Add zero-length objects check and tests

osu.Game/Rulesets/Edit/Checks/CheckZeroLengthObjects.cs

@@ -0,0 +1,47 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using osu.Game.Rulesets.Edit.Checks.Components;
+using osu.Game.Rulesets.Objects;
+using osu.Game.Rulesets.Objects.Types;
+
+namespace osu.Game.Rulesets.Edit.Checks
+{
+    public class CheckZeroLengthObjects : ICheck
+    {
+        /// <summary>
+        /// The duration can be this low before being treated as having no length, in case of precision errors. Unit is milliseconds.
+        /// </summary>
+        private const double leniency = 0.5d;
+
+        public CheckMetadata Metadata { get; } = new CheckMetadata(CheckCategory.Compose, "Zero-length hitobjects");
+
+        public IEnumerable<IssueTemplate> PossibleTemplates => new IssueTemplate[]
+        {
+            new IssueTemplateZeroLength(this)
+        };
+
+        public IEnumerable<Issue> Run(BeatmapVerifierContext context)
+        {
+            foreach (var hitObject in context.Beatmap.HitObjects)
+            {
+                if (!(hitObject is IHasDuration hasDuration))
+                    continue;
+
+                if (hasDuration.Duration < leniency)
+                    yield return new IssueTemplateZeroLength(this).Create(hitObject, hasDuration.Duration);
+            }
+        }
+
+        public class IssueTemplateZeroLength : IssueTemplate
+        {
+            public IssueTemplateZeroLength(ICheck check)
+                : base(check, IssueType.Problem, "{0} has a duration of {1:0}.")
+            {
+            }
+
+            public Issue Create(HitObject hitobject, double duration) => new Issue(hitobject, this, hitobject.GetType(), duration);
+        }
+    }
+}