using System;

namespace TBL.CSharp.Utilities.Random.Generator
{
    /// <summary>
    /// 高斯随机数发生器
    /// <para>需要正态分布的随机数序列时，适用此设施</para>
    /// <para>内部使用 <see cref="SimpleGenerator"/> 作为辅助熵</para>
    /// </summary>
    public class GaussianGenerator : IRandom<GaussianGenerator.Seed>
    {
        /// <summary>
        /// 种子值
        /// </summary>
        private int _seed;

        /// <summary>
        /// 辅助熵
        /// </summary>
        private readonly SimpleGenerator _random;

        /// <summary>
        /// 已经生成的随机数计数
        /// </summary>
        private long _numberGenerated;

        /// <summary>
        /// 下一个高斯随机数
        /// </summary>
        private double _nextGaussian;

        /// <summary>
        /// 是否使用末项
        /// </summary>
        private bool _useLast = true;

        /// <summary>
        /// 从种子构造
        /// </summary>
        /// <param name="seed">种子值</param>
        public GaussianGenerator(int seed)
        {
            _seed = seed;
            _random = new SimpleGenerator(seed);
        }

        public int NextRaw()
        {
            _numberGenerated++;
            const double num1 = 3.5;
            var num2 = (int)BoxMuller(0 + int.MaxValue / 2.0, int.MaxValue / 2.0 / num1);
            return num2;
        }

        public Seed DumpSeed() => new Seed(_seed, _numberGenerated);

        public void RestoreBySeed(Seed seed)
        {
            _seed = seed.SeedValue;
            _random.RestoreBySeed(new SimpleGenerator.Seed(seed.SeedValue));
            _numberGenerated = 0L;
            _nextGaussian = 0.0;
            _useLast = true;
            for (long index = 0; index < seed.NumberGenerated; ++index)
                this.Next();
        }

        /// <summary>
        /// Box Muller 算法
        /// </summary>
        private double BoxMuller()
        {
            if (_useLast)
            {
                _useLast = false;
                return _nextGaussian;
            }

            double num1;
            double num2;
            double d;
            do
            {
                num1 = 2.0 * _random.NextDouble() - 1.0;
                num2 = 2.0 * _random.NextDouble() - 1.0;
                d = num1 * num1 + num2 * num2;
            } while (d >= 1.0 || d == 0.0);

            double num3 = Math.Sqrt(-2.0 * Math.Log(d) / d);
            _nextGaussian = num2 * num3;
            _useLast = true;
            return num1 * num3;
        }

        /// <summary>
        /// Box Muller 算法
        /// </summary>
        private double BoxMuller(double mean, double standardDeviation) => mean + BoxMuller() * standardDeviation;

        /// <summary>
        /// 种子
        /// </summary>
        public readonly struct Seed
        {
            /// <summary>
            /// 种子值
            /// </summary>
            public readonly int SeedValue;

            /// <summary>
            /// 已经生成的随机数计数
            /// </summary>
            public readonly long NumberGenerated;

            /// <summary>
            /// 构造
            /// </summary>
            public Seed(int seed, long numberGenerated)
            {
                SeedValue = seed;
                NumberGenerated = numberGenerated;
            }
        }
    }
}