std::vector<geometry::jarvis::Point> points = {{0, 3},
+                                                   {2, 2},
+                                                   {1, 1},
+                                                   {2, 1},
+                                                   {3, 0},
+                                                   {0, 0},
+                                                   {3, 3}
+    };
+    geometry::jarvis::Convexhull hull(points);
+    std::vector<geometry::jarvis::Point> actualPoint;
+    actualPoint = hull.getConvexHull();
+
+    std::vector<geometry::jarvis::Point> expectedPoint = {{0, 3},
+                                                          {0, 0},
+                                                          {3, 0},
+                                                          {3, 3}};
+    for (int i = 0; i < expectedPoint.size(); i++) {
+        assert(actualPoint[i].x == expectedPoint[i].x);
+        assert(actualPoint[i].y == expectedPoint[i].y);
+    }
+    std::cout << "Test implementations passed!\n";
+}
+
+/** Driver Code */
+int main() {
+    test();
+    return 0;
+}