osu/osu.Game/Utils/StatelessRNG.cs

80 lines
2.7 KiB
C#

// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
// See the LICENCE file in the repository root for full licence text.
using System;
namespace osu.Game.Utils
{
/// <summary>
/// Provides a fast stateless function that can be used in randomly-looking visual elements.
/// </summary>
public static class StatelessRNG
{
private static ulong mix(ulong x)
{
unchecked
{
x ^= x >> 33;
x *= 0xff51afd7ed558ccd;
x ^= x >> 33;
x *= 0xc4ceb9fe1a85ec53;
x ^= x >> 33;
return x;
}
}
/// <summary>
/// Generate a random 64-bit unsigned integer from given seed.
/// </summary>
/// <param name="seed">
/// The seed value of this random number generator.
/// </param>
/// <param name="series">
/// The series number.
/// Different values are computed for the same seed in different series.
/// </param>
public static ulong NextULong(int seed, int series = 0)
{
unchecked
{
var combined = ((ulong)(uint)series << 32) | (uint)seed;
// The xor operation is to not map (0, 0) to 0.
return mix(combined ^ 0x12345678);
}
}
/// <summary>
/// Generate a random integer in range [0, maxValue) from given seed.
/// </summary>
/// <param name="maxValue">
/// The number of possible results.
/// </param>
/// <param name="seed">
/// The seed value of this random number generator.
/// </param>
/// <param name="series">
/// The series number.
/// Different values are computed for the same seed in different series.
/// </param>
public static int NextInt(int maxValue, int seed, int series = 0)
{
if (maxValue <= 0) throw new ArgumentOutOfRangeException(nameof(maxValue));
return (int)(NextULong(seed, series) % (ulong)maxValue);
}
/// <summary>
/// Compute a random floating point value between 0 and 1 (excluding 1) from given seed and series number.
/// </summary>
/// <param name="seed">
/// The seed value of this random number generator.
/// </param>
/// <param name="series">
/// The series number.
/// Different values are computed for the same seed in different series.
/// </param>
public static float NextSingle(int seed, int series = 0) =>
(float)(NextULong(seed, series) & ((1 << 24) - 1)) / (1 << 24); // float has 24-bit precision
}
}