// -----------------------------------------------------------------------------
// ReplayHashService.cs
// Sprint: SPRINT_20251228_007_BE_sbom_lineage_graph_ii (LIN-BE-023)
// Task: Compute replay hash per lineage node
// Description: Implements deterministic replay hash computation per spec:
//              Hash = SHA256(sbom_digest + feeds_snapshot_digest + 
//                            policy_version + vex_verdicts_digest + timestamp)
// -----------------------------------------------------------------------------

using System.Diagnostics;
using System.Security.Cryptography;
using System.Text;
using Microsoft.Extensions.Logging;

namespace StellaOps.SbomService.Services;

/// <summary>
/// Implementation of <see cref="IReplayHashService"/>.
/// Computes deterministic replay hashes for lineage nodes.
/// </summary>
internal sealed class ReplayHashService : IReplayHashService
{
    private readonly IFeedsSnapshotService? _feedsService;
    private readonly IPolicyVersionService? _policyService;
    private readonly IVexVerdictsDigestService? _vexService;
    private readonly ILogger<ReplayHashService> _logger;
    private readonly TimeProvider _timeProvider;
    private readonly ActivitySource _activitySource = new("StellaOps.SbomService.ReplayHash");

    // Delimiter used between hash components for determinism
    private const char Delimiter = '|';

    // Timestamp format for reproducibility (minute precision)
    private const string TimestampFormat = "yyyy-MM-ddTHH:mm";

    public ReplayHashService(
        ILogger<ReplayHashService> logger,
        IFeedsSnapshotService? feedsService = null,
        IPolicyVersionService? policyService = null,
        IVexVerdictsDigestService? vexService = null,
        TimeProvider? timeProvider = null)
    {
        _logger = logger;
        _feedsService = feedsService;
        _policyService = policyService;
        _vexService = vexService;
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    /// <inheritdoc/>
    public string ComputeHash(ReplayHashInputs inputs)
    {
        ArgumentNullException.ThrowIfNull(inputs);

        using var activity = _activitySource.StartActivity("ComputeReplayHash");
        activity?.SetTag("sbom_digest", inputs.SbomDigest);

        // Build canonical string representation
        // Order: sbom_digest | feeds_snapshot_digest | policy_version | vex_verdicts_digest | timestamp
        var canonicalString = BuildCanonicalString(inputs);

        // Compute SHA256 hash
        var bytes = Encoding.UTF8.GetBytes(canonicalString);
        var hash = SHA256.HashData(bytes);
        var hexHash = Convert.ToHexStringLower(hash);

        activity?.SetTag("replay_hash", hexHash);

        _logger.LogDebug(
            "Computed replay hash {Hash} for SBOM {SbomDigest}",
            hexHash,
            inputs.SbomDigest);

        return hexHash;
    }

    /// <inheritdoc/>
    public async Task<ReplayHashResult> ComputeHashAsync(
        string sbomDigest,
        string tenantId,
        CancellationToken ct = default)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(sbomDigest);
        ArgumentException.ThrowIfNullOrWhiteSpace(tenantId);

        using var activity = _activitySource.StartActivity("ComputeReplayHashAsync");
        activity?.SetTag("sbom_digest", sbomDigest);
        activity?.SetTag("tenant_id", tenantId);

        var now = _timeProvider.GetUtcNow();

        // Gather inputs from services
        var feedsDigest = await GetFeedsSnapshotDigestAsync(tenantId, ct);
        var policyVersion = await GetPolicyVersionAsync(tenantId, ct);
        var vexDigest = await GetVexVerdictsDigestAsync(sbomDigest, tenantId, ct);

        var inputs = new ReplayHashInputs
        {
            SbomDigest = sbomDigest,
            FeedsSnapshotDigest = feedsDigest,
            PolicyVersion = policyVersion,
            VexVerdictsDigest = vexDigest,
            Timestamp = TruncateToMinute(now)
        };

        var hash = ComputeHash(inputs);

        return new ReplayHashResult
        {
            Hash = hash,
            Inputs = inputs,
            ComputedAt = now
        };
    }

    /// <inheritdoc/>
    public bool VerifyHash(string expectedHash, ReplayHashInputs inputs)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(expectedHash);
        ArgumentNullException.ThrowIfNull(inputs);

        var computedHash = ComputeHash(inputs);
        var matches = string.Equals(expectedHash, computedHash, StringComparison.OrdinalIgnoreCase);

        if (!matches)
        {
            _logger.LogWarning(
                "Replay hash verification failed. Expected={Expected}, Computed={Computed}",
                expectedHash,
                computedHash);
        }

        return matches;
    }

    /// <summary>
    /// Builds the canonical string representation for hashing.
    /// Components are sorted and concatenated with delimiters for determinism.
    /// </summary>
    private static string BuildCanonicalString(ReplayHashInputs inputs)
    {
        var sb = new StringBuilder(512);

