osu-lazer/osu.Game/Rulesets/Difficulty/Skills/StrainSkill.cs

// 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.Linq;
using osu.Game.Rulesets.Difficulty.Preprocessing;
using osu.Game.Rulesets.Mods;

namespace osu.Game.Rulesets.Difficulty.Skills
{
    /// <summary>
    /// Used to processes strain values of <see cref="DifficultyHitObject"/>s, keep track of strain levels caused by the processed objects
    /// and to calculate a final difficulty value representing the difficulty of hitting all the processed objects.
    /// </summary>
    public abstract class StrainSkill : GraphSkill
    {
        /// <summary>
        /// The weight by which each strain value decays.
        /// </summary>
        protected virtual double DecayWeight => 0.9;

        protected StrainSkill(Mod[] mods)
            : base(mods)
        {
        }

        /// <summary>
        /// Returns the strain value at <see cref="DifficultyHitObject"/>. This value is calculated with or without respect to previous objects.
        /// </summary>
        protected abstract double StrainValueAt(DifficultyHitObject current);

        /// <summary>
        /// Process a <see cref="DifficultyHitObject"/> and update current strain values accordingly.
        /// </summary>
        public sealed override void Process(DifficultyHitObject current)
        {
            // The first object doesn't generate a strain, so we begin with an incremented section end
            if (current.Index == 0)
                CurrentSectionEnd = Math.Ceiling(current.StartTime / SectionLength) * SectionLength;

            while (current.StartTime > CurrentSectionEnd)
            {
                saveCurrentPeak();
                startNewSectionFrom(CurrentSectionEnd, current);
                CurrentSectionEnd += SectionLength;
            }

            CurrentSectionPeak = Math.Max(StrainValueAt(current), CurrentSectionPeak);
        }

        /// <summary>
        /// Saves the current peak strain level to the list of strain peaks, which will be used to calculate an overall difficulty.
        /// </summary>
        private void saveCurrentPeak()
        {
            StrainPeaks.Add(CurrentSectionPeak);
        }

        /// <summary>
        /// Sets the initial strain level for a new section.
        /// </summary>
        /// <param name="time">The beginning of the new section in milliseconds.</param>
        /// <param name="current">The current hit object.</param>
        private void startNewSectionFrom(double time, DifficultyHitObject current)
        {
            // The maximum strain of the new section is not zero by default
            // This means we need to capture the strain level at the beginning of the new section, and use that as the initial peak level.
            CurrentSectionPeak = CalculateInitialStrain(time, current);
        }

        /// <summary>
        /// Retrieves the peak strain at a point in time.
        /// </summary>
        /// <param name="time">The time to retrieve the peak strain at.</param>
        /// <param name="current">The current hit object.</param>
        /// <returns>The peak strain.</returns>
        protected abstract double CalculateInitialStrain(double time, DifficultyHitObject current);

        /// <summary>
        /// Returns the calculated difficulty value representing all <see cref="DifficultyHitObject"/>s that have been processed up to this point.
        /// </summary>
        public override double DifficultyValue()
        {
            double difficulty = 0;
            double weight = 1;

            // Sections with 0 strain are excluded to avoid worst-case time complexity of the following sort (e.g. /b/2351871).
            // These sections will not contribute to the difficulty.
            var peaks = GetCurrentStrainPeaks().Where(p => p > 0);

            // Difficulty is the weighted sum of the highest strains from every section.
            // We're sorting from highest to lowest strain.
            foreach (double strain in peaks.OrderDescending())
            {
                difficulty += strain * weight;
                weight *= DecayWeight;
            }

            return difficulty;
        }
    }
}