Move beat snapping to its own interface

osu.Game/Rulesets/Edit/IBeatSnapProvider.cs

@@ -0,0 +1,30 @@
+// 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.
+
+namespace osu.Game.Rulesets.Edit
+{
+    public interface IBeatSnapProvider
+    {
+        /// <summary>
+        /// Snaps a duration to the closest beat of a timing point applicable at the reference time.
+        /// </summary>
+        /// <param name="referenceTime">The time of the timing point which <paramref name="duration"/> resides in.</param>
+        /// <param name="duration">The duration to snap.</param>
+        /// <param name="beatDivisor">The divisor to use for snapping purposes.</param>
+        /// <returns>A value that represents <paramref name="duration"/> snapped to the closest beat of the timing point.</returns>
+        double SnapTime(double referenceTime, double duration, int beatDivisor);
+
+        /// <summary>
+        /// Get the most appropriate beat length at a given time.
+        /// </summary>
+        /// <param name="referenceTime">A reference time used for lookup.</param>
+        /// <param name="beatDivisor">The divisor to use for snapping purposes.</param>
+        /// <returns>The most appropriate beat length.</returns>
+        double GetBeatLengthAtTime(double referenceTime, int beatDivisor);
+
+        /// <summary>
+        /// Returns the current beat divisor.
+        /// </summary>
+        int BeatDivisor { get; }
+    }
+}