new rhythm, introduce rhythm per-object difficulty

osu.Game.Rulesets.Taiko/Difficulty/Evaluators/RhythmEvaluator.cs

@@ -0,0 +1,145 @@
+// 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 osu.Game.Rulesets.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Difficulty.Utils;
+using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing;
+using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Rhythm;
+using osu.Game.Rulesets.Taiko.Difficulty.Preprocessing.Rhythm.Data;
+
+namespace osu.Game.Rulesets.Taiko.Difficulty.Evaluators
+{
+    public class RhythmEvaluator
+    {
+        /// <summary>
+        /// Multiplier for a given denominator term.
+        /// </summary>
+        private static double termPenalty(double ratio, int denominator, double power, double multiplier)
+        {
+            return -multiplier * Math.Pow(Math.Cos(denominator * Math.PI * ratio), power);
+        }
+
+        /// <summary>
+        /// Gives a bonus for target ratio using a bell-shaped function.
+        /// </summary>
+        private static double bellCurve(double ratio, double targetRatio, double width, double multiplier)
+        {
+            return multiplier * Math.Exp(Math.E * -(Math.Pow(ratio - targetRatio, 2) / Math.Pow(width, 2)));
+        }
+
+        /// <summary>
+        /// Calculates the difficulty of a given ratio using a combination of periodic penalties and bonuses.
+        /// </summary>
+        private static double ratioDifficulty(double ratio, int terms = 8)
+        {
+            // Sum of n = 8 terms of periodic penalty.
+            double difficulty = 0;
+
+            for (int i = 1; i <= terms; ++i)
+            {
+                difficulty += termPenalty(ratio, i, 2, 1);
+            }
+
+            difficulty += terms;
+
+            // Give bonus to near-1 ratios
+            difficulty += bellCurve(ratio, 1, 0.5, 1);
+
+            // Penalise ratios that are VERY near 1
+            difficulty -= bellCurve(ratio, 1, 0.3, 1);
+
+            return difficulty / Math.Sqrt(8);
+        }
+
+        /// <summary>
+        /// Determines if the pattern of hit object intervals is consistent based on a given threshold.
+        /// </summary>
+        private static bool isConsistentPattern(EvenHitObjects evenHitObjects, double threshold = 0.1)
+        {
+            // Collect the last 4 intervals (current and the last 3 previous).
+            List<double?> intervals = new List<double?>();
+            var currentObject = evenHitObjects;
+            const int interval_count = 4;
+
+            for (int i = 0; i < interval_count && currentObject != null; i++)
+            {
+                intervals.Add(currentObject.HitObjectInterval);
+                currentObject = currentObject.Previous;
+            }
+
+            intervals.RemoveAll(interval => interval == null);
+
+            // If there are fewer than 4 valid intervals, skip the consistency check.
+            if (intervals.Count < interval_count)
+                return false;
+
+            for (int i = 0; i < intervals.Count; i++)
+            {
+                for (int j = i + 1; j < intervals.Count; j++)
+                {
+                    double ratio = intervals[i]!.Value / intervals[j]!.Value;
+                    if (Math.Abs(1 - ratio) <= threshold) // If any two intervals are similar, return true.
+                        return true;
+                }
+            }
+
+            // No similar intervals were found.
+            return false;
+        }
+
+        private static double evaluateDifficultyOf(EvenHitObjects evenHitObjects, double hitWindow)
+        {
+            double intervalDifficulty = ratioDifficulty(evenHitObjects.HitObjectIntervalRatio);
+            double? previousInterval = evenHitObjects.Previous?.HitObjectInterval;
+
+            // If a previous interval exists and there are multiple hit objects in the sequence:
+            if (previousInterval != null && evenHitObjects.Children.Count > 1)
+            {
+                double expectedDurationFromPrevious = (double)previousInterval * evenHitObjects.Children.Count;
+                double durationDifference = Math.Abs(evenHitObjects.Duration - expectedDurationFromPrevious);
+
+                intervalDifficulty *= DifficultyCalculationUtils.Logistic(
+                    durationDifference / hitWindow,
+                    midpointOffset: 0.5,
+                    multiplier: 1.5,
+                    maxValue: 1);
+            }
+
+            // Penalise regular intervals within the last four intervals.
+            if (isConsistentPattern(evenHitObjects))
+            {
+                intervalDifficulty *= 0.4;
+            }
+
+            // Penalise patterns that can be hit within a single hit window.
+            intervalDifficulty *= DifficultyCalculationUtils.Logistic(
+                evenHitObjects.Duration / hitWindow,
+                midpointOffset: 0.5,
+                multiplier: 1,
+                maxValue: 1);
+
+            return intervalDifficulty;
+        }
+
+        private static double evaluateDifficultyOf(EvenPatterns evenPatterns)
+        {
+            return ratioDifficulty(evenPatterns.IntervalRatio);
+        }
+
+        public static double EvaluateDifficultyOf(DifficultyHitObject hitObject, double hitWindow)
+        {
+            TaikoDifficultyHitObjectRhythm rhythm = ((TaikoDifficultyHitObject)hitObject).Rhythm;
+            double difficulty = 0.0d;
+
+            if (rhythm.EvenHitObjects?.FirstHitObject == hitObject) // Difficulty for EvenHitObjects
+                difficulty += evaluateDifficultyOf(rhythm.EvenHitObjects, hitWindow);
+
+            if (rhythm.EvenPatterns?.FirstHitObject == hitObject) // Difficulty for EvenPatterns
+                difficulty += evaluateDifficultyOf(rhythm.EvenPatterns) * rhythm.Difficulty;
+
+            return difficulty;
+        }
+    }
+}