Realization on Catch Star Rating

osu.Game.Rulesets.Catch/Difficulty/CatchDifficultyHitObject.cs

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+using System;
+using osu.Game.Rulesets.Catch.Objects;
+using osu.Game.Rulesets.Catch.UI;
+using OpenTK;
+
+namespace osu.Game.Rulesets.Catch.Difficulty
+{
+    class CatchDifficultyHitObject
+    {
+        internal static readonly double DECAY_BASE = 0.20;
+        private const float NORMALIZED_HITOBJECT_RADIUS = 41.0f;
+        private const float ABSOLUTE_PLAYER_POSITIONING_ERROR = 16f;
+        private float playerPositioningError;
+
+        internal CatchHitObject BaseHitObject;
+
+        /// <summary>
+        /// Measures jump difficulty. CtB doesn't have something like button pressing speed or accuracy
+        /// </summary>
+        internal double Strain = 1;
+
+        /// <summary>
+        /// This is required to keep track of lazy player movement (always moving only as far as necessary)
+        /// Without this quick repeat sliders / weirdly shaped streams might become ridiculously overrated
+        /// </summary>
+        internal float PlayerPositionOffset;
+        internal float LastMovement;
+
+        internal float NormalizedPosition;
+        internal float ActualNormalizedPosition => NormalizedPosition + PlayerPositionOffset;
+
+        internal CatchDifficultyHitObject(CatchHitObject baseHitObject, float catcherWidthHalf)
+        {
+            BaseHitObject = baseHitObject;
+
+            // We will scale everything by this factor, so we can assume a uniform CircleSize among beatmaps.
+            float scalingFactor = NORMALIZED_HITOBJECT_RADIUS / catcherWidthHalf;
+
+            playerPositioningError = ABSOLUTE_PLAYER_POSITIONING_ERROR;// * scalingFactor;
+            NormalizedPosition = baseHitObject.X * CatchPlayfield.BASE_WIDTH * scalingFactor;
+        }
+
+        private const double DIRECTION_CHANGE_BONUS = 12.5;
+        internal void CalculateStrains(CatchDifficultyHitObject previousHitObject, double timeRate)
+        {
+            // Rather simple, but more specialized things are inherently inaccurate due to the big difference playstyles and opinions make.
+            // See Taiko feedback thread.
+            double timeElapsed = (BaseHitObject.StartTime - previousHitObject.BaseHitObject.StartTime) / timeRate;
+            double decay = Math.Pow(DECAY_BASE, timeElapsed / 1000);
+
+            // Update new position with lazy movement.
+            PlayerPositionOffset =
+                MathHelper.Clamp(
+                    previousHitObject.ActualNormalizedPosition,
+                    NormalizedPosition - (NORMALIZED_HITOBJECT_RADIUS - playerPositioningError),
+                    NormalizedPosition + (NORMALIZED_HITOBJECT_RADIUS - playerPositioningError)) // Obtain new lazy position, but be stricter by allowing for an error of a certain degree of the player.
+                - NormalizedPosition; // Subtract HitObject position to obtain offset
+
+            LastMovement = DistanceTo(previousHitObject);
+            double addition = spacingWeight(LastMovement);
+
+            if (NormalizedPosition < previousHitObject.NormalizedPosition)
+            {
+                LastMovement = -LastMovement;
+            }
+
+            CatchHitObject previousHitCircle = previousHitObject.BaseHitObject;
+
+            double additionBonus = 0;
+            double sqrtTime = Math.Sqrt(Math.Max(timeElapsed, 25));
+
+            // Direction changes give an extra point!
+            if (Math.Abs(LastMovement) > 0.1)
+            {
+                if (Math.Abs(previousHitObject.LastMovement) > 0.1 && Math.Sign(LastMovement) != Math.Sign(previousHitObject.LastMovement))
+                {
+                    double bonus = DIRECTION_CHANGE_BONUS / sqrtTime;
+
+                    // Weight bonus by how 
+                    double bonusFactor = Math.Min(playerPositioningError, Math.Abs(LastMovement)) / playerPositioningError;
+
+                    // We want time to play a role twice here!
+                    addition += bonus * bonusFactor;
+
+                    // Bonus for tougher direction switches and "almost" hyperdashes at this point
+                    if (previousHitCircle != null && previousHitCircle.DistanceToHyperDash <= 10.0f / CatchPlayfield.BASE_WIDTH)
+                    {
+                        additionBonus += 0.3 * bonusFactor;
+                    }
+                }
+
+                // Base bonus for every movement, giving some weight to streams.
+                addition += 7.5 * Math.Min(Math.Abs(LastMovement), NORMALIZED_HITOBJECT_RADIUS * 2) / (NORMALIZED_HITOBJECT_RADIUS * 6) / sqrtTime;
+            }
+
+            // Bonus for "almost" hyperdashes at corner points
+            if (previousHitCircle != null && previousHitCircle.DistanceToHyperDash <= 10.0f / CatchPlayfield.BASE_WIDTH)
+            {
+                if (!previousHitCircle.HyperDash)
+                {
+                    additionBonus += 1.0;
+                }
+                else
+                {
+                    // After a hyperdash we ARE in the correct position. Always!
+                    PlayerPositionOffset = 0;
+                }
+
+                addition *= 1.0 + additionBonus * ((10 - previousHitCircle.DistanceToHyperDash * CatchPlayfield.BASE_WIDTH) / 10);
+            }
+
+            addition *= 850.0 / Math.Max(timeElapsed, 25);
+
+            Strain = previousHitObject.Strain * decay + addition;
+        }
+
+        private static double spacingWeight(float distance)
+        {
+            return Math.Pow(distance, 1.3) / 500;
+        }
+
+        internal float DistanceTo(CatchDifficultyHitObject other)
+        {
+            return Math.Abs(ActualNormalizedPosition - other.ActualNormalizedPosition);
+        }
+    }
+}