From 8ca571d8870b20f1a15ca355bf612385291b9c5c Mon Sep 17 00:00:00 2001
From: Amarildo Aliaj <82412439+amarildoaliaj@users.noreply.github.com>
Date: Mon, 26 Sep 2022 06:51:21 +0200
Subject: [PATCH] Add Collatz Conjecture (#3290)

---
 .../maths/CollatzConjecture.java              | 42 +++++++++++++++++++
 .../maths/CollatzConjectureTest.java          | 40 ++++++++++++++++++
 2 files changed, 82 insertions(+)
 create mode 100644 src/main/java/com/thealgorithms/maths/CollatzConjecture.java
 create mode 100644 src/test/java/com/thealgorithms/maths/CollatzConjectureTest.java

diff --git a/src/main/java/com/thealgorithms/maths/CollatzConjecture.java b/src/main/java/com/thealgorithms/maths/CollatzConjecture.java
new file mode 100644
index 000000000..980dc89b7
--- /dev/null
+++ b/src/main/java/com/thealgorithms/maths/CollatzConjecture.java
@@ -0,0 +1,42 @@
+package com.thealgorithms.maths;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * <a href="https://en.wikipedia.org/wiki/Collatz_conjecture">...</a>
+ */
+public class CollatzConjecture {
+
+    /**
+     * Calculate the next number of the sequence.
+     *
+     * @param n current number of the sequence
+     * @return next number of the sequence
+     */
+    public int nextNumber(final int n) {
+        if (n % 2 == 0) {
+            return n / 2;
+        }
+        return 3 * n + 1;
+    }
+
+    /**
+     * Calculate the Collatz sequence of any natural number.
+     *
+     * @param firstNumber starting number of the sequence
+     * @return sequence of the Collatz Conjecture
+     */
+    public List<Integer> collatzConjecture(int firstNumber) {
+        if (firstNumber < 1) {
+            throw new IllegalArgumentException("Must be a natural number");
+        }
+        ArrayList<Integer> result = new ArrayList<>();
+        result.add(firstNumber);
+        while (firstNumber != 1) {
+            result.add(nextNumber(firstNumber));
+            firstNumber = nextNumber(firstNumber);
+        }
+        return result;
+    }
+}
diff --git a/src/test/java/com/thealgorithms/maths/CollatzConjectureTest.java b/src/test/java/com/thealgorithms/maths/CollatzConjectureTest.java
new file mode 100644
index 000000000..e63a956dc
--- /dev/null
+++ b/src/test/java/com/thealgorithms/maths/CollatzConjectureTest.java
@@ -0,0 +1,40 @@
+package com.thealgorithms.maths;
+
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class CollatzConjectureTest {
+
+    static CollatzConjecture cConjecture;
+
+    @BeforeAll
+    static void setUp() {
+        cConjecture = new CollatzConjecture();
+    }
+
+    @Test
+    void nextNumberFromEvenNumber() {
+        assertEquals(25, cConjecture.nextNumber(50));
+    }
+
+    @Test
+    void nextNumberFromOddNumber() {
+        assertEquals(154, cConjecture.nextNumber(51));
+    }
+
+    @Test
+    void collatzConjecture() {
+        final List<Integer> expected = List.of(35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1);
+        assertIterableEquals(expected, cConjecture.collatzConjecture(35));
+    }
+
+    @Test
+    void sequenceOfNotNaturalFirstNumber() {
+        assertThrows(IllegalArgumentException.class, () -> cConjecture.collatzConjecture(0));
+        assertThrows(IllegalArgumentException.class, () -> cConjecture.collatzConjecture(-1));
+    }
+}
\ No newline at end of file