git.stella-ops.org/src/VexLens/StellaOps.VexLens/Trust/SourceTrust/TrustDecayService.cs

namespace StellaOps.VexLens.Trust.SourceTrust;

/// <summary>
/// Default implementation of <see cref="ITrustDecayService"/>.
/// Applies time-based trust decay, recency bonuses, and revocation penalties.
/// </summary>
public sealed class TrustDecayService : ITrustDecayService
{
    public DecayResult ApplyDecay(
        double baseScore,
        DateTimeOffset statementTimestamp,
        DecayContext context)
    {
        var age = context.EvaluationTime - statementTimestamp;
        var ageDays = age.TotalDays;
        var config = context.Configuration;

        var (decayFactor, category) = CalculateDecayFactor(age, config);

        // Reduce decay for statements with updates
        if (context.HasUpdates && context.UpdateCount > 0)
        {
            // Each update reduces effective age by 10%, up to 50% reduction
            var updateReduction = Math.Min(0.5, context.UpdateCount * 0.1);
            decayFactor = Math.Min(1.0, decayFactor + (1.0 - decayFactor) * updateReduction);
        }

        var decayedScore = baseScore * decayFactor;

        return new DecayResult
        {
            BaseScore = baseScore,
            DecayFactor = decayFactor,
            DecayedScore = decayedScore,
            AgeDays = ageDays,
            Category = category
        };
    }

    public double CalculateRecencyBonus(
        DateTimeOffset lastUpdateTimestamp,
        RecencyBonusContext context)
    {
        var age = context.EvaluationTime - lastUpdateTimestamp;

        if (age > context.RecencyWindow)
        {
            return 0.0;
        }

        // Linear decrease from max bonus to 0 over the recency window
        var ratio = 1.0 - (age.TotalSeconds / context.RecencyWindow.TotalSeconds);
        return context.MaxBonus * ratio;
    }

    public RevocationImpact CalculateRevocationImpact(
        RevocationInfo revocation,
        RevocationContext context)
    {
        if (!revocation.IsRevoked)
        {
            return new RevocationImpact
            {
                ShouldExclude = false,
                Penalty = 0.0,
                Explanation = "Statement is not revoked",
                RecommendedAction = RevocationAction.None
            };
        }

        // Determine impact based on revocation type
        return revocation.RevocationType switch
        {
            RevocationType.Superseded when revocation.WasSuperseded => new RevocationImpact
            {
                ShouldExclude = true,
                Penalty = context.SupersededPenalty,
                Explanation = $"Statement superseded by {revocation.SupersededBy ?? "newer statement"}",
                RecommendedAction = RevocationAction.Replace
            },

            RevocationType.Correction => new RevocationImpact
            {
                ShouldExclude = false,
                Penalty = context.CorrectionPenalty,
                Explanation = $"Statement corrected: {revocation.RevocationReason ?? "unspecified reason"}",
                RecommendedAction = RevocationAction.Penalize
            },

            RevocationType.Withdrawn => new RevocationImpact
            {
                ShouldExclude = true,
                Penalty = context.RevocationPenalty,
                Explanation = $"Statement withdrawn: {revocation.RevocationReason ?? "unspecified reason"}",
                RecommendedAction = RevocationAction.Exclude
            },

            RevocationType.Expired => new RevocationImpact
            {
                ShouldExclude = false,
                Penalty = context.RevocationPenalty * 0.5,
                Explanation = "Statement expired and was not renewed",
                RecommendedAction = RevocationAction.Review
            },

            RevocationType.SourceRevoked => new RevocationImpact
            {
                ShouldExclude = true,
                Penalty = context.RevocationPenalty,
                Explanation = "Source has been revoked",
                RecommendedAction = RevocationAction.Exclude
            },

            _ => new RevocationImpact
            {
                ShouldExclude = false,
                Penalty = context.RevocationPenalty * 0.75,
                Explanation = $"Statement revoked: {revocation.RevocationReason ?? "unknown reason"}",
                RecommendedAction = RevocationAction.Review
            }
        };
    }

    public EffectiveTrustScore GetEffectiveScore(
        double baseScore,
        TrustScoreFactors factors,
        DateTimeOffset evaluationTime)
    {
        var adjustments = new List<TrustAdjustment>();
        var shouldExclude = false;

