// <copyright file="Xorshift.cs" company="Math.NET">
// Math.NET Numerics, part of the Math.NET Project
// http://numerics.mathdotnet.com
// http://github.com/mathnet/mathnet-numerics
// http://mathnetnumerics.codeplex.com
//
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using MathNet.Numerics.Properties;
using System;

namespace MathNet.Numerics.Random
{
    /// <summary>
    /// Implements a multiply-with-carry Xorshift pseudo random number generator (RNG) specified in Marsaglia, George. (2003). Xorshift RNGs.
    /// <code>Xn = a * Xn−3 + c mod 2^32</code>
    /// http://www.jstatsoft.org/v08/i14/paper
    /// </summary>
    public class Xorshift : AbstractRandomNumberGenerator
    {
        /// <summary>
        /// The default value for X1.
        /// </summary>
        private const uint YSeed = 362436069;

        /// <summary>
        /// The default value for X2.
        /// </summary>
        private const uint ZSeed = 77465321;

        /// <summary>
        /// The default value for the multiplier.
        /// </summary>
        private const uint ASeed = 916905990;

        /// <summary>
        /// The default value for the carry over.
        /// </summary>
        private const uint CSeed = 13579;

        /// <summary>
        /// The multiplier to compute a double-precision floating point number [0, 1)
        /// </summary>
        private const double UlongToDoubleMultiplier = 1.0 / (uint.MaxValue + 1.0);

        /// <summary>
        /// Seed or last but three unsigned random number. 
        /// </summary>
        private ulong _x;

        /// <summary>
        /// Last but two unsigned random number. 
        /// </summary>
        private ulong _y;

        /// <summary>
        /// Last but one unsigned random number. 
        /// </summary>
        private ulong _z;

        /// <summary>
        /// The value of the carry over. 
        /// </summary>
        private ulong _c;

        /// <summary>
        /// The multiplier.
        /// </summary>
        private readonly ulong _a;

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class using
        /// the current time as the seed.
        /// </summary>
        /// <remarks>If the seed value is zero, it is set to one. Uses the
        /// value of <see cref="Control.ThreadSafeRandomNumberGenerators"/> to
        /// set whether the instance is thread safe.
        /// Uses the default values of:
        /// <list>
        /// <item>a = 916905990</item>
        /// <item>c = 13579</item>
        /// <item>X1 = 77465321</item>
        /// <item>X2 = 362436069</item>
        /// </list></remarks>
        public Xorshift() : this((int)DateTime.Now.Ticks)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class using
        /// the current time as the seed.
        /// </summary>
        /// <param name="a">The multiply value</param>
        /// <param name="c">The initial carry value.</param>
        /// <param name="x1">The initial value if X1.</param>
        /// <param name="x2">The initial value if X2.</param>
        /// <remarks>If the seed value is zero, it is set to one. Uses the
        /// value of <see cref="Control.ThreadSafeRandomNumberGenerators"/> to
        /// set whether the instance is thread safe.
        /// Note: <paramref name="c"/> must be less than <paramref name="a"/>.
        /// </remarks>
        public Xorshift(long a, long c, long x1, long x2)
            : this((int)DateTime.Now.Ticks, a, c, x1, x2)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class using
        /// the current time as the seed.
        /// </summary>
        /// <param name="threadSafe">if set to <c>true</c> , the class is thread safe.</param>
        /// <remarks>
        /// Uses the default values of:
        /// <list>
        /// <item>a = 916905990</item>
        /// <item>c = 13579</item>
        /// <item>X1 = 77465321</item>
        /// <item>X2 = 362436069</item>
        /// </list></remarks>
        public Xorshift(bool threadSafe)
            : this((int)DateTime.Now.Ticks, threadSafe)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class using
        /// the current time as the seed.
        /// </summary>
        /// <param name="threadSafe">if set to <c>true</c> , the class is thread safe.</param>
        /// <param name="a">The multiply value</param>
        /// <param name="c">The initial carry value.</param>
        /// <param name="x1">The initial value if X1.</param>
        /// <param name="x2">The initial value if X2.</param>
        /// <remarks><paramref name="c"/> must be less than <paramref name="a"/>.</remarks>
        public Xorshift(bool threadSafe, long a, long c, long x1, long x2)
            : this((int)DateTime.Now.Ticks, threadSafe, a, c, x1, x2)
        {
        }
         
