using FluentAssertions;
using StellaOps.Policy.Determinization.Models;
using Xunit;

namespace StellaOps.Policy.Determinization.Tests.Models;

public class ObservationDecayTests
{
    [Fact]
    public void Fresh_Should_CreateZeroAgeDecay()
    {
        // Arrange
        var now = DateTimeOffset.UtcNow;

        // Act
        var decay = ObservationDecay.Fresh(now);

        // Assert
        decay.ObservedAt.Should().Be(now);
        decay.RefreshedAt.Should().Be(now);
        decay.CalculateDecay(now).Should().Be(1.0);
    }

    [Fact]
    public void CalculateDecay_Should_ApplyHalfLifeFormula()
    {
        // Arrange
        var observedAt = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
        var decay = ObservationDecay.Create(observedAt, observedAt);

        // After 14 days (one half-life), decay should be ~0.5
        var after14Days = observedAt.AddDays(14);

        // Act
        var decayValue = decay.CalculateDecay(after14Days);

        // Assert
        decayValue.Should().BeApproximately(0.5, 0.01);
    }

    [Fact]
    public void CalculateDecay_Should_NotDropBelowFloor()
    {
        // Arrange
        var observedAt = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
        var decay = ObservationDecay.Create(observedAt, observedAt);

        // Very old observation (1 year)
        var afterYear = observedAt.AddDays(365);

        // Act
        var decayValue = decay.CalculateDecay(afterYear);

        // Assert
        decayValue.Should().BeGreaterThanOrEqualTo(decay.Floor);
    }

    [Fact]
    public void IsStale_Should_DetectStaleObservations()
    {
        // Arrange
        var observedAt = new DateTimeOffset(2026, 1, 1, 0, 0, 0, TimeSpan.Zero);
        var decay = ObservationDecay.Create(observedAt, observedAt);

        // Decay drops below 0.5 threshold around 14 days
        var before = observedAt.AddDays(10);
        var after = observedAt.AddDays(20);

        // Act & Assert
        decay.CheckIsStale(before).Should().BeFalse();
        decay.CheckIsStale(after).Should().BeTrue();
    }

    [Fact]
    public void CalculateDecay_Should_ReturnOneForFutureDates()
    {
        // Arrange
        var now = DateTimeOffset.UtcNow;
        var decay = ObservationDecay.Fresh(now);

        // Act (future date, should not decay)
        var futureDecay = decay.CalculateDecay(now.AddDays(-1));

        // Assert
        futureDecay.Should().Be(1.0);
    }
}