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osu-lazer/osu.Game.Rulesets.Taiko/TaikoDifficultyCalculator.cs

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// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
using osu.Game.Beatmaps;
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using osu.Game.Rulesets.Taiko.Beatmaps;
using osu.Game.Rulesets.Taiko.Objects;
using System.Collections.Generic;
using System;
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namespace osu.Game.Rulesets.Taiko
{
internal class TaikoDifficultyCalculator : DifficultyCalculator<TaikoHitObject>
{
private const double star_scaling_factor = 0.04125;
/// <summary>
/// In milliseconds. For difficulty calculation we will only look at the highest strain value in each time interval of size STRAIN_STEP.
/// This is to eliminate higher influence of stream over aim by simply having more HitObjects with high strain.
/// The higher this value, the less strains there will be, indirectly giving long beatmaps an advantage.
/// </summary>
private const double strain_step = 400;
/// <summary>
/// The weighting of each strain value decays to this number * it's previous value
/// </summary>
private const double decay_weight = 0.9;
/// <summary>
/// HitObjects are stored as a member variable.
/// </summary>
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private readonly List<TaikoHitObjectDifficulty> difficultyHitObjects = new List<TaikoHitObjectDifficulty>();
public TaikoDifficultyCalculator(Beatmap beatmap)
: base(beatmap)
{
}
public override double Calculate(Dictionary<string, double> categoryDifficulty = null)
{
// Fill our custom DifficultyHitObject class, that carries additional information
difficultyHitObjects.Clear();
foreach (var hitObject in Beatmap.HitObjects)
difficultyHitObjects.Add(new TaikoHitObjectDifficulty(hitObject));
// Sort DifficultyHitObjects by StartTime of the HitObjects - just to make sure.
difficultyHitObjects.Sort((a, b) => a.BaseHitObject.StartTime.CompareTo(b.BaseHitObject.StartTime));
if (!calculateStrainValues()) return 0;
double starRating = calculateDifficulty() * star_scaling_factor;
if (categoryDifficulty != null)
{
categoryDifficulty.Add("Strain", starRating);
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categoryDifficulty.Add("Hit window 300", 35 /*HitObjectManager.HitWindow300*/ / TimeRate);
}
return starRating;
}
private bool calculateStrainValues()
{
// Traverse hitObjects in pairs to calculate the strain value of NextHitObject from the strain value of CurrentHitObject and environment.
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using (List<TaikoHitObjectDifficulty>.Enumerator hitObjectsEnumerator = difficultyHitObjects.GetEnumerator())
{
if (!hitObjectsEnumerator.MoveNext()) return false;
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TaikoHitObjectDifficulty current = hitObjectsEnumerator.Current;
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// First hitObject starts at strain 1. 1 is the default for strain values, so we don't need to set it here. See DifficultyHitObject.
while (hitObjectsEnumerator.MoveNext())
{
var next = hitObjectsEnumerator.Current;
next?.CalculateStrains(current, TimeRate);
current = next;
}
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return true;
}
}
private double calculateDifficulty()
{
double actualStrainStep = strain_step * TimeRate;
// Find the highest strain value within each strain step
List<double> highestStrains = new List<double>();
double intervalEndTime = actualStrainStep;
double maximumStrain = 0; // We need to keep track of the maximum strain in the current interval
TaikoHitObjectDifficulty previousHitObject = null;
foreach (var hitObject in difficultyHitObjects)
{
// While we are beyond the current interval push the currently available maximum to our strain list
while (hitObject.BaseHitObject.StartTime > intervalEndTime)
{
highestStrains.Add(maximumStrain);
// The maximum strain of the next interval is not zero by default! We need to take the last hitObject we encountered, take its strain and apply the decay
// until the beginning of the next interval.
if (previousHitObject == null)
{
maximumStrain = 0;
}
else
{
double decay = Math.Pow(TaikoHitObjectDifficulty.DECAY_BASE, (intervalEndTime - previousHitObject.BaseHitObject.StartTime) / 1000);
maximumStrain = previousHitObject.Strain * decay;
}
// Go to the next time interval
intervalEndTime += actualStrainStep;
}
// Obtain maximum strain
maximumStrain = Math.Max(hitObject.Strain, maximumStrain);
previousHitObject = hitObject;
}
// Build the weighted sum over the highest strains for each interval
double difficulty = 0;
double weight = 1;
highestStrains.Sort((a, b) => b.CompareTo(a)); // Sort from highest to lowest strain.
foreach (double strain in highestStrains)
{
difficulty += weight * strain;
weight *= decay_weight;
}
return difficulty;
}
protected override BeatmapConverter<TaikoHitObject> CreateBeatmapConverter(Beatmap beatmap) => new TaikoBeatmapConverter(true);
}
}