File size: 1,538 Bytes
3ce1a40 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 | using System.Text;
namespace Repoverse.Core.Generation;
/// <summary>
/// Deterministic seed derivation (§8). string.GetHashCode is randomized per process,
/// so seeds use FNV-1a over UTF-8 followed by a SplitMix64 finalizer.
/// </summary>
public readonly record struct Seed(ulong Value)
{
public Seed Derive(string scope)
{
var h = 14695981039346656037UL ^ Value;
foreach (var b in Encoding.UTF8.GetBytes(scope))
{
h ^= b;
h *= 1099511628211UL;
}
return new Seed(Mix(h));
}
public SeededRandom Random() => new(Value);
internal static ulong Mix(ulong z)
{
z += 0x9E3779B97F4A7C15UL;
z = (z ^ (z >> 30)) * 0xBF58476D1CE4E5B9UL;
z = (z ^ (z >> 27)) * 0x94D049BB133111EBUL;
return z ^ (z >> 31);
}
public override string ToString() => Value.ToString("x16");
}
/// <summary>SplitMix64 stream. Same seed → same sequence on every platform and runtime.</summary>
public sealed class SeededRandom(ulong state)
{
private ulong _state = state;
public ulong NextULong()
{
_state += 0x9E3779B97F4A7C15UL;
return Seed.Mix(_state - 0x9E3779B97F4A7C15UL);
}
/// <summary>Uniform integer in [min, maxExclusive).</summary>
public int Next(int min, int maxExclusive)
{
if (maxExclusive <= min) return min;
return min + (int)(NextULong() % (ulong)(maxExclusive - min));
}
public T Pick<T>(IReadOnlyList<T> items) => items[Next(0, items.Count)];
}
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