Added SkipLast method.

Author: igor-tkachev

Main/src/CodeJam.Main.csproj

@@ -93,6 +93,7 @@
 			<AutoGen>True</AutoGen>
 		</Compile>
 		<Compile Include="Algorithms\Algorithms.EqualRange.IComparable.cs" />
+		<Compile Include="Collections\Enumerable\EnumerableExtensions.SkipLast.cs" />
 		<Compile Include="Collections\IntervalTree.cs" />
 		<None Include="Algorithms\Algorithms.EqualRange.tt">
 			<Generator>TextTemplatingFileGenerator</Generator>

Main/src/Collections/Enumerable/EnumerableExtensions.SkipLast.cs

@@ -0,0 +1,52 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+using JetBrains.Annotations;
+
+namespace CodeJam.Collections
+{
+	partial class EnumerableExtensions
+	{
+		/// <summary>
+		/// Skips a specified number of contiguous elements from the end of a sequence.
+		/// </summary>
+		/// <remarks>
+		/// This operator uses deferred execution and streams its results.
+		/// </remarks>
+		/// <typeparam name="T">The type of the elements of <paramref name="source"/>.</typeparam>
+		/// <param name="source">The sequence to skip the last element of.</param>
+		/// <param name="count">The number of elements to skip.</param>
+		/// <returns>
+		/// An <see cref="IEnumerable{T}"/> that does not contains the specified number of elements from the end of the input sequence.
+		/// </returns>
+		[NotNull, Pure]
+		public static IEnumerable<T> SkipLast<T>([NotNull] this IEnumerable<T> source, int count = 1)
+		{
+			Code.NotNull(source, nameof (source));
+
+			if (count <= 0)
+				return Enumerable.Empty<T>();
+
+			if (source is ICollection<T> collection)
+				return count < collection.Count
+					? source.Take(collection.Count - count)
+					: Enumerable.Empty<T>();
+
+			return SkipLastImpl(source, count);
+		}
+
+		private static IEnumerable<T> SkipLastImpl<T>(IEnumerable<T> source, int count)
+		{
+			var queue = new Queue<T>(count);
+
+			foreach (var item in source)
+			{
+				if (queue.Count == count)
+					yield return queue.Dequeue();
+
+				queue.Enqueue(item);
+			}
+		}
+	}
+}

Main/src/Collections/Enumerable/EnumerableExtensions.TakeLast.cs

@@ -28,10 +28,10 @@ public static IEnumerable<T> TakeLast<T>([NotNull] this IEnumerable<T> source, i
 			if (count <= 0)
 				return Enumerable.Empty<T>();
 
-		if (source is IList<T> list)
-			return count < list.Count
-				? TakeLastImpl(list, count)
-				: list;
+			if (source is IList<T> list)
+				return count < list.Count
+					? TakeLastImpl(list, count)
+					: list;
 
 			return TakeLastImpl(source, count);
 		}

Main/tests/Collections/Enumerable/EnumerableExtensionTests.SkipLast.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Linq;
+
+using CodeJam.Strings;
+
+using NUnit.Framework;
+
+namespace CodeJam.Collections
+{
+	partial class EnumerableExtensionTests
+	{
+		[TestCase(new[] {3, 1, 8, 0, 6}, ExpectedResult = "3, 1, 8", TestName = nameof(SkipLastTest) + "1")]
+		[TestCase(new[] {1},             ExpectedResult = "",        TestName = nameof(SkipLastTest) + "2")]
+		[TestCase(new int[0],            ExpectedResult = "",        TestName = nameof(SkipLastTest) + "3")]
+		public string SkipLastTest(int[] source) => source.SkipLast(2).Join(", ");
+
+		[TestCase(new[] {3, 1, 8, 0, 6}, ExpectedResult = "3, 1, 8", TestName = nameof(SkipLastEnumerableTest) + "1")]
+		[TestCase(new[] {1},             ExpectedResult = "",        TestName = nameof(SkipLastEnumerableTest) + "2")]
+		[TestCase(new int[0],            ExpectedResult = "",        TestName = nameof(SkipLastEnumerableTest) + "3")]
+		public string SkipLastEnumerableTest(int[] source) => source.Select(i => i).SkipLast(2).Join(", ");
+	}
+}