using System;

namespace RobotAndDonkey.Game.Utils;

/// <summary>
///     Implements a XorShift* PRNG, with 64 bits of internal state.
///     See http://en.wikipedia.org/wiki/Xorshift
/// </summary>
public struct SRandom
{
    public SRandom(ulong seed)
    {
        if (seed == 0)
            throw new InvalidOperationException("Seed needs to be bigger than zero.");

        Seed = seed;
    }

    public void Shuffle<T>(Span<T> span)
    {
        var n = span.Length;

        for (var i = 0; i < n; i++)
        {
            var r = i + Next(n - i);
            (span[r], span[i]) = (span[i], span[r]);
        }
    }

    public ulong Next()
    {
        Seed ^= Seed >> 12;
        Seed ^= Seed << 25;
        Seed ^= Seed >> 27;
        return Seed * 2685821657736338717UL; // multiplier taken from wikipedia article on XorShift PRNGs
    }

    public ulong Next(ulong upperLimit)
    {
        if (upperLimit == 0)
            return 0;

        return Next() % upperLimit;
    }

    public uint Next(uint upperLimit)
    {
        if (upperLimit == 0)
            return 0;

        return (uint)(Next() % upperLimit);
    }

    public uint Next(uint lowerLimit, uint upperLimit)
    {
        if (upperLimit == 0)
            return 0;

        if (lowerLimit == upperLimit)
            return lowerLimit;

        return lowerLimit + (uint)(Next() % (upperLimit - lowerLimit));
    }

    public int Next(int upperLimit)
    {
        if (upperLimit == 0)
            return 0;

        return (int)(Next() % (uint)(upperLimit & 0x7fffffff));
    }

    public int Next(int lowerLimit, int upperLimit)
    {
        if (upperLimit == 0)
            return 0;

        if (lowerLimit == upperLimit)
            return lowerLimit;

        return lowerLimit + (int)(Next() % (uint)((upperLimit - lowerLimit) & 0x7fffffff));
    }

    public double NextDouble()
    {
        return Next(int.MaxValue) * (1.0 / int.MaxValue);
    }

    public float NextSingle()
    {
        return (float)NextDouble();
    }

    public readonly override string ToString()
    {
        return $"0x{Seed:X}";
    }

    public ulong Seed { get; internal set; }
}