osu/osu.Game.Modes.Osu/OsuDifficultyCalculator.cs

192 lines
9.4 KiB
C#

// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
using osu.Game.Beatmaps;
using osu.Game.Modes.Osu.Beatmaps;
using osu.Game.Modes.Osu.Objects;
using System;
using System.Collections.Generic;
namespace osu.Game.Modes.Osu
{
public class OsuDifficultyCalculator : DifficultyCalculator<OsuHitObject>
{
private const double star_scaling_factor = 0.0675;
private const double extreme_scaling_factor = 0.5;
/// <summary>
/// HitObjects are stored as a member variable.
/// </summary>
internal List<OsuHitObjectDifficulty> DifficultyHitObjects = new List<OsuHitObjectDifficulty>();
public OsuDifficultyCalculator(Beatmap beatmap) : base(beatmap)
{
}
protected override void PreprocessHitObjects()
{
foreach (var h in Objects)
if (h.Type == HitObjectType.Slider)
((Slider)h).Curve.Calculate();
}
protected override double CalculateInternal(Dictionary<string, string> categoryDifficulty)
{
// Fill our custom DifficultyHitObject class, that carries additional information
DifficultyHitObjects.Clear();
foreach (var hitObject in Objects)
DifficultyHitObjects.Add(new OsuHitObjectDifficulty(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 speedDifficulty = CalculateDifficulty(DifficultyType.Speed);
double aimDifficulty = CalculateDifficulty(DifficultyType.Aim);
// OverallDifficulty is not considered in this algorithm and neither is HpDrainRate. That means, that in this form the algorithm determines how hard it physically is
// to play the map, assuming, that too much of an error will not lead to a death.
// It might be desirable to include OverallDifficulty into map difficulty, but in my personal opinion it belongs more to the weighting of the actual peformance
// and is superfluous in the beatmap difficulty rating.
// If it were to be considered, then I would look at the hit window of normal HitCircles only, since Sliders and Spinners are (almost) "free" 300s and take map length
// into account as well.
// The difficulty can be scaled by any desired metric.
// In osu!tp it gets squared to account for the rapid increase in difficulty as the limit of a human is approached. (Of course it also gets scaled afterwards.)
// It would not be suitable for a star rating, therefore:
// The following is a proposal to forge a star rating from 0 to 5. It consists of taking the square root of the difficulty, since by simply scaling the easier
// 5-star maps would end up with one star.
double speedStars = Math.Sqrt(speedDifficulty) * star_scaling_factor;
double aimStars = Math.Sqrt(aimDifficulty) * star_scaling_factor;
if (categoryDifficulty != null)
{
categoryDifficulty.Add("Aim", aimStars.ToString("0.00"));
categoryDifficulty.Add("Speed", speedStars.ToString("0.00"));
double hitWindow300 = 30/*HitObjectManager.HitWindow300*/ / TimeRate;
double preEmpt = 450/*HitObjectManager.PreEmpt*/ / TimeRate;
categoryDifficulty.Add("OD", (-(hitWindow300 - 80.0) / 6.0).ToString("0.00"));
categoryDifficulty.Add("AR", (preEmpt > 1200.0 ? -(preEmpt - 1800.0) / 120.0 : -(preEmpt - 1200.0) / 150.0 + 5.0).ToString("0.00"));
int maxCombo = 0;
foreach (OsuHitObjectDifficulty hitObject in DifficultyHitObjects)
maxCombo += hitObject.MaxCombo;
categoryDifficulty.Add("Max combo", maxCombo.ToString());
}
// Again, from own observations and from the general opinion of the community a map with high speed and low aim (or vice versa) difficulty is harder,
// than a map with mediocre difficulty in both. Therefore we can not just add both difficulties together, but will introduce a scaling that favors extremes.
double starRating = speedStars + aimStars + Math.Abs(speedStars - aimStars) * extreme_scaling_factor;
// Another approach to this would be taking Speed and Aim separately to a chosen power, which again would be equivalent. This would be more convenient if
// the hit window size is to be considered as well.
// Note: The star rating is tuned extremely tight! Airman (/b/104229) and Freedom Dive (/b/126645), two of the hardest ranked maps, both score ~4.66 stars.
// Expect the easier kind of maps that officially get 5 stars to obtain around 2 by this metric. The tutorial still scores about half a star.
// Tune by yourself as you please. ;)
return starRating;
}
protected bool CalculateStrainValues()
{
// Traverse hitObjects in pairs to calculate the strain value of NextHitObject from the strain value of CurrentHitObject and environment.
using (List<OsuHitObjectDifficulty>.Enumerator hitObjectsEnumerator = DifficultyHitObjects.GetEnumerator())
{
if (!hitObjectsEnumerator.MoveNext()) return false;
OsuHitObjectDifficulty current = hitObjectsEnumerator.Current;
// 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;
}
return true;
}
}
/// <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>
protected const double STRAIN_STEP = 400;
/// <summary>
/// The weighting of each strain value decays to this number * it's previous value
/// </summary>
protected const double DECAY_WEIGHT = 0.9;
protected double CalculateDifficulty(DifficultyType type)
{
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
OsuHitObjectDifficulty previousHitObject = null;
foreach (OsuHitObjectDifficulty 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(OsuHitObjectDifficulty.DECAY_BASE[(int)type], (intervalEndTime - previousHitObject.BaseHitObject.StartTime) / 1000);
maximumStrain = previousHitObject.Strains[(int)type] * decay;
}
// Go to the next time interval
intervalEndTime += actualStrainStep;
}
// Obtain maximum strain
maximumStrain = Math.Max(hitObject.Strains[(int)type], 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 IBeatmapConverter<OsuHitObject> CreateBeatmapConverter() => new OsuBeatmapConverter();
// Those values are used as array indices. Be careful when changing them!
public enum DifficultyType
{
Speed = 0,
Aim,
};
}
}