using System;

namespace NLangDetect.Core.Extensions
{
    public static class RandomExtensions
    {
        private const double _Epsilon = 2.22044604925031E-15;

        private static readonly object _mutex = new object();

        private static double _nextNextGaussian;
        private static bool _hasNextNextGaussian;

        /// <summary>
        /// Returns the next pseudorandom, Gaussian ("normally") distributed double value with mean 0.0 and standard deviation 1.0 from this random number generator's sequence.
        /// The general contract of nextGaussian is that one double value, chosen from (approximately) the usual normal distribution with mean 0.0 and standard deviation 1.0, is pseudorandomly generated and returned.
        /// </summary>
        /// <remarks>
        /// Taken from: http://download.oracle.com/javase/6/docs/api/java/util/Random.html (nextGaussian())
        /// </remarks>
        public static double NextGaussian(this Random random)
        {
            lock (_mutex)
            {
                if (_hasNextNextGaussian)
                {
                    _hasNextNextGaussian = false;

                    return _nextNextGaussian;
                }

                double v1, v2, s;

                do
                {
                    v1 = 2.0 * random.NextDouble() - 1.0; // between -1.0 and 1.0
                    v2 = 2.0 * random.NextDouble() - 1.0; // between -1.0 and 1.0
                    s = v1 * v1 + v2 * v2;
                }
                while (s >= 1.0 || Math.Abs(s - 0.0) < _Epsilon);

                double multiplier = Math.Sqrt(-2.0 * Math.Log(s) / s);

                _nextNextGaussian = v2 * multiplier;
                _hasNextNextGaussian = true;

                return v1 * multiplier;
            }
        }
    }
}