// -----------------------------------------------------------------------------
// DeterminismPropertyTests.cs
// Sprint: SPRINT_20260106_001_002_LB_determinization_scoring
// Task: DCS-023 - Write determinism tests: same snapshot same entropy
// Description: Property-based tests ensuring identical inputs produce identical
//              outputs across multiple invocations and calculator instances.
// -----------------------------------------------------------------------------

using FluentAssertions;
using Microsoft.Extensions.Logging.Abstractions;
using StellaOps.Policy.Determinization.Evidence;
using StellaOps.Policy.Determinization.Models;
using StellaOps.Policy.Determinization.Scoring;
using Xunit;

namespace StellaOps.Policy.Determinization.Tests.PropertyTests;

/// <summary>
/// Property tests verifying determinism.
/// DCS-023: same inputs must yield same outputs, always.
/// </summary>
[Trait("Category", "Unit")]
[Trait("Property", "Determinism")]
public class DeterminismPropertyTests
{
    private readonly DateTimeOffset _fixedTime = new(2026, 1, 7, 12, 0, 0, TimeSpan.Zero);

    /// <summary>
    /// Property: Same snapshot produces same entropy on repeated calls.
    /// </summary>
    [Fact]
    public void Entropy_SameSnapshot_ProducesSameResult()
    {
        // Arrange
        var calculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);
        var snapshot = CreateDeterministicSnapshot();

        // Act - calculate 10 times
        var results = new List<double>();
        for (var i = 0; i < 10; i++)
        {
            results.Add(calculator.CalculateEntropy(snapshot));
        }

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "same input should always produce same entropy");
    }

    /// <summary>
    /// Property: Different calculator instances produce same entropy for same snapshot.
    /// </summary>
    [Fact]
    public void Entropy_DifferentInstances_ProduceSameResult()
    {
        // Arrange
        var snapshot = CreateDeterministicSnapshot();

        // Act - create multiple instances and calculate
        var results = new List<double>();
        for (var i = 0; i < 5; i++)
        {
            var calculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);
            results.Add(calculator.CalculateEntropy(snapshot));
        }

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "different instances should produce same entropy for same input");
    }

    /// <summary>
    /// Property: Parallel execution produces consistent results.
    /// </summary>
    [Fact]
    public void Entropy_ParallelExecution_ProducesConsistentResults()
    {
        // Arrange
        var calculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);
        var snapshot = CreateDeterministicSnapshot();

        // Act - calculate in parallel
        var tasks = Enumerable.Range(0, 100)
            .Select(_ => Task.Run(() => calculator.CalculateEntropy(snapshot)))
            .ToArray();

        Task.WaitAll(tasks);
        var results = tasks.Select(t => t.Result).ToList();

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "parallel execution should produce consistent results");
    }

    /// <summary>
    /// Property: Same decay calculation produces same result.
    /// </summary>
    [Fact]
    public void Decay_SameInputs_ProducesSameResult()
    {
        // Arrange
        var calculator = new DecayedConfidenceCalculator(NullLogger<DecayedConfidenceCalculator>.Instance);
        var ageDays = 7.0;
        var halfLifeDays = 14.0;

        // Act - calculate 10 times
        var results = new List<double>();
        for (var i = 0; i < 10; i++)
        {
            results.Add(calculator.CalculateDecayFactor(ageDays, halfLifeDays));
        }

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "same input should always produce same decay factor");
    }

    /// <summary>
    /// Property: Same snapshot with same weights produces same entropy.
    /// </summary>
    [Theory]
    [InlineData(0.25, 0.15, 0.25, 0.15, 0.10, 0.10)]
    [InlineData(0.30, 0.20, 0.20, 0.10, 0.10, 0.10)]
    [InlineData(0.16, 0.16, 0.16, 0.16, 0.18, 0.18)]
    public void Entropy_SameSnapshotSameWeights_ProducesSameResult(
        double vex, double epss, double reach, double runtime, double backport, double sbom)
    {
        // Arrange
        var calculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);
        var snapshot = CreateDeterministicSnapshot();
        var weights = new SignalWeights
        {
            VexWeight = vex,
            EpssWeight = epss,
            ReachabilityWeight = reach,
            RuntimeWeight = runtime,
            BackportWeight = backport,
            SbomLineageWeight = sbom
        };

        // Act - calculate 5 times
        var results = new List<double>();
        for (var i = 0; i < 5; i++)
        {
            results.Add(calculator.CalculateEntropy(snapshot, weights));
        }

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "same snapshot + weights should always produce same entropy");
    }

    /// <summary>
    /// Property: Order of snapshot construction doesn't affect entropy.
    /// </summary>
    [Fact]
    public void Entropy_EquivalentSnapshots_ProduceSameResult()
    {
        // Arrange
        var calculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);

