osukey/osu.Game/Rulesets/Difficulty/DifficultyCalculator.cs

// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE

using System.Collections.Generic;
using System.Linq;
using osu.Framework.Extensions.IEnumerableExtensions;
using osu.Framework.Timing;
using osu.Game.Beatmaps;
using osu.Game.Rulesets.Mods;

namespace osu.Game.Rulesets.Difficulty
{
    public abstract class DifficultyCalculator
    {
        protected readonly IBeatmap Beatmap;
        protected readonly Mod[] Mods;

        protected double TimeRate { get; private set; } = 1;

        protected DifficultyCalculator(IBeatmap beatmap, Mod[] mods = null)
        {
            Beatmap = beatmap;
            Mods = mods ?? new Mod[0];

            ApplyMods(Mods);
        }

        protected virtual void ApplyMods(Mod[] mods)
        {
            var clock = new StopwatchClock();
            mods.OfType<IApplicableToClock>().ForEach(m => m.ApplyToClock(clock));
            TimeRate = clock.Rate;
        }

        protected virtual void PreprocessHitObjects()
        {
        }

        public abstract double Calculate(Dictionary<string, double> categoryDifficulty = null);

        protected double StrainCalculator(List<double> highestStrains, double decayWeight)
        {
            // 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 *= decayWeight;
            }
            return difficulty;
        }
    }
}