From bea72d0a8a62998de1c375f76ac4db0120b3da0a Mon Sep 17 00:00:00 2001
From: zhabinka <sergeykarpuk@yandex.by>
Date: Mon, 21 Jan 2019 20:11:06 +0300
Subject: [PATCH] Added Collatz Conjecture

---
 .../collatz-conjecture/collatz-conjecture.js  | 22 +++++++++++++
 .../collatz-conjecture.spec.js                | 31 +++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 javascript/collatz-conjecture/collatz-conjecture.js
 create mode 100644 javascript/collatz-conjecture/collatz-conjecture.spec.js

diff --git a/javascript/collatz-conjecture/collatz-conjecture.js b/javascript/collatz-conjecture/collatz-conjecture.js
new file mode 100644
index 0000000..7c25960
--- /dev/null
+++ b/javascript/collatz-conjecture/collatz-conjecture.js
@@ -0,0 +1,22 @@
+// If n is even, divide n by 2 to get n / 2.
+// If n is odd, multiply n by 3 and add 1 to get 3n + 1.
+
+const operation = n => (n % 2 === 0 ? n / 2 : 3 * n + 1);
+
+const steps = (number) => {
+  if (number <= 0) {
+    throw new Error('Only positive numbers are allowed');
+  }
+
+  const iter = (num, count) => {
+    if (num === 1) {
+      return count;
+    }
+
+    return iter(operation(num), count + 1);
+  };
+
+  return iter(number, 0);
+};
+
+export { steps };
diff --git a/javascript/collatz-conjecture/collatz-conjecture.spec.js b/javascript/collatz-conjecture/collatz-conjecture.spec.js
new file mode 100644
index 0000000..53c45ba
--- /dev/null
+++ b/javascript/collatz-conjecture/collatz-conjecture.spec.js
@@ -0,0 +1,31 @@
+import { steps } from './collatz-conjecture';
+
+describe('steps()', () => {
+  test('zero steps for one', () => {
+    expect(steps(1)).toEqual(0);
+  });
+
+  test('divide if even', () => {
+    expect(steps(16)).toEqual(4);
+  });
+
+  test('even and odd steps', () => {
+    expect(steps(12)).toEqual(9);
+  });
+
+  test('Large number of even and odd steps', () => {
+    expect(steps(1000000)).toEqual(152);
+  });
+
+  test('zero is an error', () => {
+    expect(() => {
+      steps(0);
+    }).toThrow(new Error('Only positive numbers are allowed'));
+  });
+
+  test('negative value is an error', () => {
+    expect(() => {
+      steps(-15);
+    }).toThrow(new Error('Only positive numbers are allowed'));
+  });
+});