        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class.
        /// </summary>
        /// <param name="seed">The seed value.</param>
        /// <remarks>If the seed value is zero, it is set to one. Uses the
        /// value of <see cref="Control.ThreadSafeRandomNumberGenerators"/> to
        /// set whether the instance is thread safe.
        /// Uses the default values of:
        /// <list>
        /// <item>a = 916905990</item>
        /// <item>c = 13579</item>
        /// <item>X1 = 77465321</item>
        /// <item>X2 = 362436069</item>
        /// </list></remarks>
        public Xorshift(int seed) : this(seed, Control.ThreadSafeRandomNumberGenerators)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class.
        /// </summary>
        /// <param name="seed">The seed value.</param>
        /// <remarks>If the seed value is zero, it is set to one. Uses the
        /// value of <see cref="Control.ThreadSafeRandomNumberGenerators"/> to
        /// set whether the instance is thread safe.</remarks>
        /// <param name="a">The multiply value</param>
        /// <param name="c">The initial carry value.</param>
        /// <param name="x1">The initial value if X1.</param>
        /// <param name="x2">The initial value if X2.</param>
        /// <remarks><paramref name="c"/> must be less than <paramref name="a"/>.</remarks>
        public Xorshift(int seed, long a, long c, long x1, long x2)
            : this(seed, Control.ThreadSafeRandomNumberGenerators, a, c, x1, x2)
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class.
        /// </summary>
        /// <param name="seed">The seed value.</param>
        /// <param name="threadSafe">if set to <c>true</c>, the class is thread safe.</param>
        /// <remarks>
        /// Uses the default values of:
        /// <list>
        /// <item>a = 916905990</item>
        /// <item>c = 13579</item>
        /// <item>X1 = 77465321</item>
        /// <item>X2 = 362436069</item>
        /// </list></remarks>
        public Xorshift(int seed, bool threadSafe) : base(threadSafe)
        {
            if (seed == 0)
            {
                seed = 1;
            }

            _x = (uint)seed;
            _y = YSeed;
            _z = ZSeed;
            _c = CSeed;
            _a = ASeed;
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="Xorshift"/> class.
        /// </summary>
        /// <param name="seed">The seed value.</param>
        /// <param name="threadSafe">if set to <c>true</c>, the class is thread safe.</param>
        /// <param name="a">The multiply value</param>
        /// <param name="c">The initial carry value.</param>
        /// <param name="x1">The initial value if X1.</param>
        /// <param name="x2">The initial value if X2.</param>
        /// <remarks><paramref name="c"/> must be less than <paramref name="a"/>.</remarks>
        public Xorshift(int seed, bool threadSafe, long a, long c, long x1, long x2)
            : base(threadSafe)
        {
            if (seed == 0)
            {
                seed = 1;
            }

            if (a <= c)
            {
                throw new ArgumentException(string.Format(Resources.ArgumentOutOfRangeGreater, "a", "c"), "a");
            }

            _x = (uint)seed;
            _y = (ulong)x1;
            _z = (ulong)x2;
            _a = (ulong)a;
            _c = (ulong)c;
        }

        /// <summary>
        /// Returns a random number between 0.0 and 1.0.
        /// </summary>
        /// <returns>
        /// A double-precision floating point number greater than or equal to 0.0, and less than 1.0.
        /// </returns>
        protected override double DoSample()
        {
            var t = (_a * _x) + _c;
            _x = _y;
            _y = _z;
            _c = t >> 32;
            _z = t & 0xffffffff;
            return _z * UlongToDoubleMultiplier;
        }
    }
}