From 49b9a04e20da2f72424ead4932194332445009a4 Mon Sep 17 00:00:00 2001
From: Leefrost <evertodante@gmail.com>
Date: Sun, 7 Oct 2018 15:16:46 +0300
Subject: [PATCH] Catalan numbers

Due to Contribution.md I added Catalan numbers generator. Test included
---
 .../CatalanNumbersTests.cs                    | 48 +++++++++++++++++++
 .../CatalanNumbers/CatalanNumberGenerator.cs  | 26 ++++++++++
 2 files changed, 74 insertions(+)
 create mode 100644 Algorithms/Algorithms.Tests/CatalanNumbersTests/CatalanNumbersTests.cs
 create mode 100644 Algorithms/Algorithms/CatalanNumbers/CatalanNumberGenerator.cs

diff --git a/Algorithms/Algorithms.Tests/CatalanNumbersTests/CatalanNumbersTests.cs b/Algorithms/Algorithms.Tests/CatalanNumbersTests/CatalanNumbersTests.cs
new file mode 100644
index 0000000..6c2b75f
--- /dev/null
+++ b/Algorithms/Algorithms.Tests/CatalanNumbersTests/CatalanNumbersTests.cs
@@ -0,0 +1,48 @@
+using FluentAssertions;
+using Xunit;
+
+namespace Algorithms.Tests.CatalanNumbersTests
+{
+    public class CatalanNumbersTests
+    {
+        [Fact]
+        public void GetNumbers_Generate3rdCatalanNumber_Return5()
+        {
+            var expected = 5;
+
+            var number = CatalanNumberGenerator.GetNumberOn(3);
+
+            number.Should().Be(expected);
+        }
+
+        [Fact]
+        public void GetNumbers_Generate15rdCatalanNumber_Return9694845()
+        {
+            var expected = 9694845;
+
+            var number = CatalanNumberGenerator.GetNumberOn(15);
+
+            number.Should().Be(expected);
+        }
+
+        [Fact]
+        public void GetNumbers_Generate0rdCatalanNumber_Return1()
+        {
+            var expected = 0;
+
+            var number = CatalanNumberGenerator.GetNumberOn(0);
+
+            number.Should().Be(expected);
+        }
+
+        [Fact]
+        public void GetNumbers_GenerateNegativePos_Return0()
+        {
+            var expected = 0;
+
+            var number = CatalanNumberGenerator.GetNumberOn(-1);
+
+            number.Should().Be(expected);
+        }
+    }
+}
diff --git a/Algorithms/Algorithms/CatalanNumbers/CatalanNumberGenerator.cs b/Algorithms/Algorithms/CatalanNumbers/CatalanNumberGenerator.cs
new file mode 100644
index 0000000..afb7ae8
--- /dev/null
+++ b/Algorithms/Algorithms/CatalanNumbers/CatalanNumberGenerator.cs
@@ -0,0 +1,26 @@
+using System;
+
+namespace Algorithms
+{
+    public static class CatalanNumberGenerator
+    {
+        private static double Factorial(double n)
+        {
+            if (n <= 0.0)
+                return 1;
+
+            return n * Factorial(n - 1);
+        }
+
+        public static double GetNumberOn(int n)
+        {
+            if (n <= 0)
+                return 0;
+
+            const double topMultiplier = 2.0;
+            var result = Factorial(topMultiplier * n) / (Factorial(n + 1) * Factorial(n));
+
+            return Math.Ceiling(result);
+        }
+    }
+}