osu-lazer/osu.Game.Rulesets.Osu/Difficulty/Skills/Speed.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 osu.Game.Rulesets.Difficulty.Preprocessing;
using osu.Game.Rulesets.Mods;
using osu.Game.Rulesets.Osu.Difficulty.Preprocessing;
using osu.Game.Rulesets.Osu.Objects;

namespace osu.Game.Rulesets.Osu.Difficulty.Skills
{
    /// <summary>
    /// Represents the skill required to press keys with regards to keeping up with the speed at which objects need to be hit.
    /// </summary>
    public class Speed : OsuStrainSkill
    {
        private const double single_spacing_threshold = 125;

        private const double angle_bonus_begin = 5 * Math.PI / 6;
        private const double pi_over_4 = Math.PI / 4;
        private const double pi_over_2 = Math.PI / 2;

        private const double rhythm_multiplier = 2.5;
        private const int history_time_max = 3000; // 3 seconds of calculatingRhythmBonus max.

        private double skillMultiplier => 1375;
        private double strainDecayBase => 0.3;

        private double currentTapStrain = 1;
        private double currentMovementStrain = 1;
        private double currentRhythm = 1;

        protected override int ReducedSectionCount => 5;
        protected override double DifficultyMultiplier => 1.04;

        private const double min_speed_bonus = 75; // ~200BPM
        private const double max_speed_bonus = 45; // ~330BPM
        private const double speed_balancing_factor = 40;

        protected override int HistoryLength => 32;

        public Speed(Mod[] mods)
            : base(mods)
        {
        }

        private bool isRatioEqual(double ratio, double a, double b)
        {
            return a + 15 > ratio * b && a - 15 < ratio * b;
        }

        /// <summary>
        /// Calculates a rhythm multiplier for the difficulty of the tap associated with historic data of the current <see cref="OsuDifficultyHitObject"/>.
        /// </summary>
        private double calculateRhythmBonus(DifficultyHitObject current)
        {
            if (current.BaseObject is Spinner)
                return 0;

            int previousIslandSize = -1;
            double rhythmComplexitySum = 0;
            int islandSize = 0;

            bool firstDeltaSwitch = false;

            for (int i = Previous.Count - 2; i > 0; i--)
            {
                double currHistoricalDecay = Math.Max(0, (history_time_max - (current.StartTime - Previous[i - 1].StartTime))) / history_time_max; // scales note 0 to 1 from history to now

                if (currHistoricalDecay != 0)
                {
                    currHistoricalDecay = Math.Min(currHistoricalDecay, (double)(Previous.Count - i) / Previous.Count); // either we're limited by time or limited by object count.

                    double currDelta = ((OsuDifficultyHitObject)Previous[i - 1]).StrainTime;
                    double prevDelta = ((OsuDifficultyHitObject)Previous[i]).StrainTime;
                    double prevPrevDelta = ((OsuDifficultyHitObject)Previous[i + 1]).StrainTime;
                    double effectiveRatio = Math.Min(prevDelta, currDelta) / Math.Max(prevDelta, currDelta);

                    if (effectiveRatio > 0.5)
                        effectiveRatio = 0.5 + (effectiveRatio - 0.5) * 10; // large buff for 1/3 -> 1/4 type transitions.

                    effectiveRatio *= Math.Sqrt(100 / ((currDelta + prevDelta) / 2)) * currHistoricalDecay; // scale with bpm slightly and with time

                    if (firstDeltaSwitch)
                    {
                        if (isRatioEqual(1.0, prevDelta, currDelta))
                        {
                            islandSize++; // island is still progressing, count size.
                        }
                        else
                        {
                            if (islandSize > 6)
                                islandSize = 6;

                            if (Previous[i - 1].BaseObject is Slider) // bpm change is into slider, this is easy acc window
                                effectiveRatio *= 0.25;

                            if (Previous[i].BaseObject is Slider) // bpm change was from a slider, this is easier typically than circle -> circle
                                effectiveRatio *= 0.5;

                            if (previousIslandSize == islandSize) // repeated island size (ex: triplet -> triplet)
                                effectiveRatio *= 0.25;

                            if (prevPrevDelta > prevDelta + 10 && prevDelta > currDelta + 10) // previous increase happened a note ago, 1/1->1/2-1/4, dont want to buff this.
                                effectiveRatio *= 0.125;

                            rhythmComplexitySum += effectiveRatio;

                            previousIslandSize = islandSize; // log the last island size.

                            if (prevDelta * 1.25 < currDelta) // we're slowing down, stop counting
                                firstDeltaSwitch = false; // if we're speeding up, this stays true and  we keep counting island size.

                            islandSize = 0;
                        }
                    }
                    else if (prevDelta > 1.25 * currDelta) // we want to be speeding up.
                    {
                        // Begin counting island until we change speed again.
                        firstDeltaSwitch = true;
                        islandSize = 0;
                    }
                }
            }

            return Math.Sqrt(4 + rhythmComplexitySum * rhythm_multiplier) / 2; //produces multiplier that can be applied to strain. range [1, infinity)
        }

        private double tapStrainOf(DifficultyHitObject current, double speedBonus)
        {
            if (current.BaseObject is Spinner)
                return 0;

            var osuCurrObj = (OsuDifficultyHitObject)current;

            return speedBonus / osuCurrObj.StrainTime;
        }

        private double movementStrainOf(DifficultyHitObject current, double speedBonus)
        {
            if (current.BaseObject is Spinner)
                return 0;

            var osuCurrObj = (OsuDifficultyHitObject)current;

            double distance = Math.Min(single_spacing_threshold, osuCurrObj.TravelDistance + osuCurrObj.JumpDistance);

            double angleBonus = 1.0;

            if (osuCurrObj.Angle != null)
            {
                double angle = osuCurrObj.Angle.Value;

                if (angle < pi_over_2)
                    angleBonus = 1.25;
                else if (angle < angle_bonus_begin)
                    angleBonus = 1 + Math.Pow(Math.Sin(1.5 * (angle_bonus_begin - angle)), 2) / 4;
            }

            return (angleBonus * speedBonus * Math.Pow(distance / single_spacing_threshold, 3.5)) / osuCurrObj.StrainTime;
        }

        private double strainDecay(double ms) => Math.Pow(strainDecayBase, ms / 1000);

        protected override double CalculateInitialStrain(double time) => (currentMovementStrain + currentTapStrain * currentRhythm) * strainDecay(time - Previous[0].StartTime);

        protected override double StrainValueAt(DifficultyHitObject current)
        {
            double speedBonus = 1.0;
            double deltaTime = Math.Max(max_speed_bonus, current.DeltaTime);

            if (deltaTime < min_speed_bonus)
                speedBonus = 1 + 0.75 * Math.Pow((min_speed_bonus - deltaTime) / speed_balancing_factor, 2);

            currentRhythm = calculateRhythmBonus(current);

            currentTapStrain *= strainDecay(current.DeltaTime);
            currentTapStrain += tapStrainOf(current, speedBonus) * skillMultiplier;

            currentMovementStrain *= strainDecay(current.DeltaTime);
            currentMovementStrain += movementStrainOf(current, speedBonus) * skillMultiplier;

            return currentMovementStrain + currentTapStrain * currentRhythm;
        }
    }
}