Colour compression preprocessing implementation

osu.Game.Rulesets.Taiko/Difficulty/Preprocessing/TaikoDifficultyHitObjectColour.cs

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+namespace osu.Game.Rulesets.Taiko.Difficulty.Preprocessing
+{
+    /// <summary>
+    /// Stores colour compression information for a <see cref="TaikoDifficultyHitObject"/>.
+    /// </summary>
+    public class TaikoDifficultyHitObjectColour
+    {
+        const int max_repetition_interval = 16;
+
+        private TaikoDifficultyHitObjectColour previous;
+
+        /// <summary>
+        /// True if the current colour is different from the previous colour.
+        /// </summary>
+        public bool Delta { get; private set; }
+
+        /// <summary>
+        /// How many notes are Delta repeated
+        /// </summary>
+        public int DeltaRunLength { get; private set; }
+
+        /// <summary>
+        /// How many notes between the current and previous identical <see cref="TaikoDifficultyHitObjectColour"/>.
+        /// Negative number means that there is no repetition in range.
+        /// </summary>
+        public int RepetitionInterval { get; private set; }
+
+        /// <summary>
+        /// Get the <see cref="TaikoDifficultyHitObjectColour"/> instance for the given hitObject. This is implemented
+        /// as a static function instead of constructor to allow for reusing existing instances.
+        /// TODO: findRepetitionInterval needs to be called a final time after all hitObjects have been processed.
+        /// </summary>
+        public static TaikoDifficultyHitObjectColour GetInstanceFor(
+            TaikoDifficultyHitObject hitObject, TaikoDifficultyHitObject lastObject, TaikoDifficultyHitObjectColour previous)
+        {
+            bool delta = lastObject == null || hitObject.HitType != lastObject.HitType;
+            if (delta == previous.Delta)
+            {
+                previous.DeltaRunLength += 1;
+                return previous;
+            }
+            else
+            {
+                // Calculate RepetitionInterval for previous
+                previous.RepetitionInterval = findRepetitionInterval(previous);
+
+                return new TaikoDifficultyHitObjectColour()
+                {
+                    Delta = delta,
+                    DeltaRunLength = 1,
+                    RepetitionInterval = -1,
+                    previous = previous
+                };
+            }
+        }
+
+        /// <summary>
+        /// Finds the closest previous <see cref="TaikoDifficultyHitObjectColour"/> that has the identical delta value 
+        /// and run length to target, and returns the amount of notes between them.
+        /// </summary>
+        private static int findRepetitionInterval(TaikoDifficultyHitObjectColour target) {
+            if (target.previous == null || target.previous.previous == null)
+                return -1;
+
+            int interval = target.previous.DeltaRunLength;
+            TaikoDifficultyHitObjectColour other = target.previous.previous;
+            while(other != null && interval < max_repetition_interval) {
+                if (other.Delta == target.Delta && other.DeltaRunLength == target.DeltaRunLength)
+                    return interval;
+                else
+                    interval += other.DeltaRunLength;
+                other = other.previous;
+            }
+
+            return -1;
+        }
+    }
+}