Implement Taiko difficulty calculation.

osu.Game.Rulesets.Taiko/Objects/TaikoHitObjectDifficulty.cs

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+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System;
+
+namespace osu.Game.Rulesets.Taiko.Objects
+{
+    internal class TaikoHitObjectDifficulty
+    {
+        /// <summary>
+        /// Factor by how much individual / overall strain decays per second.
+        /// </summary>
+        /// <remarks>
+        /// These values are results of tweaking a lot and taking into account general feedback.
+        /// </remarks>
+        internal const double DECAY_BASE = 0.30;
+
+        private const double type_change_bonus = 0.75;
+        private const double rhythm_change_bonus = 1.0;
+        private const double rhythm_change_base_threshold = 0.2;
+        private const double rhythm_change_base = 2.0;
+
+        internal TaikoHitObject BaseHitObject;
+
+        /// <summary>
+        /// Measures note density in a way
+        /// </summary>
+        internal double Strain = 1;
+
+        private double timeElapsed = 0;
+        private int sameTypeSince = 1;
+
+        private bool isRim => BaseHitObject is RimHit;
+
+        public TaikoHitObjectDifficulty(TaikoHitObject baseHitObject)
+        {
+            this.BaseHitObject = baseHitObject;
+        }
+
+        internal void CalculateStrains(TaikoHitObjectDifficulty 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.
+            timeElapsed = (BaseHitObject.StartTime - previousHitObject.BaseHitObject.StartTime) / timeRate;
+            double decay = Math.Pow(DECAY_BASE, timeElapsed / 1000);
+
+            double addition = 1;
+
+            // Only if we are no slider or spinner we get an extra addition
+            if (previousHitObject.BaseHitObject is Hit && BaseHitObject is Hit
+                && BaseHitObject.StartTime - previousHitObject.BaseHitObject.StartTime < 1000) // And we only want to check out hitobjects which aren't so far in the past
+            {
+                addition += typeChangeAddition(previousHitObject);
+                addition += rhythmChangeAddition(previousHitObject);
+            }
+
+            double additionFactor = 1.0;
+            // Scale AdditionFactor linearly from 0.4 to 1 for TimeElapsed from 0 to 50
+            if (timeElapsed < 50.0)
+                additionFactor = 0.4 + 0.6 * timeElapsed / 50.0;
+
+            Strain = previousHitObject.Strain * decay + addition * additionFactor;
+        }
+
+        private TypeSwitch lastTypeSwitchEven = TypeSwitch.None;
+        private double typeChangeAddition(TaikoHitObjectDifficulty previousHitObject)
+        {
+            // If we don't have the same hit type, trigger a type change!
+            if (previousHitObject.isRim != isRim)
+            {
+                lastTypeSwitchEven = previousHitObject.sameTypeSince % 2 == 0 ? TypeSwitch.Even : TypeSwitch.Odd;
+
+                // We only want a bonus if the parity of the type switch changes!
+                switch (previousHitObject.lastTypeSwitchEven)
+                {
+                    case TypeSwitch.Even:
+                        if (lastTypeSwitchEven == TypeSwitch.Odd)
+                            return type_change_bonus;
+                        break;
+                    case TypeSwitch.Odd:
+                        if (lastTypeSwitchEven == TypeSwitch.Even)
+                            return type_change_bonus;
+                        break;
+                }
+            }
+            // No type change? Increment counter and keep track of last type switch
+            else
+            {
+                lastTypeSwitchEven = previousHitObject.lastTypeSwitchEven;
+                sameTypeSince = previousHitObject.sameTypeSince + 1;
+            }
+
+            return 0;
+        }
+
+        private double rhythmChangeAddition(TaikoHitObjectDifficulty previousHitObject)
+        {
+            // We don't want a division by zero if some random mapper decides to put 2 HitObjects at the same time.
+            if (timeElapsed == 0 || previousHitObject.timeElapsed == 0)
+                return 0;
+
+            double timeElapsedRatio = Math.Max(previousHitObject.timeElapsed / timeElapsed, timeElapsed / previousHitObject.timeElapsed);
+
+            if (timeElapsedRatio >= 8)
+                return 0;
+
+            double difference = Math.Log(timeElapsedRatio, rhythm_change_base) % 1.0;
+
+            if (isWithinChangeThreshold(difference))
+                return rhythm_change_bonus;
+
+            return 0;
+        }
+
+        private bool isWithinChangeThreshold(double value)
+        {
+            return value > rhythm_change_base_threshold && value < 1 - rhythm_change_base_threshold;
+        }
+
+        private enum TypeSwitch
+        {
+            None,
+            Even,
+            Odd
+        }
+    }
+}