        // Create two snapshots with same values but constructed differently
        var snapshot1 = CreateSnapshotWithVexFirst();
        var snapshot2 = CreateSnapshotWithEpssFirst();

        // Act
        var entropy1 = calculator.CalculateEntropy(snapshot1);
        var entropy2 = calculator.CalculateEntropy(snapshot2);

        // Assert
        entropy1.Should().Be(entropy2, "equivalent snapshots should produce identical entropy");
    }

    /// <summary>
    /// Property: Decay with floor is deterministic.
    /// </summary>
    [Theory]
    [InlineData(1.0, 30, 14.0, 0.1)]
    [InlineData(0.8, 7, 7.0, 0.05)]
    [InlineData(0.5, 100, 30.0, 0.2)]
    public void Decay_WithFloor_IsDeterministic(double baseConfidence, int ageDays, double halfLifeDays, double floor)
    {
        // Arrange
        var calculator = new DecayedConfidenceCalculator(NullLogger<DecayedConfidenceCalculator>.Instance);

        // Act - calculate 10 times
        var results = new List<double>();
        for (var i = 0; i < 10; i++)
        {
            results.Add(calculator.Calculate(baseConfidence, ageDays, halfLifeDays, floor));
        }

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "decay with floor should be deterministic");
    }

    /// <summary>
    /// Property: Entropy calculation is independent of external state.
    /// </summary>
    [Fact]
    public void Entropy_IndependentOfGlobalState_ProducesConsistentResults()
    {
        // Arrange
        var snapshot = CreateDeterministicSnapshot();

        // Act - interleave calculations with some "noise"
        var results = new List<double>();
        for (var i = 0; i < 10; i++)
        {
            // Create new calculator each time to verify no shared state issues
            var calculator = new UncertaintyScoreCalculator(NullLogger<UncertaintyScoreCalculator>.Instance);

            // Do some unrelated operations
            _ = Guid.NewGuid();
            _ = DateTime.UtcNow;

            results.Add(calculator.CalculateEntropy(snapshot));
        }

        // Assert - all results should be identical
        results.Distinct().Should().HaveCount(1, "entropy should be independent of external state");
    }

    #region Helper Methods

    private SignalSnapshot CreateDeterministicSnapshot()
    {
        return new SignalSnapshot
        {
            Cve = "CVE-2024-1234",
            Purl = "pkg:test@1.0.0",
            Vex = SignalState<VexClaimSummary>.Queried(
                new VexClaimSummary { Status = "affected", Confidence = 0.9, StatementCount = 1, ComputedAt = _fixedTime },
                _fixedTime),
            Epss = SignalState<EpssEvidence>.Queried(
                new EpssEvidence { Cve = "CVE-2024-1234", Epss = 0.5, Percentile = 0.8, PublishedAt = _fixedTime },
                _fixedTime),
            Reachability = SignalState<ReachabilityEvidence>.Queried(
                new ReachabilityEvidence { Status = ReachabilityStatus.Reachable, AnalyzedAt = _fixedTime },
                _fixedTime),
            Runtime = SignalState<RuntimeEvidence>.NotQueried(),
            Backport = SignalState<BackportEvidence>.NotQueried(),
            Sbom = SignalState<SbomLineageEvidence>.NotQueried(),
            Cvss = SignalState<CvssEvidence>.Queried(
                new CvssEvidence { Vector = "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H", Version = "3.1", BaseScore = 9.8, Severity = "CRITICAL", Source = "NVD", PublishedAt = _fixedTime },
                _fixedTime),
            SnapshotAt = _fixedTime
        };
    }

    private SignalSnapshot CreateSnapshotWithVexFirst()
    {
        var snapshot = SignalSnapshot.Empty("CVE-2024-1234", "pkg:test@1.0", _fixedTime);
        return snapshot with
        {
            Vex = SignalState<VexClaimSummary>.Queried(
                new VexClaimSummary { Status = "affected", Confidence = 0.9, StatementCount = 1, ComputedAt = _fixedTime },
                _fixedTime),
            Epss = SignalState<EpssEvidence>.Queried(
                new EpssEvidence { Cve = "CVE-2024-1234", Epss = 0.5, Percentile = 0.8, PublishedAt = _fixedTime },
                _fixedTime)
        };
    }

    private SignalSnapshot CreateSnapshotWithEpssFirst()
    {
        var snapshot = SignalSnapshot.Empty("CVE-2024-1234", "pkg:test@1.0", _fixedTime);
        return snapshot with
        {
            Epss = SignalState<EpssEvidence>.Queried(
                new EpssEvidence { Cve = "CVE-2024-1234", Epss = 0.5, Percentile = 0.8, PublishedAt = _fixedTime },
                _fixedTime),
            Vex = SignalState<VexClaimSummary>.Queried(
                new VexClaimSummary { Status = "affected", Confidence = 0.9, StatementCount = 1, ComputedAt = _fixedTime },
                _fixedTime)
        };
    }

    #endregion
}