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SecureBank.Extensions/IEnumerableExtensions.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using static System.Runtime.InteropServices.JavaScript.JSType;
+
+namespace SecureBank.Extensions
+{
+    public static class IEnumerableExtensions
+    {
+        #region PUBLIC METHODS
+
+        public static IEnumerable<IEnumerable<T>> GetCombinations<T>(this IEnumerable<T> elements, int n)
+        {
+            if (elements.Count() < n)
+            {
+                throw new ArgumentException("Array length can't be less than number of selected elements");
+            }
+
+            if (n < 1)
+            {
+                throw new ArgumentException("Number of selected elements can't be less than 1");
+            }
+
+            if (elements.Count() == n)
+            {
+                return new List<IEnumerable<T>> { elements.ToList() };
+            }
+
+            List<List<T>> result = new List<List<T>>();
+
+            int[] indices = Enumerable.Range(0, n).ToArray();
+
+            while (indices[0] <= elements.Count() - n)
+            {
+                List<T> combination = new List<T>();
+
+                for (int i = 0; i < n; i++)
+                {
+                    combination.Add(elements.ElementAt(indices[i]));
+                }
+
+                result.Add(combination);
+
+                int j = n - 1;
+                while (j >= 0 && indices[j] == elements.Count() - n + j)
+                {
+                    j--;
+                }
+
+                if (j < 0)
+                {
+                    break;
+                }
+
+                indices[j]++;
+                for (int k = j + 1; k < n; k++)
+                {
+                    indices[k] = indices[k - 1] + 1;
+                }
+            }
+
+            return result;
+
+        }
+
+        #endregion
+
+
+
+        #region PRIVATE METHODS
+
+
+
+        #endregion
+    }
+}