using Shouldly;
using Xunit;

namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;

/// <summary>Gitea #694: <see cref="EventHorizon.TryClamp"/> — the pure window arithmetic of <c>ServerHistorian:EventReadHorizon</c>.</summary>
public sealed class EventHorizonTests
{
    private static readonly DateTime Now = new(2026, 10, 13, 12, 0, 0, DateTimeKind.Utc);
    private static readonly TimeSpan H = TimeSpan.FromDays(30);
    private static readonly DateTime Floor = Now - H;

    [Fact]
    public void No_horizon_changes_nothing()
    {
        var (s, e) = (DateTime.MinValue, Now);
        EventHorizon.TryClamp(ref s, ref e, reverse: false, TimeSpan.Zero, Now).ShouldBeTrue();
        (s, e).ShouldBe((DateTime.MinValue, Now));
        EventHorizon.TryClamp(ref s, ref e, reverse: false, TimeSpan.FromDays(-1), Now).ShouldBeTrue();
        (s, e).ShouldBe((DateTime.MinValue, Now));
    }

    [Fact]
    public void A_forward_window_starts_at_the_floor()
    {
        var (s, e) = (DateTime.UnixEpoch, Now);
        EventHorizon.TryClamp(ref s, ref e, reverse: false, H, Now).ShouldBeTrue();
        (s, e).ShouldBe((Floor, Now));

        var (s2, e2) = (Floor.AddDays(1), Now);
        EventHorizon.TryClamp(ref s2, ref e2, reverse: false, H, Now).ShouldBeTrue();
        s2.ShouldBe(Floor.AddDays(1)); // already inside: unchanged
    }

    [Fact]
    public void A_reverse_window_keeps_the_floor_instant()
    {
        var (s, e) = (Now, new DateTime(0, DateTimeKind.Utc));
        EventHorizon.TryClamp(ref s, ref e, reverse: true, H, Now).ShouldBeTrue();
        (s, e).ShouldBe((Now, Floor.AddTicks(-1))); // (end, start] — the floor itself is read
    }

    [Fact]
    public void A_window_wholly_before_the_floor_is_empty()
    {
        var (s, e) = (Floor.AddDays(-2), Floor);
        EventHorizon.TryClamp(ref s, ref e, reverse: false, H, Now).ShouldBeFalse(); // [start, end) ends at the floor

        var (rs, re) = (Floor.AddTicks(-1), Floor.AddDays(-2));
        EventHorizon.TryClamp(ref rs, ref re, reverse: true, H, Now).ShouldBeFalse();

        var (ok, oe) = (Floor, Floor.AddDays(-2));
        EventHorizon.TryClamp(ref ok, ref oe, reverse: true, H, Now).ShouldBeTrue(); // (…, floor] holds the floor
    }

    [Fact]
    public void A_horizon_longer_than_the_calendar_is_no_horizon()
    {
        var (s, e) = (DateTime.MinValue, Now);
        EventHorizon.TryClamp(ref s, ref e, reverse: false, TimeSpan.MaxValue, Now).ShouldBeTrue();
        s.ShouldBe(DateTime.MinValue);
    }
}