using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using StellaOps.Concelier.Models;

namespace StellaOps.Concelier.Core.Unknown;

/// <summary>
/// Default implementation that derives unknown-state markers from canonical advisories.
/// </summary>
public sealed class UnknownStateLedger : IUnknownStateLedger
{
    private static readonly ImmutableHashSet<string> ImpactStatuses = ImmutableHashSet.Create(
        StringComparer.Ordinal,
        AffectedPackageStatusCatalog.KnownAffected,
        AffectedPackageStatusCatalog.Affected,
        AffectedPackageStatusCatalog.UnderInvestigation,
        AffectedPackageStatusCatalog.Pending,
        AffectedPackageStatusCatalog.Unknown);

    private static readonly ImmutableHashSet<string> FixStatuses = ImmutableHashSet.Create(
        StringComparer.Ordinal,
        AffectedPackageStatusCatalog.Fixed,
        AffectedPackageStatusCatalog.FirstFixed,
        AffectedPackageStatusCatalog.Mitigated);

    private static readonly ImmutableDictionary<string, UnknownMarkerSeed> MarkerSeeds = new Dictionary<string, UnknownMarkerSeed>(StringComparer.Ordinal)
    {
        [UnknownStateMarkerKinds.UnknownVulnerabilityRange] = new UnknownMarkerSeed(0.8, "high"),
        [UnknownStateMarkerKinds.UnknownOrigin] = new UnknownMarkerSeed(0.6, "medium"),
        [UnknownStateMarkerKinds.AmbiguousFix] = new UnknownMarkerSeed(0.45, "medium"),
    }.ToImmutableDictionary(StringComparer.Ordinal);

    private readonly IUnknownStateRepository _repository;
    private readonly TimeProvider _timeProvider;

    public UnknownStateLedger(IUnknownStateRepository repository, TimeProvider? timeProvider = null)
    {
        _repository = repository ?? throw new ArgumentNullException(nameof(repository));
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public async ValueTask<UnknownStateLedgerResult> RecordAsync(
        UnknownStateLedgerRequest request,
        CancellationToken cancellationToken)
    {
        ArgumentNullException.ThrowIfNull(request);

        var recordedAt = _timeProvider.GetUtcNow();
        var markers = EvaluateMarkers(request.Advisory, request.AsOf, recordedAt);

        await _repository.UpsertAsync(request.VulnerabilityKey, markers, cancellationToken).ConfigureAwait(false);

        return new UnknownStateLedgerResult(request.VulnerabilityKey, request.AsOf, markers);
    }

    public ValueTask<IReadOnlyList<UnknownStateSnapshot>> GetByVulnerabilityAsync(
        string vulnerabilityKey,
        CancellationToken cancellationToken)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(vulnerabilityKey);
        var normalizedKey = vulnerabilityKey.Trim().ToLowerInvariant();
        return _repository.GetByVulnerabilityAsync(normalizedKey, cancellationToken);
    }

    private static ImmutableArray<UnknownStateSnapshot> EvaluateMarkers(
        Advisory advisory,
        DateTimeOffset observedAt,
        DateTimeOffset recordedAt)
    {
        var builder = ImmutableArray.CreateBuilder<UnknownStateSnapshot>(initialCapacity: 3);

        if (advisory is not null)
        {
            if (TryCreateUnknownVulnerabilityRangeMarker(advisory, observedAt, recordedAt, out var unknownRange))
            {
                builder.Add(unknownRange);
            }

            if (TryCreateUnknownOriginMarker(advisory, observedAt, recordedAt, out var unknownOrigin))
            {
                builder.Add(unknownOrigin);
            }

            if (TryCreateAmbiguousFixMarker(advisory, observedAt, recordedAt, out var ambiguousFix))
            {
                builder.Add(ambiguousFix);
            }
        }

        if (builder.Count == 0)
        {
            return ImmutableArray<UnknownStateSnapshot>.Empty;
        }

        builder.Sort(static (left, right) => StringComparer.Ordinal.Compare(left.Marker, right.Marker));
        return builder.ToImmutable();
    }

    private static bool TryCreateUnknownVulnerabilityRangeMarker(
        Advisory advisory,
        DateTimeOffset observedAt,
        DateTimeOffset recordedAt,
        out UnknownStateSnapshot snapshot)
    {
        snapshot = null!;

        if (advisory.AffectedPackages.IsDefaultOrEmpty)
        {
            return false;
        }

        var lackingPackages = 0;

        foreach (var package in advisory.AffectedPackages)
        {
            if (package is null)
            {
                continue;
            }

            if (!HasImpactStatus(package))
            {
                continue;
            }

