TheriapolisV3/Theriapolis.Tests/Determinism/SeededRngTests.cs

using Theriapolis.Core.Util;
using Xunit;

namespace Theriapolis.Tests.Determinism;

/// <summary>
/// Phase 0 smoke tests: two SeededRng instances with the same seed must produce
/// identical outputs, and different seeds must diverge.
/// </summary>
public sealed class SeededRngTests
{
    [Fact]
    public void SameSeed_ProducesSameSequence()
    {
        var a = new SeededRng(123);
        var b = new SeededRng(123);

        for (int i = 0; i < 1000; i++)
            Assert.Equal(a.NextUInt64(), b.NextUInt64());
    }

    [Fact]
    public void DifferentSeeds_ProduceDifferentSequences()
    {
        var a = new SeededRng(123);
        var b = new SeededRng(456);

        bool anyDifferent = false;
        for (int i = 0; i < 10; i++)
            if (a.NextUInt64() != b.NextUInt64()) { anyDifferent = true; break; }

        Assert.True(anyDifferent, "Different seeds should produce different sequences.");
    }

    [Fact]
    public void ZeroSeed_DoesNotGetStuck()
    {
        var rng = new SeededRng(0);
        ulong prev = rng.NextUInt64();
        bool moved = false;
        for (int i = 0; i < 5; i++)
        {
            ulong next = rng.NextUInt64();
            if (next != prev) { moved = true; break; }
            prev = next;
        }
        Assert.True(moved, "RNG must advance even from seed 0.");
    }

    [Fact]
    public void NextFloat_InRange()
    {
        var rng = new SeededRng(999);
        for (int i = 0; i < 10_000; i++)
        {
            float f = rng.NextFloat();
            Assert.True(f >= 0f && f < 1f, $"float {f} out of [0,1)");
        }
    }

    [Fact]
    public void ForSubsystem_DifferentTagsProduceDifferentStreams()
    {
        var a = SeededRng.ForSubsystem(0xDEADBEEF, Theriapolis.Core.C.RNG_TERRAIN);
        var b = SeededRng.ForSubsystem(0xDEADBEEF, Theriapolis.Core.C.RNG_MOISTURE);
        Assert.NotEqual(a.NextUInt64(), b.NextUInt64());
    }

    [Fact]
    public void Seed123_FirstValue_IsReproducible()
    {
        // Two independent instances must produce the exact same first value.
        var x = new SeededRng(123).NextUInt64();
        var y = new SeededRng(123).NextUInt64();
        Assert.Equal(x, y);
    }
}