/*
 * Copyright 2016 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.Common;

/// <summary>
/// Signals when a sample should be taken from a stream of data to select a
/// uniformly distributed fraction of the data.
/// </summary>
public class FixedRatioSampler
{
    /// <summary>
    /// Sampling occurs if the proportion of samples to pulses drops below this number.
    /// </summary>
    private readonly double _ratio;

    private long _numPulses = 0;
    private long _numSamples = 0;

    /// <summary>
    /// Creates a fixed ratio sampler with the given ratio.
    /// </summary>
    /// <param name="ratio">Ratio between 0.0 and 1.0. Sampling occurs if the proportion of samples to pulses drops below this number.</param>
    public FixedRatioSampler(double ratio)
    {
        if (ratio < 0.0 || ratio > 1.0)
        {
            throw new ArgumentException($"Ratio must be between 0.0 and 1.0, got {ratio}", nameof(ratio));
        }
        
        if (ratio == 0.0)
        {
            // Warning: FixedRatioSampler is dropping all data
        }
        
        _ratio = ratio;
    }

    /// <summary>
    /// Returns true if this pulse should result in a sample.
    /// </summary>
    public bool Pulse()
    {
        _numPulses++;
        if (_numSamples / _numPulses < _ratio)
        {
            _numSamples++;
            return true;
        }
        return false;
    }

    /// <summary>
    /// Returns a debug string describing the current ratio of samples to pulses.
    /// </summary>
    public string DebugString()
    {
        var percentage = _numPulses > 0 ? 100.0 * _numSamples / _numPulses : 0.0;
        return $"{_numSamples} ({percentage:F2}%)";
    }
}