/*
 * Copyright 2018 The Cartographer Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace CartographerSharp.Metrics;

/// <summary>
/// Histogram metric for tracking value distributions.
/// </summary>
public abstract class Histogram
{
    /// <summary>
    /// Bucket boundaries for histogram.
    /// </summary>
    public class BucketBoundaries : List<double>
    {
        public BucketBoundaries() : base() { }
        public BucketBoundaries(IEnumerable<double> collection) : base(collection) { }
    }

    /// <summary>
    /// Histogram instance that does nothing. Safe for use in static initializers.
    /// </summary>
    public static Histogram Null()
    {
        return new NullHistogram();
    }

    /// <summary>
    /// Creates fixed-width bucket boundaries.
    /// </summary>
    public static BucketBoundaries FixedWidth(double width, int numFiniteBuckets)
    {
        var result = new BucketBoundaries();
        double boundary = 0;
        for (int i = 0; i < numFiniteBuckets; i++)
        {
            boundary += width;
            result.Add(boundary);
        }
        return result;
    }

    /// <summary>
    /// Creates scaled powers-of bucket boundaries.
    /// </summary>
    public static BucketBoundaries ScaledPowersOf(double baseValue, double scaleFactor, double maxValue)
    {
        if (baseValue <= 1)
            throw new ArgumentException("Base must be greater than 1", nameof(baseValue));
        if (scaleFactor <= 0)
            throw new ArgumentException("Scale factor must be greater than 0", nameof(scaleFactor));

        var result = new BucketBoundaries();
        double boundary = scaleFactor;
        while (boundary < maxValue)
        {
            result.Add(boundary);
            boundary *= baseValue;
        }
        return result;
    }

    /// <summary>
    /// Observes a value in the histogram.
    /// </summary>
    public abstract void Observe(double value);
}

/// <summary>
/// Null implementation of histogram that does nothing.
/// </summary>
internal class NullHistogram : Histogram
{
    public override void Observe(double value)
    {
        // Do nothing
    }
}