            if (!HasConcreteRange(package))
            {
                lackingPackages++;
            }
        }

        if (lackingPackages == 0)
        {
            return false;
        }

        var seed = MarkerSeeds[UnknownStateMarkerKinds.UnknownVulnerabilityRange];
        var evidence = lackingPackages == 1
            ? "1 affected package lacks explicit version ranges."
            : $"{lackingPackages} affected packages lack explicit version ranges.";

        snapshot = new UnknownStateSnapshot(
            UnknownStateMarkerKinds.UnknownVulnerabilityRange,
            seed.Confidence,
            seed.Band,
            observedAt,
            recordedAt,
            evidence);
        return true;
    }

    private static bool TryCreateUnknownOriginMarker(
        Advisory advisory,
        DateTimeOffset observedAt,
        DateTimeOffset recordedAt,
        out UnknownStateSnapshot snapshot)
    {
        snapshot = null!;

        if (ContainsKnownProvenance(advisory.Provenance))
        {
            return false;
        }

        var seed = MarkerSeeds[UnknownStateMarkerKinds.UnknownOrigin];
        var evidence = advisory.Provenance.IsDefaultOrEmpty
            ? "Advisory provenance is missing; falling back to inferred sources."
            : "All advisory provenance sources resolve to 'unknown'.";

        snapshot = new UnknownStateSnapshot(
            UnknownStateMarkerKinds.UnknownOrigin,
            seed.Confidence,
            seed.Band,
            observedAt,
            recordedAt,
            evidence);
        return true;
    }

    private static bool TryCreateAmbiguousFixMarker(
        Advisory advisory,
        DateTimeOffset observedAt,
        DateTimeOffset recordedAt,
        out UnknownStateSnapshot snapshot)
    {
        snapshot = null!;

        if (advisory.AffectedPackages.IsDefaultOrEmpty)
        {
            return false;
        }

        var ambiguousPackages = 0;

        foreach (var package in advisory.AffectedPackages)
        {
            if (package is null)
            {
                continue;
            }

            if (!package.Statuses.IsDefaultOrEmpty && package.Statuses.Any(status => FixStatuses.Contains(status.Status)))
            {
                var hasFixedRange = package.VersionRanges.Any(static range => !string.IsNullOrWhiteSpace(range.FixedVersion));
                if (!hasFixedRange)
                {
                    ambiguousPackages++;
                }
            }
        }

        if (ambiguousPackages == 0)
        {
            return false;
        }

        var seed = MarkerSeeds[UnknownStateMarkerKinds.AmbiguousFix];
        var evidence = ambiguousPackages == 1
            ? "Fix status published without explicit fixed version details."
            : $"Fix status published without explicit fixed versions for {ambiguousPackages} packages.";

        snapshot = new UnknownStateSnapshot(
            UnknownStateMarkerKinds.AmbiguousFix,
            seed.Confidence,
            seed.Band,
            observedAt,
            recordedAt,
            evidence);
        return true;
    }

    private static bool HasImpactStatus(AffectedPackage package)
    {
        if (package.Statuses.IsDefaultOrEmpty)
        {
            return false;
        }

        foreach (var status in package.Statuses)
        {
            if (status is null)
            {
                continue;
            }

            if (ImpactStatuses.Contains(status.Status))
            {
                return true;
            }
        }

        return false;
    }

    private static bool HasConcreteRange(AffectedPackage package)
    {
        if (package.VersionRanges.IsDefaultOrEmpty)
        {
            return false;
        }

        foreach (var range in package.VersionRanges)
        {
            if (range is null)
            {
                continue;
            }

            if (!string.IsNullOrWhiteSpace(range.IntroducedVersion) ||
                !string.IsNullOrWhiteSpace(range.FixedVersion) ||
                !string.IsNullOrWhiteSpace(range.LastAffectedVersion) ||
                !string.IsNullOrWhiteSpace(range.RangeExpression))
            {
                return true;
            }
        }

        return false;
    }

    private static bool ContainsKnownProvenance(ImmutableArray<AdvisoryProvenance> provenance)
    {
        if (provenance.IsDefaultOrEmpty)
        {
            return false;
        }

        foreach (var entry in provenance)
        {
            if (entry is null)
            {
                continue;
            }

            if (IsKnownSource(entry.Source))
            {
                return true;
            }
        }

        return false;
    }

    private static bool IsKnownSource(string? source)
        => !string.IsNullOrWhiteSpace(source) &&
           !string.Equals(source, "unknown", StringComparison.OrdinalIgnoreCase);

    private readonly record struct UnknownMarkerSeed(double Confidence, string Band);
}