From f111af62b61ad008e43253dca3f8a62dac8d4e7d Mon Sep 17 00:00:00 2001
From: Dithi <heydithi88@gmail.com>
Date: Thu, 17 Apr 2025 19:23:33 +0530
Subject: [PATCH] Added RandomizedClosestPair code and test

---
 .../randomized/RandomizedClosestPair.java     | 108 ++++++++++++++++++
 .../randomized/RandomizedClosestPairTest.java |  45 ++++++++
 2 files changed, 153 insertions(+)
 create mode 100644 src/main/java/com/thealgorithms/randomized/RandomizedClosestPair.java
 create mode 100644 src/test/java/com/thealgorithms/randomized/RandomizedClosestPairTest.java

diff --git a/src/main/java/com/thealgorithms/randomized/RandomizedClosestPair.java b/src/main/java/com/thealgorithms/randomized/RandomizedClosestPair.java
new file mode 100644
index 000000000000..92b28be75223
--- /dev/null
+++ b/src/main/java/com/thealgorithms/randomized/RandomizedClosestPair.java
@@ -0,0 +1,108 @@
+package com.thealgorithms.randomized;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.Comparator;
+import java.util.List;
+import java.util.ArrayList;
+import java.util.Random;
+
+/**
+ * Randomized Closest Pair of Points Algorithm
+ *
+ * Use Case:
+ * - Efficiently finds the closest pair of points in a 2D plane.
+ * - Applicable in computational geometry, clustering, and graphics.
+ *
+ * Time Complexity:
+ * - Expected: O(n log n) using randomized divide and conquer
+ *
+ * @see <a href="https://en.wikipedia.org/wiki/Closest_pair_of_points_problem">Closest Pair of Points - Wikipedia</a>
+ */
+public final class RandomizedClosestPair {
+
+    // Prevent instantiation of utility class
+    private RandomizedClosestPair() {
+        throw new UnsupportedOperationException("Utility class");
+    }
+
+    public static class Point {
+        public final double x, y;
+        public Point(double x, double y) {
+            this.x = x;
+            this.y = y;
+        }
+    }
+
+    public static double findClosestPairDistance(Point[] points) {
+        List<Point> shuffled = new ArrayList<>(Arrays.asList(points));
+        Collections.shuffle(shuffled, new Random());
+
+        Point[] px = shuffled.toArray(new Point[0]);
+        Arrays.sort(px, Comparator.comparingDouble(p -> p.x));
+
+        Point[] py = px.clone();
+        Arrays.sort(py, Comparator.comparingDouble(p -> p.y));
+
+        return closestPair(px, py);
+    }
+
+    private static double closestPair(Point[] px, Point[] py) {
+        int n = px.length;
+        if (n <= 3) {
+            return bruteForce(px);
+        }
+
+        int mid = n / 2;
+        Point midPoint = px[mid];
+
+        Point[] Qx = Arrays.copyOfRange(px, 0, mid);
+        Point[] Rx = Arrays.copyOfRange(px, mid, n);
+
+        List<Point> Qy = new ArrayList<>();
+        List<Point> Ry = new ArrayList<>();
+        for (Point p : py) {
+            if (p.x <= midPoint.x) Qy.add(p);
+            else Ry.add(p);
+        }
+
+        double d1 = closestPair(Qx, Qy.toArray(new Point[0]));
+        double d2 = closestPair(Rx, Ry.toArray(new Point[0]));
+
+        double d = Math.min(d1, d2);
+
+        List<Point> strip = new ArrayList<>();
+        for (Point p : py) {
+            if (Math.abs(p.x - midPoint.x) < d) {
+                strip.add(p);
+            }
+        }
+
+        return Math.min(d, stripClosest(strip, d));
+    }
+
+    private static double bruteForce(Point[] points) {
+        double min = Double.POSITIVE_INFINITY;
+        for (int i = 0; i < points.length; i++) {
+            for (int j = i + 1; j < points.length; j++) {
+                min = Math.min(min, distance(points[i], points[j]));
+            }
+        }
+        return min;
+    }
+
+    private static double stripClosest(List<Point> strip, double d) {
+        double min = d;
+        int n = strip.size();
+        for (int i = 0; i < n; i++) {
+            for (int j = i + 1; j < n && (strip.get(j).y - strip.get(i).y) < min; j++) {
+                min = Math.min(min, distance(strip.get(i), strip.get(j)));
+            }
+        }
+        return min;
+    }
+
+    private static double distance(Point p1, Point p2) {
+        return Math.hypot(p1.x - p2.x, p1.y - p2.y);
+    }
+} 
diff --git a/src/test/java/com/thealgorithms/randomized/RandomizedClosestPairTest.java b/src/test/java/com/thealgorithms/randomized/RandomizedClosestPairTest.java
new file mode 100644
index 000000000000..5b402f3be90d
--- /dev/null
+++ b/src/test/java/com/thealgorithms/randomized/RandomizedClosestPairTest.java
@@ -0,0 +1,45 @@
+package com.thealgorithms.randomized;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import com.thealgorithms.randomized.RandomizedClosestPair.Point;
+import org.junit.jupiter.api.Test;
+
+public class RandomizedClosestPairTest {
+
+    @Test
+    public void testClosestPairBasic() {
+        Point[] points = new Point[] {
+            new Point(2, 3),
+            new Point(12, 30),
+            new Point(40, 50),
+            new Point(5, 1),
+            new Point(12, 10),
+            new Point(3, 4)
+        };
+        double result = RandomizedClosestPair.findClosestPairDistance(points);
+        assertEquals(Math.hypot(1, 1), result, 0.01); // Closest pair: (2,3) and (3,4)
+    }
+
+    @Test
+    public void testIdenticalPoints() {
+        Point[] points = new Point[] {
+            new Point(0, 0),
+            new Point(0, 0),
+            new Point(1, 1),
+        };
+        double result = RandomizedClosestPair.findClosestPairDistance(points);
+        assertEquals(0.0, result, 0.00001);
+    }
+
+    @Test
+    public void testMinimalCase() {
+        Point[] points = new Point[] {
+            new Point(0, 0),
+            new Point(3, 4)
+        };
+        double result = RandomizedClosestPair.findClosestPairDistance(points);
+        assertEquals(5.0, result, 0.00001);
+    }
+} 
+