osu-lazer/osu.Game/Rulesets/Difficulty/DifficultyCalculator.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.Collections.Generic;
using System.Linq;
using osu.Framework.Audio.Track;
using osu.Framework.Extensions.IEnumerableExtensions;
using osu.Game.Beatmaps;
using osu.Game.Rulesets.Difficulty.Preprocessing;
using osu.Game.Rulesets.Difficulty.Skills;
using osu.Game.Rulesets.Mods;
using osu.Game.Rulesets.Objects;

namespace osu.Game.Rulesets.Difficulty
{
    public abstract class DifficultyCalculator
    {
        /// <summary>
        /// The length of each strain section.
        /// </summary>
        protected virtual int SectionLength => 400;

        private readonly Ruleset ruleset;
        private readonly WorkingBeatmap beatmap;

        protected DifficultyCalculator(Ruleset ruleset, WorkingBeatmap beatmap)
        {
            this.ruleset = ruleset;
            this.beatmap = beatmap;
        }

        /// <summary>
        /// Calculates the difficulty of the beatmap using a specific mod combination.
        /// </summary>
        /// <param name="mods">The mods that should be applied to the beatmap.</param>
        /// <returns>A structure describing the difficulty of the beatmap.</returns>
        public DifficultyAttributes Calculate(params Mod[] mods)
        {
            mods = mods.Select(m => m.CreateCopy()).ToArray();

            IBeatmap playableBeatmap = beatmap.GetPlayableBeatmap(ruleset.RulesetInfo, mods);

            var track = new TrackVirtual(10000);
            mods.OfType<IApplicableToTrack>().ForEach(m => m.ApplyToTrack(track));

            return calculate(playableBeatmap, mods, track.Rate);
        }

        /// <summary>
        /// Calculates the difficulty of the beatmap using all mod combinations applicable to the beatmap.
        /// </summary>
        /// <returns>A collection of structures describing the difficulty of the beatmap for each mod combination.</returns>
        public IEnumerable<DifficultyAttributes> CalculateAll()
        {
            foreach (var combination in CreateDifficultyAdjustmentModCombinations())
            {
                if (combination is MultiMod multi)
                    yield return Calculate(multi.Mods);
                else
                    yield return Calculate(combination);
            }
        }

        private DifficultyAttributes calculate(IBeatmap beatmap, Mod[] mods, double clockRate)
        {
            var skills = CreateSkills(beatmap);

            if (!beatmap.HitObjects.Any())
                return CreateDifficultyAttributes(beatmap, mods, skills, clockRate);

            var difficultyHitObjects = SortObjects(CreateDifficultyHitObjects(beatmap, clockRate)).ToList();

            double sectionLength = SectionLength * clockRate;

            // The first object doesn't generate a strain, so we begin with an incremented section end
            double currentSectionEnd = Math.Ceiling(beatmap.HitObjects.First().StartTime / sectionLength) * sectionLength;

            foreach (DifficultyHitObject h in difficultyHitObjects)
            {
                while (h.BaseObject.StartTime > currentSectionEnd)
                {
                    foreach (Skill s in skills)
                    {
                        s.SaveCurrentPeak();
                        s.StartNewSectionFrom(currentSectionEnd);
                    }

                    currentSectionEnd += sectionLength;
                }

                foreach (Skill s in skills)
                    s.Process(h);
            }

            // The peak strain will not be saved for the last section in the above loop
            foreach (Skill s in skills)
                s.SaveCurrentPeak();

            return CreateDifficultyAttributes(beatmap, mods, skills, clockRate);
        }

        /// <summary>
        /// Sorts a given set of <see cref="DifficultyHitObject"/>s.
        /// </summary>
        /// <param name="input">The <see cref="DifficultyHitObject"/>s to sort.</param>
        /// <returns>The sorted <see cref="DifficultyHitObject"/>s.</returns>
        protected virtual IEnumerable<DifficultyHitObject> SortObjects(IEnumerable<DifficultyHitObject> input)
            => input.OrderBy(h => h.BaseObject.StartTime);

