From b60fdc495b98b3aaa299c89da6bf062027e667c3 Mon Sep 17 00:00:00 2001
From: Oliver Booth <me@olivr.me>
Date: Thu, 1 Dec 2022 13:51:30 +0000
Subject: [PATCH] Add IReadOnlyList<T>.Slice

---
 CHANGELOG.md                            |  1 +
 X10D.Tests/src/Collections/ListTests.cs | 52 +++++++++++++++++
 X10D/src/Collections/ListExtensions.cs  | 75 ++++++++++++++++++++++++-
 3 files changed, 127 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index dfdf9a8..e4354cf 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,7 @@
 - X10D: Added `IList<T>.RemoveRange(Range)`
 - X10D: Added `IList<T>.Swap(IList<T>)` (#62)
 - X10D: Added `IReadOnlyList<T>.IndexOf(T[, int[, int]])`
+- X10D: Added `IReadOnlyList<T>.Slice(int[, int]])`
 - X10D: Added `Point.IsOnLine(LineF)`, `Point.IsOnLine(PointF, PointF)`, and `Point.IsOnLine(Vector2, Vector2)`
 - X10D: Added `PointF.IsOnLine(LineF)`, `PointF.IsOnLine(PointF, PointF)`, and `PointF.IsOnLine(Vector2, Vector2)`
 - X10D: Added `Point.ToSize()`
diff --git a/X10D.Tests/src/Collections/ListTests.cs b/X10D.Tests/src/Collections/ListTests.cs
index 2f17998..06c079a 100644
--- a/X10D.Tests/src/Collections/ListTests.cs
+++ b/X10D.Tests/src/Collections/ListTests.cs
@@ -216,6 +216,58 @@ public class ListTests
         Assert.ThrowsException<ArgumentNullException>(() => ((List<int>?)null)!.Shuffle());
     }
 
+    [TestMethod]
+    public void Slice_ShouldReturnCorrectValue_GivenStartIndex()
+    {
+        int[] array = {0, 1, 2, 3, 4, 5};
+        CollectionAssert.AreEqual(new[] {2, 3, 4, 5}, array.Slice(2).ToArray());
+    }
+
+    [TestMethod]
+    public void Slice_ShouldReturnCorrectValue_GivenStartIndexAndLength()
+    {
+        int[] array = {0, 1, 2, 3, 4, 5};
+        CollectionAssert.AreEqual(new[] {2, 3, 4}, array.Slice(2, 3).ToArray());
+    }
+
+    [TestMethod]
+    public void Slice_ShouldReturnEmptyList_ForEmptyList()
+    {
+        int[] array = Array.Empty<int>();
+        CollectionAssert.AreEqual(Array.Empty<int>(), array.Slice(0).ToArray());
+        CollectionAssert.AreEqual(Array.Empty<int>(), array.Slice(0, 0).ToArray());
+    }
+
+    [TestMethod]
+    public void Slice_ShouldThrowArgumentNullException_GivenNullList()
+    {
+        int[]? array = null;
+        Assert.ThrowsException<ArgumentNullException>(() => array!.Slice(0));
+        Assert.ThrowsException<ArgumentNullException>(() => array!.Slice(0, 0));
+    }
+
+    [TestMethod]
+    public void Slice_ShouldThrowArgumentOutOfRangeException_GivenNegativeCount()
+    {
+        int[] array = Array.Empty<int>();
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.Slice(0, -1));
+    }
+
+    [TestMethod]
+    public void Slice_ShouldThrowArgumentOutOfRangeException_GivenNegativeStartIndex()
+    {
+        int[] array = Array.Empty<int>();
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.Slice(-1));
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.Slice(-1, 0));
+    }
+
+    [TestMethod]
+    public void Slice_ShouldThrowArgumentOutOfRangeException_GivenInvalidStartIndexCountPair()
+    {
+        int[] array = {0, 1, 2};
+        Assert.ThrowsException<ArgumentOutOfRangeException>(() => array.Slice(2, 4));
+    }
+
     [TestMethod]
     public void Swap_ShouldThrowArgumentNullException_GivenNullSource()
     {
diff --git a/X10D/src/Collections/ListExtensions.cs b/X10D/src/Collections/ListExtensions.cs
index 086b70e..6e18207 100644
--- a/X10D/src/Collections/ListExtensions.cs
+++ b/X10D/src/Collections/ListExtensions.cs
@@ -1,4 +1,3 @@
-using System.Diagnostics;
 using System.Diagnostics.Contracts;
 using X10D.Core;
 
@@ -320,6 +319,80 @@ public static class ListExtensions
         }
     }
 
+    /// <summary>
+    ///     Forms a slice out of the current list that begins at a specified index.
+    /// </summary>
+    /// <param name="source">The list to slice.</param>
+    /// <param name="start">The index at which to begin the slice.</param>
+    /// <typeparam name="T">The type of elements in <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     A list that consists of all elements of the current list from <paramref name="start" /> to the end of the list.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
+    /// <exception cref="ArgumentOutOfRangeException">
+    ///     <paramref name="start" /> is less than zero or greater than <see cref="IReadOnlyList{T}.Count" />.
+    /// </exception>
+    public static IReadOnlyList<T> Slice<T>(this IReadOnlyList<T> source, int start)
+    {
+#if NET6_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(source);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+#endif
+
+        return source.Slice(start, source.Count - start);
+    }
+
+    /// <summary>
+    ///     Forms a slice out of the current list that begins at a specified index for a specified length.
+    /// </summary>
+    /// <param name="source">The list to slice.</param>
+    /// <param name="start">The index at which to begin the slice.</param>
+    /// <param name="length">The desired length for the slice.</param>
+    /// <typeparam name="T">The type of elements in <paramref name="source" />.</typeparam>
+    /// <returns>
+    ///     A list that consists of all elements of the current list from <paramref name="start" /> to the end of the list.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
+    /// <exception cref="ArgumentOutOfRangeException">
+    ///     <paramref name="start" /> or <paramref name="start" /> + <paramref name="length" /> is less than zero or greater than
+    ///     <see cref="IReadOnlyList{T}.Count" />.
+    /// </exception>
+    public static IReadOnlyList<T> Slice<T>(this IReadOnlyList<T> source, int start, int length)
+    {
+#if NET6_0_OR_GREATER
+        ArgumentNullException.ThrowIfNull(source);
+#else
+        if (source is null)
+        {
+            throw new ArgumentNullException(nameof(source));
+        }
+#endif
+
+        if (start < 0 || start > source.Count)
+        {
+            throw new ArgumentOutOfRangeException(nameof(start), ExceptionMessages.IndexOutOfRange);
+        }
+
+        if (length < 0 || length > source.Count - start)
+        {
+            throw new ArgumentOutOfRangeException(nameof(length), ExceptionMessages.CountMustBeInRange);
+        }
+
+        var sliced = new List<T>();
+
+        int endIndex = start + length;
+        for (int index = start; index < endIndex; index++)
+        {
+            sliced.Add(source[index]);
+        }
+
+        return sliced.AsReadOnly();
+    }
+
     /// <summary>
     ///     Swaps all elements in a list with the elements in another list.
     /// </summary>