chore(CSharp): added big mod (#498)

algorithms/CSharp/README.md

@@ -4,6 +4,9 @@ For running the `.cs` file please using [.Net Finddle](https://dotnetfiddle.net/
 ## Arrays
 1. [Single Number](src/Arrays/single-number.cs)
 
+## Number Theory
+1. [Big Mod Algorithm](src/Number-Theory/big-mod.cs)
+
 ## Sorts
 
 1. [Bubble Sort](src/Sorts/bubble-sort.cs)

algorithms/CSharp/src/Number-Theory/big-mod.cs

@@ -0,0 +1,28 @@
+using System;
+
+namespace Algorithms.NumberTheory
+{
+    public class BigMod
+    {
+        // (a ^ p) % m
+        public static long Mod(long a, long p, long m)
+        {
+            if(p == 0)
+            {
+                return 1;
+            }
+            if(p % 2 == 1)
+            {
+               return ((a % m) * Mod(a, p - 1, m)) % m;
+            }
+
+            long temp = Mod(a, p / 2, m);
+            return (temp * temp) % m;
+        }
+
+        public static void Main()
+        {
+            Console.WriteLine($"{Mod(4, 5, 3)}");
+        }
+    }
+}

algorithms/CSharp/test/Number-Theory/big-mod.cs

@@ -0,0 +1,21 @@
+using NUnit.Framework;
+
+namespace Algorithms.Tests.NumberTheory
+{
+    [TestFixture]
+    public class BigMod
+    {
+        [TestCase(4, 5, 3, 1)]
+        [TestCase(100001, 122, 13, 12)]
+        [TestCase(10000, 1000, 118, 36)]
+        [TestCase(333, 31, 17, 12)]
+        [TestCase(411, 56, 107, 9)]
+        [TestCase(100000, 0, 100, 1)]
+        [TestCase(99999999, 99999, 13, 5)]
+        public void BigMod_ShouldReturnExpectedResult(long a, long p, long m, long expected)
+        {
+            long result = Algorithms.NumberTheory.BigMod.Mod(a, p, m);
+            Assert.AreEqual(expected, result);
+        }
+    }
+}