Add Keith Number (#111) (#259)

Author: iamstan13y

Algorithms.Tests/Numeric/KeithNumberTest.cs

@@ -0,0 +1,33 @@
+using Algorithms.Numeric;
+using NUnit.Framework;
+using System;
+
+namespace Algorithms.Tests.Numeric
+{
+    public static class KeithNumberTest
+    {
+        [Test]
+        [TestCase(14)]
+        [TestCase(47)]
+        [TestCase(197)]
+        [TestCase(7909)]
+        public static void KeithNumberWork(int number)
+        {
+            // Act
+            var result = KeithNumberChecker.IsKeithNumber(number);
+
+            // Assert
+            Assert.IsTrue(result);
+        }
+
+        [Test]
+        [TestCase(-2)]
+        public static void KeithNumberShouldThrowEx(int number)
+        {
+            // Arrange
+
+            // Assert
+            Assert.Throws<ArgumentException>(() => KeithNumberChecker.IsKeithNumber(number));
+        }
+    }
+}

Algorithms/Numeric/KeithNumberChecker.cs

@@ -0,0 +1,57 @@
+using System;
+
+namespace Algorithms.Numeric
+{
+    /// <summary>
+    ///  In number theory, a Keith number or repfigit number is a natural number n in a given number base b with k digits such that
+    ///  when a sequence is created such that the first k terms are the k digits of n and each subsequent term is the sum of the
+    ///  previous k terms, n is part of the sequence.
+    /// </summary>
+    public static class KeithNumberChecker
+    {
+        /// <summary>
+        ///     Checks if a number is a Keith number or not.
+        /// </summary>
+        /// <param name="number">Number to check.</param>
+        /// <returns>True if it is a Keith number; False otherwise.</returns>
+        public static bool IsKeithNumber(int number)
+        {
+            if (number < 0)
+            {
+                throw new ArgumentException($"{nameof(number)} cannot be negative");
+            }
+
+            var tempNumber = number;
+
+            var stringNumber = number.ToString();
+
+            var digitsInNumber = stringNumber.Length;
+
+            /* storing the terms of the series */
+            var termsArray = new int[number];
+
+            for (var i = digitsInNumber - 1; i >= 0; i--)
+            {
+                termsArray[i] = tempNumber % 10;
+                tempNumber /= 10;
+            }
+
+            var sum = 0;
+            var k = digitsInNumber;
+            while (sum < number)
+            {
+                sum = 0;
+
+                for (var j = 1; j <= digitsInNumber; j++)
+                {
+                    sum += termsArray[k - j];
+                }
+
+                termsArray[k] = sum;
+                k++;
+            }
+
+            return sum == number;
+        }
+    }
+}

README.md

@@ -42,6 +42,7 @@ This repository contains algorithms and data structures implemented in C# for ed
     * [Series](./Algorithms/Numeric/Series)
       * [Maclaurin Series](./Algorithms/Numeric/Series/Maclaurin.cs)
     * [Gauss-Jordan Elimination](./Algorithms/Numeric/GaussJordanElimination.cs)
+    * [Keith Number Checker](./Algorithms/Numeric/KeithNumberChecker.cs)
     * [Pseudo-Inverse](./Algorithms/Numeric/Pseudoinverse/PseudoInverse.cs)
     * [Narcissistic Number Checker](./Algorithms/Numeric/NarcissisticNumberChecker.cs)
     * [Perfect Number Checker](./Algorithms/Numeric/PerfectNumberChecker.cs)