        // Component 1: SBOM digest (normalized to lowercase)
        sb.Append(NormalizeDigest(inputs.SbomDigest));
        sb.Append(Delimiter);

        // Component 2: Feeds snapshot digest
        sb.Append(NormalizeDigest(inputs.FeedsSnapshotDigest));
        sb.Append(Delimiter);

        // Component 3: Policy version (trimmed, lowercase for consistency)
        sb.Append(inputs.PolicyVersion.Trim().ToLowerInvariant());
        sb.Append(Delimiter);

        // Component 4: VEX verdicts digest
        sb.Append(NormalizeDigest(inputs.VexVerdictsDigest));
        sb.Append(Delimiter);

        // Component 5: Timestamp (minute precision, UTC)
        sb.Append(inputs.Timestamp.ToUniversalTime().ToString(TimestampFormat));

        return sb.ToString();
    }

    /// <summary>
    /// Normalizes a digest string for consistent hashing.
    /// </summary>
    private static string NormalizeDigest(string digest)
    {
        // Handle digest formats: sha256:xxxx, xxxx (bare)
        var normalized = digest.Trim().ToLowerInvariant();

        // If it doesn't have algorithm prefix, assume sha256
        if (!normalized.Contains(':'))
        {
            normalized = $"sha256:{normalized}";
        }

        return normalized;
    }

    /// <summary>
    /// Truncates a timestamp to minute precision for reproducibility.
    /// </summary>
    private static DateTimeOffset TruncateToMinute(DateTimeOffset timestamp)
    {
        return new DateTimeOffset(
            timestamp.Year,
            timestamp.Month,
            timestamp.Day,
            timestamp.Hour,
            timestamp.Minute,
            0,
            TimeSpan.Zero);
    }

    private async Task<string> GetFeedsSnapshotDigestAsync(string tenantId, CancellationToken ct)
    {
        if (_feedsService is not null)
        {
            return await _feedsService.GetCurrentSnapshotDigestAsync(tenantId, ct);
        }

        // Fallback: return a placeholder indicating feeds service not available
        _logger.LogDebug("FeedsSnapshotService not available, using placeholder digest");
        return "sha256:feeds-snapshot-unavailable";
    }

    private async Task<string> GetPolicyVersionAsync(string tenantId, CancellationToken ct)
    {
        if (_policyService is not null)
        {
            return await _policyService.GetCurrentVersionAsync(tenantId, ct);
        }

        // Fallback: return default policy version
        _logger.LogDebug("PolicyVersionService not available, using default version");
        return "v1.0.0-default";
    }

    private async Task<string> GetVexVerdictsDigestAsync(
        string sbomDigest,
        string tenantId,
        CancellationToken ct)
    {
        if (_vexService is not null)
        {
            return await _vexService.ComputeVerdictsDigestAsync(sbomDigest, tenantId, ct);
        }

        // Fallback: return a placeholder
        _logger.LogDebug("VexVerdictsDigestService not available, using placeholder digest");
        return "sha256:vex-verdicts-unavailable";
    }
}

/// <summary>
/// Service for getting the current feeds snapshot digest.
/// </summary>
internal interface IFeedsSnapshotService
{
    /// <summary>
    /// Gets the content digest of the current vulnerability feeds snapshot.
    /// </summary>
    Task<string> GetCurrentSnapshotDigestAsync(string tenantId, CancellationToken ct = default);
}

/// <summary>
/// Service for getting the current policy version.
/// </summary>
internal interface IPolicyVersionService
{
    /// <summary>
    /// Gets the current policy bundle version for a tenant.
    /// </summary>
    Task<string> GetCurrentVersionAsync(string tenantId, CancellationToken ct = default);
}

/// <summary>
/// Service for computing VEX verdicts digest.
/// </summary>
internal interface IVexVerdictsDigestService
{
    /// <summary>
    /// Computes a content digest of all VEX verdicts applicable to an SBOM.
    /// </summary>
    Task<string> ComputeVerdictsDigestAsync(
        string sbomDigest,
        string tenantId,
        CancellationToken ct = default);
}