        /// <summary>
        /// Creates all <see cref="Mod"/> combinations which adjust the <see cref="Beatmap"/> difficulty.
        /// </summary>
        public Mod[] CreateDifficultyAdjustmentModCombinations()
        {
            return createDifficultyAdjustmentModCombinations(DifficultyAdjustmentMods, Array.Empty<Mod>()).ToArray();

            static IEnumerable<Mod> createDifficultyAdjustmentModCombinations(ReadOnlyMemory<Mod> remainingMods, IEnumerable<Mod> currentSet, int currentSetCount = 0)
            {
                // Return the current set.
                switch (currentSetCount)
                {
                    case 0:
                        // Initial-case: Empty current set
                        yield return new ModNoMod();

                        break;

                    case 1:
                        yield return currentSet.Single();

                        break;

                    default:
                        yield return new MultiMod(currentSet.ToArray());

                        break;
                }

                // Apply the rest of the remaining mods recursively.
                for (int i = 0; i < remainingMods.Length; i++)
                {
                    var (nextSet, nextCount) = flatten(remainingMods.Span[i]);

                    // Check if any mods in the next set are incompatible with any of the current set.
                    if (currentSet.SelectMany(m => m.IncompatibleMods).Any(c => nextSet.Any(c.IsInstanceOfType)))
                        continue;

                    // Check if any mods in the next set are the same type as the current set. Mods of the exact same type are not incompatible with themselves.
                    if (currentSet.Any(c => nextSet.Any(n => c.GetType() == n.GetType())))
                        continue;

                    // If all's good, attach the next set to the current set and recurse further.
                    foreach (var combo in createDifficultyAdjustmentModCombinations(remainingMods.Slice(i + 1), currentSet.Concat(nextSet), currentSetCount + nextCount))
                        yield return combo;
                }
            }

            // Flattens a mod hierarchy (through MultiMod) as an IEnumerable<Mod>
            static (IEnumerable<Mod> set, int count) flatten(Mod mod)
            {
                if (!(mod is MultiMod multi))
                    return (mod.Yield(), 1);

                IEnumerable<Mod> set = Enumerable.Empty<Mod>();
                int count = 0;

                foreach (var nested in multi.Mods)
                {
                    var (nestedSet, nestedCount) = flatten(nested);
                    set = set.Concat(nestedSet);
                    count += nestedCount;
                }

                return (set, count);
            }
        }

        /// <summary>
        /// Retrieves all <see cref="Mod"/>s which adjust the <see cref="Beatmap"/> difficulty.
        /// </summary>
        protected virtual Mod[] DifficultyAdjustmentMods => Array.Empty<Mod>();

        /// <summary>
        /// Creates <see cref="DifficultyAttributes"/> to describe beatmap's calculated difficulty.
        /// </summary>
        /// <param name="beatmap">The <see cref="IBeatmap"/> whose difficulty was calculated.</param>
        /// <param name="mods">The <see cref="Mod"/>s that difficulty was calculated with.</param>
        /// <param name="skills">The skills which processed the beatmap.</param>
        /// <param name="clockRate">The rate at which the gameplay clock is run at.</param>
        protected abstract DifficultyAttributes CreateDifficultyAttributes(IBeatmap beatmap, Mod[] mods, Skill[] skills, double clockRate);

        /// <summary>
        /// Enumerates <see cref="DifficultyHitObject"/>s to be processed from <see cref="HitObject"/>s in the <see cref="IBeatmap"/>.
        /// </summary>
        /// <param name="beatmap">The <see cref="IBeatmap"/> providing the <see cref="HitObject"/>s to enumerate.</param>
        /// <param name="clockRate">The rate at which the gameplay clock is run at.</param>
        /// <returns>The enumerated <see cref="DifficultyHitObject"/>s.</returns>
        protected abstract IEnumerable<DifficultyHitObject> CreateDifficultyHitObjects(IBeatmap beatmap, double clockRate);

        /// <summary>
        /// Creates the <see cref="Skill"/>s to calculate the difficulty of an <see cref="IBeatmap"/>.
        /// </summary>
        /// <param name="beatmap">The <see cref="IBeatmap"/> whose difficulty will be calculated.</param>
        /// <returns>The <see cref="Skill"/>s.</returns>
        protected abstract Skill[] CreateSkills(IBeatmap beatmap);
    }
}