Add algorithm SumOfDigits (#571)

Author: Lê Nam Khánh

Algorithms.Tests/Numeric/SumOfDigitsTest.cs

@@ -0,0 +1,49 @@
+using Algorithms.Numeric;
+using NUnit.Framework;
+using System;
+
+namespace Algorithms.Tests.Numeric;
+
+/// <summary>
+/// Tests for the SumOfDigits class.
+/// </summary>
+public static class SumOfDigitsTests
+{
+    /// <summary>
+    /// Tests the calculation of the sum of digits for various non-negative integers.
+    /// </summary>
+    /// <param name="input">The input number.</param>
+    /// <param name="expectedSum">The expected sum of its digits.</param>
+    [TestCase(0, 0)]
+    [TestCase(7, 7)]
+    [TestCase(10, 1)]
+    [TestCase(42, 6)]
+    [TestCase(12345, 15)]
+    [TestCase(9999, 36)]
+    [TestCase(8675309, 38)]
+    [TestCase(2147483647, 46)] // Max value for int
+    public static void GetsCorrectSumOfDigits(int input, int expectedSum)
+    {
+        // Act
+        var result = SumOfDigits.Calculate(input);
+
+        // Assert
+        Assert.That(result, Is.EqualTo(expectedSum));
+    }
+
+    /// <summary>
+    /// Tests that the method throws an ArgumentException when a negative number is provided.
+    /// </summary>
+    /// <param name="num">The negative input number.</param>
+    [TestCase(-1)]
+    [TestCase(-100)]
+    [TestCase(-54321)]
+    public static void ThrowsExceptionForNegativeNumbers(int num)
+    {
+        // Act
+        void Act() => SumOfDigits.Calculate(num);
+
+        // Assert
+        _ = Assert.Throws<ArgumentException>(Act);
+    }
+}

Algorithms/Numeric/SumOfDigits.cs

@@ -0,0 +1,48 @@
+using System;
+
+namespace Algorithms.Numeric;
+
+/// <summary>
+///     Provides functionality to calculate the sum of the digits of an integer.
+/// </summary>
+public static class SumOfDigits
+{
+    /// <summary>
+    ///     Calculates the sum of the digits of a non-negative integer.
+    ///     The method iteratively uses the modulus operator (%) to get the last digit
+    ///     and the division operator (/) to drop the last digit until the number is 0.
+    /// </summary>
+    /// <param name="number">The non-negative integer whose digits are to be summed.</param>
+    /// <returns>The sum of the digits of the input number.</returns>
+    /// <exception cref="ArgumentException">Thrown if the input number is negative.</exception>
+    public static int Calculate(int number)
+    {
+        if (number < 0)
+        {
+            throw new ArgumentException("Input must be a non-negative integer.", nameof(number));
+        }
+
+        if (number == 0)
+        {
+            return 0;
+        }
+
+        int sum = 0;
+        int currentNumber = number;
+
+        // Loop until the number becomes 0
+        while (currentNumber > 0)
+        {
+            // Get the last digit (e.g., 123 % 10 = 3)
+            int digit = currentNumber % 10;
+
+            // Add the digit to the sum
+            sum += digit;
+
+            // Remove the last digit (e.g., 123 / 10 = 12)
+            currentNumber /= 10;
+        }
+
+        return sum;
+    }
+}