From bc6b64b1d7a2ffc5310a69a69a08a8b1a01f0be7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bart=C5=82omiej=20Dach?= <dach.bartlomiej@gmail.com>
Date: Sun, 17 May 2020 21:55:01 +0200
Subject: [PATCH 1/2] Add failing test

---
 .../NonVisual/BarLineGeneratorTest.cs         | 73 +++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 osu.Game.Tests/NonVisual/BarLineGeneratorTest.cs

diff --git a/osu.Game.Tests/NonVisual/BarLineGeneratorTest.cs b/osu.Game.Tests/NonVisual/BarLineGeneratorTest.cs
new file mode 100644
index 0000000000..e663e1128e
--- /dev/null
+++ b/osu.Game.Tests/NonVisual/BarLineGeneratorTest.cs
@@ -0,0 +1,73 @@
+// 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 NUnit.Framework;
+using osu.Framework.Utils;
+using osu.Game.Beatmaps;
+using osu.Game.Beatmaps.ControlPoints;
+using osu.Game.Beatmaps.Timing;
+using osu.Game.Rulesets.Objects;
+
+namespace osu.Game.Tests.NonVisual
+{
+    public class BarLineGeneratorTest
+    {
+        [Test]
+        public void TestRoundingErrorCompensation()
+        {
+            // The aim of this test is to make sure bar line generation compensates for floating-point errors.
+            // The premise of the test is that we have a single timing point that should result in bar lines
+            // that start at a time point that is a whole number every seventh beat.
+
+            // The fact it's every seventh beat is important - it's a number indivisible by 2, which makes
+            // it susceptible to rounding inaccuracies. In fact this was originally spotted in cases of maps
+            // that met exactly this criteria.
+
+            const int beat_length_numerator = 2000;
+            const int beat_length_denominator = 7;
+            const TimeSignatures signature = TimeSignatures.SimpleQuadruple;
+
+            var beatmap = new Beatmap
+            {
+                HitObjects = new List<HitObject>
+                {
+                    new HitObject { StartTime = 0 },
+                    new HitObject { StartTime = 120_000 }
+                },
+                ControlPointInfo = new ControlPointInfo()
+            };
+
+            beatmap.ControlPointInfo.Add(0, new TimingControlPoint
+            {
+                BeatLength = (double)beat_length_numerator / beat_length_denominator,
+                TimeSignature = signature
+            });
+
+            var barLines = new BarLineGenerator<BarLine>(beatmap).BarLines;
+
+            for (int i = 0; i * beat_length_denominator < barLines.Count; i++)
+            {
+                var barLine = barLines[i * beat_length_denominator];
+                var expectedTime = beat_length_numerator * (int)signature * i;
+
+                // every seventh bar's start time should be at least greater than the whole number we expect.
+                // It cannot be less, as that can affect overlapping scroll algorithms
+                // (the previous timing point might be chosen incorrectly if this is not the case)
+                Assert.GreaterOrEqual(barLine.StartTime, expectedTime);
+
+                // on the other side, make sure we don't stray too far from the expected time either.
+                Assert.IsTrue(Precision.AlmostEquals(barLine.StartTime, expectedTime));
+
+                // check major/minor lines for good measure too
+                Assert.AreEqual(i % (int)signature == 0, barLine.Major);
+            }
+        }
+
+        private class BarLine : IBarLine
+        {
+            public double StartTime { get; set; }
+            public bool Major { get; set; }
+        }
+    }
+}

From 17ae392a759771801a8a3d325c4e630f4c60f770 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bart=C5=82omiej=20Dach?= <dach.bartlomiej@gmail.com>
Date: Sun, 17 May 2020 22:08:49 +0200
Subject: [PATCH 2/2] Apply rounding to bar line start times

---
 osu.Game/Rulesets/Objects/BarLineGenerator.cs | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/osu.Game/Rulesets/Objects/BarLineGenerator.cs b/osu.Game/Rulesets/Objects/BarLineGenerator.cs
index 5588e9c0b7..9556b52735 100644
--- a/osu.Game/Rulesets/Objects/BarLineGenerator.cs
+++ b/osu.Game/Rulesets/Objects/BarLineGenerator.cs
@@ -1,6 +1,7 @@
 // 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;
 using System.Collections.Generic;
 using System.Linq;
 using osu.Framework.Utils;
@@ -46,6 +47,16 @@ namespace osu.Game.Rulesets.Objects
 
                 for (double t = currentTimingPoint.Time; Precision.DefinitelyBigger(endTime, t); t += barLength, currentBeat++)
                 {
+                    var roundedTime = Math.Round(t, MidpointRounding.AwayFromZero);
+
+                    // in the case of some bar lengths, rounding errors can cause t to be slightly less than
+                    // the expected whole number value due to floating point inaccuracies.
+                    // if this is the case, apply rounding.
+                    if (Precision.AlmostEquals(t, roundedTime))
+                    {
+                        t = roundedTime;
+                    }
+
                     BarLines.Add(new TBarLine
                     {
                         StartTime = t,