the-living-world/tests/TheLivingWorld.Tests/WorldSimulationTests.cs

using TheLivingWorld.Api.Simulation;
using TheLivingWorld.Core.Contracts;
using TheLivingWorld.Core.Simulation;

namespace TheLivingWorld.Tests;

public sealed class WorldSimulationTests
{
    [Fact]
    public void Create_starts_at_default_morning_and_ticks_at_x1()
    {
        using var simulation = WorldSimulation.Create(ReadySummary(), catchUp: false);

        var before = simulation.SnapshotClock();
        Assert.Equal(GameTime.DefaultStart, before.GameTime);
        Assert.Equal(1, before.TimeScale);
        Assert.False(before.Paused);

        simulation.Tick(TimeSpan.FromSeconds(2));

        var after = simulation.SnapshotClock();
        Assert.Equal(GameTime.DefaultStart.AddMinutes(10), after.GameTime);
        Assert.True(simulation.IsDirty);
    }

    [Fact]
    public void Tick_does_not_advance_while_paused()
    {
        var summary = ReadySummary() with
        {
            Clock = WorldSimulation.DefaultClock() with { Paused = true },
        };
        using var simulation = WorldSimulation.Create(summary, catchUp: false);

        simulation.Tick(TimeSpan.FromSeconds(5));

        Assert.Equal(GameTime.DefaultStart, simulation.SnapshotClock().GameTime);
        Assert.False(simulation.IsDirty);
    }

    [Fact]
    public void Update_bakes_elapsed_time_before_changing_scale()
    {
        using var simulation = WorldSimulation.Create(ReadySummary(), catchUp: false);

        simulation.Tick(TimeSpan.FromSeconds(1));
        var clock = simulation.Update(new UpdateClockRequest { TimeScale = 2 });

        Assert.Equal(2, clock.TimeScale);
        // Tick advanced 5 game minutes; Update may bake a few extra wall-clock milliseconds.
        Assert.InRange(
            clock.GameTime,
            GameTime.DefaultStart.AddMinutes(5),
            GameTime.DefaultStart.AddMinutes(5).AddSeconds(30));
        Assert.True(simulation.IsDirty);
    }

    [Fact]
    public void Update_rejects_invalid_time_scale()
    {
        using var simulation = WorldSimulation.Create(ReadySummary(), catchUp: false);

        Assert.Throws<ArgumentOutOfRangeException>(() =>
            simulation.Update(new UpdateClockRequest { TimeScale = 5 }));
    }

    [Fact]
    public void Catch_up_advances_from_last_ticked_at()
    {
        var lastTickedAt = DateTimeOffset.UtcNow - TimeSpan.FromSeconds(3);
        var summary = ReadySummary() with { LastTickedAt = lastTickedAt };

        using var simulation = WorldSimulation.Create(summary, catchUp: true);

        // ~3 real seconds at x1 → ~15 game minutes (allow a little wall-clock drift).
        var actual = simulation.SnapshotClock().GameTime;
        Assert.InRange(
            actual,
            GameTime.DefaultStart.AddMinutes(14),
            GameTime.DefaultStart.AddMinutes(17));
    }

    [Fact]
    public void Catch_up_skips_a_world_that_was_paused_when_the_host_went_down()
    {
        var summary = ReadySummary() with
        {
            Clock = WorldSimulation.DefaultClock() with { Paused = true },
            LastTickedAt = DateTimeOffset.UtcNow - TimeSpan.FromHours(6),
        };

        using var simulation = WorldSimulation.Create(summary, catchUp: true);

        Assert.Equal(GameTime.DefaultStart, simulation.SnapshotClock().GameTime);

        // Unpausing must not replay the six hours spent paused offline either.
        var resumed = simulation.Update(new UpdateClockRequest { Paused = false });
        Assert.False(resumed.Paused);
        Assert.InRange(resumed.GameTime, GameTime.DefaultStart, GameTime.DefaultStart.AddMinutes(5));
    }

    [Fact]
    public void OverlayForApi_strips_last_ticked_at()
    {
        using var simulation = WorldSimulation.Create(ReadySummary(), catchUp: false);
        var overlaid = simulation.OverlayForApi(ReadySummary());

        Assert.NotNull(overlaid.Clock);
        Assert.Null(overlaid.LastTickedAt);
    }

    private static WorldSummaryDto ReadySummary() => new()
    {
        Id = "town-aaaaaaaa",
        Name = "Town",
        Latitude = 31.9,
        Longitude = -100.5,
        SizeMeters = 10_000,
        Status = WorldStatus.Ready,
        CreatedAt = DateTimeOffset.UtcNow,
        Clock = WorldSimulation.DefaultClock(),
        LastTickedAt = DateTimeOffset.UtcNow,
    };
}