68 lines
2.2 KiB
C#
68 lines
2.2 KiB
C#
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namespace Theriapolis.Core.Util;
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/// <summary>
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/// SplitMix64-based pseudo-random number generator with named sub-stream support.
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/// All game randomness must go through this class — no new System.Random() anywhere.
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/// </summary>
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public sealed class SeededRng
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{
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private ulong _state;
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public SeededRng(ulong seed)
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{
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// Mix the seed to avoid bad low-entropy states
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_state = seed == 0 ? 0x9e3779b97f4a7c15UL : seed;
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// Warm up the state
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NextUInt64();
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NextUInt64();
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}
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/// <summary>Create a sub-stream for a specific subsystem using the world seed and a named tag constant.</summary>
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public static SeededRng ForSubsystem(ulong worldSeed, ulong subsystemTag)
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=> new(worldSeed ^ subsystemTag);
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public ulong NextUInt64()
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{
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// SplitMix64 step
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_state += 0x9e3779b97f4a7c15UL;
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ulong z = _state;
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z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9UL;
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z = (z ^ (z >> 27)) * 0x94d049bb133111ebUL;
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return z ^ (z >> 31);
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}
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public uint NextUInt32() => (uint)(NextUInt64() >> 32);
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/// <summary>Returns a double in [0, 1).</summary>
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public double NextDouble() => (NextUInt64() >> 11) * (1.0 / (1UL << 53));
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/// <summary>Returns a float in [0, 1).</summary>
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public float NextFloat() => (float)NextDouble();
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/// <summary>Returns a float in [min, max).</summary>
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public float NextFloat(float min, float max) => min + NextFloat() * (max - min);
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/// <summary>Returns an int in [min, max).</summary>
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public int NextInt(int min, int max)
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{
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if (max <= min) return min;
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return min + (int)(NextUInt64() % (ulong)(max - min));
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}
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/// <summary>Returns an int in [0, max).</summary>
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public int NextInt(int max) => NextInt(0, max);
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/// <summary>Returns true with probability p (0–1).</summary>
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public bool NextBool(double p = 0.5) => NextDouble() < p;
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/// <summary>Shuffles a span in place using Fisher–Yates.</summary>
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public void Shuffle<T>(Span<T> span)
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{
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for (int i = span.Length - 1; i > 0; i--)
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{
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int j = NextInt(0, i + 1);
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(span[i], span[j]) = (span[j], span[i]);
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}
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}
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}
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