From 3c071c4e08e03910045634a80039ac47401f1a58 Mon Sep 17 00:00:00 2001
From: Ruturaj Jadhav <ruturajjadhav122@gmail.com>
Date: Thu, 9 Oct 2025 23:01:56 +0530
Subject: [PATCH] Added HappyNumber algorithm in maths section (#6571)

* Added HappyNumber algorithm in maths section

* Fix Checkstyle: remove trailing spaces in HappyNumber

* Fix formatting: remove trailing spaces and apply clang-format

* Update HappyNumberTest.java

* Removed old HappyNumbersSeq.java as replaced by new HappyNumber.java
---
 .../com/thealgorithms/maths/HappyNumber.java  | 57 +++++++++++++++++++
 .../thealgorithms/others/HappyNumbersSeq.java | 39 -------------
 .../thealgorithms/maths/HappyNumberTest.java  | 32 +++++++++++
 3 files changed, 89 insertions(+), 39 deletions(-)
 create mode 100644 src/main/java/com/thealgorithms/maths/HappyNumber.java
 delete mode 100644 src/main/java/com/thealgorithms/others/HappyNumbersSeq.java
 create mode 100644 src/test/java/com/thealgorithms/maths/HappyNumberTest.java

diff --git a/src/main/java/com/thealgorithms/maths/HappyNumber.java b/src/main/java/com/thealgorithms/maths/HappyNumber.java
new file mode 100644
index 000000000..bfe746953
--- /dev/null
+++ b/src/main/java/com/thealgorithms/maths/HappyNumber.java
@@ -0,0 +1,57 @@
+package com.thealgorithms.maths;
+
+/**
+ * A Happy Number is defined as a number which eventually reaches 1 when replaced
+ * by the sum of the squares of each digit.
+ * If it falls into a cycle that does not include 1, then it is not a happy number.
+
+ * Example:
+ * 19 → 1² + 9² = 82
+ * 82 → 8² + 2² = 68
+ * 68 → 6² + 8² = 100
+ * 100 → 1² + 0² + 0² = 1 → Happy Number!
+ */
+public final class HappyNumber {
+
+    private HappyNumber() {
+    }
+
+    /**
+     * Checks whether the given number is a Happy Number.
+     * Uses Floyd’s Cycle Detection algorithm (tortoise and hare method)
+     * to detect loops efficiently.
+     *
+     * @param n The number to check
+     * @return true if n is a Happy Number, false otherwise
+     */
+    public static boolean isHappy(int n) {
+        int slow = n;
+        int fast = n;
+
+        do {
+            slow = sumOfSquares(slow); // move 1 step
+            fast = sumOfSquares(sumOfSquares(fast)); // move 2 steps
+        } while (slow != fast);
+
+        return slow == 1; // If cycle ends in 1 → Happy number
+    }
+
+    /**
+     * Calculates the sum of squares of the digits of a number.
+     *
+     * Example:
+     * num = 82 → 8² + 2² = 64 + 4 = 68
+     *
+     * @param num The number to calculate sum of squares of digits
+     * @return The sum of squares of the digits
+     */
+    private static int sumOfSquares(int num) {
+        int sum = 0;
+        while (num > 0) {
+            int digit = num % 10;
+            sum += digit * digit;
+            num /= 10;
+        }
+        return sum;
+    }
+}
diff --git a/src/main/java/com/thealgorithms/others/HappyNumbersSeq.java b/src/main/java/com/thealgorithms/others/HappyNumbersSeq.java
deleted file mode 100644
index 0ae1e451b..000000000
--- a/src/main/java/com/thealgorithms/others/HappyNumbersSeq.java
+++ /dev/null
@@ -1,39 +0,0 @@
-package com.thealgorithms.others;
-
-import java.util.Arrays;
-import java.util.HashSet;
-import java.util.Scanner;
-import java.util.Set;
-
-public final class HappyNumbersSeq {
-    private HappyNumbersSeq() {
-    }
-
-    private static final Set<Integer> CYCLE_NUMS = new HashSet<>(Arrays.asList(4, 16, 20, 37, 58, 145));
-
-    public static void main(String[] args) {
-        Scanner in = new Scanner(System.in);
-        System.out.print("Enter number: ");
-        int n = in.nextInt();
-        while (n != 1 && !isSad(n)) {
-            System.out.print(n + " ");
-            n = sumSquares(n);
-        }
-        String res = n == 1 ? "1 Happy number" : "Sad number";
-        System.out.println(res);
-        in.close();
-    }
-
-    private static int sumSquares(int n) {
-        int s = 0;
-        for (; n > 0; n /= 10) {
-            int r = n % 10;
-            s += r * r;
-        }
-        return s;
-    }
-
-    private static boolean isSad(int n) {
-        return CYCLE_NUMS.contains(n);
-    }
-}
diff --git a/src/test/java/com/thealgorithms/maths/HappyNumberTest.java b/src/test/java/com/thealgorithms/maths/HappyNumberTest.java
new file mode 100644
index 000000000..4b7cb7954
--- /dev/null
+++ b/src/test/java/com/thealgorithms/maths/HappyNumberTest.java
@@ -0,0 +1,32 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+public class HappyNumberTest {
+
+    @Test
+    void testHappyNumbers() {
+        // Known happy numbers
+        assertTrue(HappyNumber.isHappy(1));
+        assertTrue(HappyNumber.isHappy(7));
+        assertTrue(HappyNumber.isHappy(19));
+        assertTrue(HappyNumber.isHappy(100));
+    }
+
+    @Test
+    void testUnhappyNumbers() {
+        // Known unhappy numbers
+        assertFalse(HappyNumber.isHappy(2));
+        assertFalse(HappyNumber.isHappy(4));
+        assertFalse(HappyNumber.isHappy(20));
+    }
+
+    @Test
+    void testLargeNumber() {
+        // Just to check behavior with larger input
+        assertTrue(HappyNumber.isHappy(1000000)); // reduces to 1 eventually
+    }
+}