Fix SliderPath.GetSegmentEnds

osu.Game.Tests/Visual/Gameplay/TestSceneSliderPath.cs

@@ -185,6 +185,37 @@ public void TestShortenToNegativeLength()
             AddStep("shorten to -10 length", () => path.ExpectedDistance.Value = -10);
         }
 
+        [Test]
+        public void TestGetSegmentEnds()
+        {
+            var positions = new[]
+            {
+                Vector2.Zero,
+                new Vector2(100, 0),
+                new Vector2(100),
+                new Vector2(200, 100),
+            };
+            double[] distances = { 100d, 200d, 300d };
+
+            AddStep("create path", () => path.ControlPoints.AddRange(positions.Select(p => new PathControlPoint(p, PathType.Linear))));
+            AddAssert("segment ends are correct", () => path.GetSegmentEnds(), () => Is.EqualTo(distances.Select(d => d / 300)));
+            AddAssert("segment end positions recovered", () => path.GetSegmentEnds().Select(p => path.PositionAt(p)), () => Is.EqualTo(positions.Skip(1)));
+
+            AddStep("lengthen last segment", () => path.ExpectedDistance.Value = 400);
+            AddAssert("segment ends are correct", () => path.GetSegmentEnds(), () => Is.EqualTo(distances.Select(d => d / 400)));
+            AddAssert("segment end positions recovered", () => path.GetSegmentEnds().Select(p => path.PositionAt(p)), () => Is.EqualTo(positions.Skip(1)));
+
+            AddStep("shorten last segment", () => path.ExpectedDistance.Value = 150);
+            AddAssert("segment ends are correct", () => path.GetSegmentEnds(), () => Is.EqualTo(distances.Select(d => d / 150)));
+            // see remarks in `GetSegmentEnds()` xmldoc (`SliderPath.PositionAt()` clamps progress to [0,1]).
+            AddAssert("segment end positions not recovered", () => path.GetSegmentEnds().Select(p => path.PositionAt(p)), () => Is.EqualTo(new[]
+            {
+                positions[1],
+                new Vector2(100, 50),
+                new Vector2(100, 50),
+            }));
+        }
+
         private List<PathControlPoint> createSegment(PathType type, params Vector2[] controlPoints)
         {
             var points = controlPoints.Select(p => new PathControlPoint { Position = p }).ToList();

osu.Game/Rulesets/Objects/SliderPath.cs

@@ -40,11 +40,13 @@ public class SliderPath
 
         private readonly List<Vector2> calculatedPath = new List<Vector2>();
         private readonly List<double> cumulativeLength = new List<double>();
-        private readonly List<int> segmentEnds = new List<int>();
         private readonly Cached pathCache = new Cached();
 
         private double calculatedLength;
 
+        private readonly List<int> segmentEnds = new List<int>();
+        private double[] segmentEndDistances = Array.Empty<double>();
+
         /// <summary>
         /// Creates a new <see cref="SliderPath"/>.
         /// </summary>
@@ -196,13 +198,28 @@ public List<PathControlPoint> PointsInSegment(PathControlPoint controlPoint)
         }
 
         /// <summary>
-        /// Returns the progress values at which segments of the path end.
+        /// Returns the progress values at which (control point) segments of the path end.
+        /// Ranges from 0 (beginning of the path) to 1 (end of the path) to infinity (beyond the end of the path).
         /// </summary>
+        /// <remarks>
+        /// <see cref="PositionAt"/> truncates the progression values to [0,1],
+        /// so you can't use this method in conjunction with that one to retrieve the positions of segment ends beyond the end of the path.
+        /// </remarks>
+        /// <example>
+        /// <para>
+        /// In case <see cref="Distance"/> is less than <see cref="CalculatedDistance"/>,
+        /// the last segment ends after the end of the path, hence it returns a value greater than 1.
+        /// </para>
+        /// <para>
+        /// In case <see cref="Distance"/> is greater than <see cref="CalculatedDistance"/>,
+        /// the last segment ends before the end of the path, hence it returns a value less than 1.
+        /// </para>
+        /// </example>
         public IEnumerable<double> GetSegmentEnds()
         {
             ensureValid();
 
-            return segmentEnds.Select(i => cumulativeLength[i] / calculatedLength);
+            return segmentEndDistances.Select(d => d / Distance);
         }
 
         private void invalidate()
@@ -251,8 +268,11 @@ private void calculatePath()
                         calculatedPath.Add(t);
                 }
 
-                // Remember the index of the segment end
-                segmentEnds.Add(calculatedPath.Count - 1);
+                if (i > 0)
+                {
+                    // Remember the index of the segment end
+                    segmentEnds.Add(calculatedPath.Count - 1);
+                }
 
                 // Start the new segment at the current vertex
                 start = i;
@@ -298,6 +318,14 @@ private void calculateLength()
                 cumulativeLength.Add(calculatedLength);
             }
 
+            // Store the distances of the segment ends now, because after shortening the indices may be out of range
+            segmentEndDistances = new double[segmentEnds.Count];
+
+            for (int i = 0; i < segmentEnds.Count; i++)
+            {
+                segmentEndDistances[i] = cumulativeLength[segmentEnds[i]];
+            }
+
             if (ExpectedDistance.Value is double expectedDistance && calculatedLength != expectedDistance)
             {
                 // In osu-stable, if the last two control points of a slider are equal, extension is not performed.
@@ -319,10 +347,6 @@ private void calculateLength()
                     {
                         cumulativeLength.RemoveAt(cumulativeLength.Count - 1);
                         calculatedPath.RemoveAt(pathEndIndex--);
-
-                        // Shorten the last segment to the expected distance
-                        if (segmentEnds.Count > 0)
-                            segmentEnds[^1]--;
                     }
                 }