        // Apply decay
        var decayConfig = factors.DecayConfiguration ?? DecayConfiguration.CreateDefault();
        var decayContext = new DecayContext
        {
            EvaluationTime = evaluationTime,
            Configuration = decayConfig,
            HasUpdates = factors.UpdateCount > 0,
            UpdateCount = factors.UpdateCount
        };

        var decayResult = ApplyDecay(baseScore, factors.StatementTimestamp, decayContext);

        adjustments.Add(new TrustAdjustment
        {
            Type = TrustAdjustmentType.Decay,
            Amount = decayResult.DecayedScore - baseScore,
            Reason = $"Time-based decay (age: {decayResult.AgeDays:F1} days, category: {decayResult.Category})"
        });

        var effectiveScore = decayResult.DecayedScore;

        // Apply recency bonus if recently updated
        var recencyBonus = 0.0;
        if (factors.LastUpdateTimestamp.HasValue)
        {
            var recencyContext = new RecencyBonusContext
            {
                EvaluationTime = evaluationTime
            };

            recencyBonus = CalculateRecencyBonus(factors.LastUpdateTimestamp.Value, recencyContext);

            if (recencyBonus > 0)
            {
                effectiveScore += recencyBonus;
                adjustments.Add(new TrustAdjustment
                {
                    Type = TrustAdjustmentType.RecencyBonus,
                    Amount = recencyBonus,
                    Reason = "Recently updated statement"
                });
            }
        }

        // Apply update bonus
        if (factors.UpdateCount > 1)
        {
            var updateBonus = Math.Min(0.05, factors.UpdateCount * 0.01);
            effectiveScore += updateBonus;
            adjustments.Add(new TrustAdjustment
            {
                Type = TrustAdjustmentType.UpdateBonus,
                Amount = updateBonus,
                Reason = $"Statement has been updated {factors.UpdateCount} times"
            });
        }

        // Apply revocation penalty
        var revocationPenalty = 0.0;
        if (factors.Revocation != null)
        {
            var revocationContext = new RevocationContext
            {
                EvaluationTime = evaluationTime
            };

            var revocationImpact = CalculateRevocationImpact(factors.Revocation, revocationContext);

            if (revocationImpact.ShouldExclude)
            {
                shouldExclude = true;
            }

            revocationPenalty = revocationImpact.Penalty;
            effectiveScore -= revocationPenalty;

            adjustments.Add(new TrustAdjustment
            {
                Type = TrustAdjustmentType.RevocationPenalty,
                Amount = -revocationPenalty,
                Reason = revocationImpact.Explanation
            });
        }

        // Clamp final score
        effectiveScore = Math.Clamp(effectiveScore, 0.0, 1.0);

        return new EffectiveTrustScore
        {
            BaseScore = baseScore,
            EffectiveScore = effectiveScore,
            DecayFactor = decayResult.DecayFactor,
            RecencyBonus = recencyBonus,
            RevocationPenalty = revocationPenalty,
            ShouldExclude = shouldExclude,
            StalenessCategory = decayResult.Category,
            Adjustments = adjustments
        };
    }

    private (double Factor, StalenessCategory Category) CalculateDecayFactor(
        TimeSpan age,
        DecayConfiguration config)
    {
        if (age <= TimeSpan.Zero)
        {
            return (1.0, StalenessCategory.Fresh);
        }

        if (age < config.FreshThreshold)
        {
            return (1.0, StalenessCategory.Fresh);
        }

        if (age < config.RecentThreshold)
        {
            var factor = CalculateCurveValue(
                age, config.FreshThreshold, config.RecentThreshold,
                1.0, 0.9, config.CurveType);
            return (factor, StalenessCategory.Recent);
        }

        if (age < config.StaleThreshold)
        {
            var factor = CalculateCurveValue(
                age, config.RecentThreshold, config.StaleThreshold,
                0.9, 0.7, config.CurveType);
            return (factor, StalenessCategory.Aging);
        }

        if (age < config.ExpiredThreshold)
        {
            var factor = CalculateCurveValue(
                age, config.StaleThreshold, config.ExpiredThreshold,
                0.7, config.MinDecayFactor, config.CurveType);
            return (factor, StalenessCategory.Stale);
        }

        return (config.MinDecayFactor, StalenessCategory.Expired);
    }

    private static double CalculateCurveValue(
        TimeSpan current,
        TimeSpan start,
        TimeSpan end,
        double startValue,
        double endValue,
        DecayCurveType curveType)
    {
        var progress = (current - start).TotalSeconds / (end - start).TotalSeconds;
        progress = Math.Clamp(progress, 0.0, 1.0);

        return curveType switch
        {
            DecayCurveType.Linear =>
                startValue + (endValue - startValue) * progress,

            DecayCurveType.Exponential =>
                startValue * Math.Pow(endValue / startValue, progress),

            DecayCurveType.Step =>
                progress < 0.5 ? startValue : endValue,

            _ => startValue + (endValue - startValue) * progress
        };
    }
}