Add DoWhile operator

Source/SuperLinq/DoWhile.cs

@@ -0,0 +1,42 @@
+namespace SuperLinq;
+
+public partial class SuperEnumerable
+{
+	/// <summary>
+	/// Generates an enumerable sequence by repeating a source sequence as long as the given loop postcondition holds.
+	/// </summary>
+	/// <typeparam name="TSource">Source sequence element type.</typeparam>
+	/// <param name="source">Source sequence to repeat while the condition evaluates true.</param>
+	/// <param name="condition">Loop condition.</param>
+	/// <returns>Sequence generated by repeating the given sequence until the condition evaluates to false.</returns>
+	/// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="condition"/> is <see
+	/// langword="null"/>.</exception>
+	/// <remarks>
+	/// <para>
+	/// <paramref name="condition"/> is evaluated lazily, once at the end of each loop of <paramref name="source"/>.
+	/// </para>
+	/// <para>
+	/// <paramref name="source"/> is cached via <see cref="Memoize{TSource}(IEnumerable{TSource}, bool)"/>, so that it
+	/// is only iterated once during the first loop. Successive loops will enumerate the cache instead of <paramref
+	/// name="source"/>.
+	/// </para>
+	/// </remarks>
+	public static IEnumerable<TSource> DoWhile<TSource>(this IEnumerable<TSource> source, Func<bool> condition)
+	{
+		Guard.IsNotNull(source);
+		Guard.IsNotNull(condition);
+
+		return Core(source, condition);
+
+		static IEnumerable<TSource> Core(IEnumerable<TSource> source, Func<bool> condition)
+		{
+			using var memo = source.Memoize();
+
+			do
+			{
+				foreach (var item in source)
+					yield return item;
+			} while (condition());
+		}
+	}
+}

Tests/SuperLinq.Test/DoWhileTest.cs

@@ -0,0 +1,32 @@
+namespace Test;
+
+public class DoWhileTest
+{
+	[Fact]
+	public void DoWhileIsLazy()
+	{
+		_ = new BreakingSequence<int>().DoWhile(BreakingFunc.Of<bool>());
+	}
+
+	[Fact]
+	public void DoWhileBehavior()
+	{
+		var starts = 0;
+		var seq = Enumerable.Range(1, 10)
+			.DoWhile(
+				() =>
+					starts++ switch
+					{
+						0 or 1 => true,
+						2 => false,
+						_ => throw new TestException(),
+					});
+
+		Assert.Equal(0, starts);
+		seq.AssertSequenceEqual(
+			Enumerable.Range(1, 10)
+				.Concat(Enumerable.Range(1, 10))
+				.Concat(Enumerable.Range(1, 10)));
+		Assert.Equal(3, starts);
+	}
+}