// -----------------------------------------------------------------------------
// PolicyDecisionAttestationService.cs
// Sprint: SPRINT_3801_0001_0001_policy_decision_attestation
// Description: Implementation of policy decision attestation service.
// -----------------------------------------------------------------------------

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Security.Cryptography;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using StellaOps.Scanner.WebService.Contracts;
using StellaOps.Scanner.WebService.Domain;

namespace StellaOps.Scanner.WebService.Services;

/// <summary>
/// Implementation of the policy decision attestation service.
/// </summary>
/// <remarks>
/// Creates in-toto statements for policy decisions. The actual DSSE signing
/// is deferred to the Attestor module when available.
/// </remarks>
public sealed class PolicyDecisionAttestationService : IPolicyDecisionAttestationService
{
    private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web)
    {
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
        PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower,
        WriteIndented = false
    };

    private readonly ILogger<PolicyDecisionAttestationService> _logger;
    private readonly TimeProvider _timeProvider;
    private readonly PolicyDecisionAttestationOptions _options;

    // In-memory store for attestations (production would use persistent storage)
    private readonly ConcurrentDictionary<string, PolicyDecisionAttestationResult> _attestations = new();

    public PolicyDecisionAttestationService(
        ILogger<PolicyDecisionAttestationService> logger,
        IOptions<PolicyDecisionAttestationOptions>? options = null,
        TimeProvider? timeProvider = null)
    {
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
        _timeProvider = timeProvider ?? TimeProvider.System;
        _options = options?.Value ?? new PolicyDecisionAttestationOptions();
    }

    /// <inheritdoc />
    public Task<PolicyDecisionAttestationResult> CreateAttestationAsync(
        PolicyDecisionInput input,
        CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(input);
        ArgumentException.ThrowIfNullOrWhiteSpace(input.FindingId);
        ArgumentException.ThrowIfNullOrWhiteSpace(input.Cve);
        ArgumentException.ThrowIfNullOrWhiteSpace(input.ComponentPurl);

        try
        {
            var now = _timeProvider.GetUtcNow();
            var ttl = input.DecisionTtl ?? TimeSpan.FromDays(_options.DefaultDecisionTtlDays);
            var expiresAt = now.Add(ttl);

            // Build the statement
            var statement = BuildStatement(input, now, expiresAt);

            // Compute content-addressed ID
            var attestationId = ComputeAttestationId(statement);

            // Store the attestation
            var key = BuildKey(input.ScanId, input.FindingId);
            var result = PolicyDecisionAttestationResult.Succeeded(
                statement,
                attestationId,
                dsseEnvelope: null // Signing deferred to Attestor module
            );

            _attestations[key] = result;

            _logger.LogInformation(
                "Created policy decision attestation for {FindingId}: {Decision} (score={Score}, attestation={AttestationId})",
                input.FindingId,
                input.Decision,
                input.Reasoning.FinalScore,
                attestationId);

            return Task.FromResult(result);
        }
        catch (Exception ex)
        {
            _logger.LogError(ex, "Failed to create policy decision attestation for {FindingId}", input.FindingId);
            return Task.FromResult(PolicyDecisionAttestationResult.Failed(ex.Message));
        }
    }

    /// <inheritdoc />
    public Task<PolicyDecisionAttestationResult?> GetAttestationAsync(
        ScanId scanId,
        string findingId,
        CancellationToken cancellationToken = default)
    {
        var key = BuildKey(scanId, findingId);
        if (_attestations.TryGetValue(key, out var result))
        {
            return Task.FromResult<PolicyDecisionAttestationResult?>(result);
        }

        return Task.FromResult<PolicyDecisionAttestationResult?>(null);
    }

    private PolicyDecisionStatement BuildStatement(
        PolicyDecisionInput input,
        DateTimeOffset evaluatedAt,
        DateTimeOffset expiresAt)
    {
        // Build subjects - the scan and finding are the subjects of this attestation
        var subjects = new List<PolicyDecisionSubject>
        {
            new()
            {
                Name = $"scan:{input.ScanId.Value}",
                Digest = new Dictionary<string, string>
                {
                    ["sha256"] = ComputeSha256(input.ScanId.Value)
                }
            },
            new()
            {
                Name = $"finding:{input.FindingId}",
                Digest = new Dictionary<string, string>
                {
                    ["sha256"] = ComputeSha256(input.FindingId)
                }
            }
        };

        // Build predicate
        var predicate = new PolicyDecisionPredicate
        {
            FindingId = input.FindingId,
            Cve = input.Cve,
            ComponentPurl = input.ComponentPurl,
            Decision = input.Decision,
            Reasoning = input.Reasoning,
            EvidenceRefs = input.EvidenceRefs,
            EvaluatedAt = evaluatedAt,
            ExpiresAt = expiresAt,
            PolicyVersion = input.PolicyVersion,
            PolicyHash = input.PolicyHash
        };

        return new PolicyDecisionStatement
        {
            Subject = subjects,
            Predicate = predicate
        };
    }

    private static string ComputeAttestationId(PolicyDecisionStatement statement)
    {
        var json = JsonSerializer.Serialize(statement, JsonOptions);
        var hash = ComputeSha256(json);
        return $"sha256:{hash}";
    }

    private static string ComputeSha256(string input)
    {
        var bytes = Encoding.UTF8.GetBytes(input);
        var hashBytes = SHA256.HashData(bytes);
        return Convert.ToHexStringLower(hashBytes);
    }

    private static string BuildKey(ScanId scanId, string findingId)
        => $"{scanId.Value}:{findingId}";
}

/// <summary>
/// Configuration options for policy decision attestations.
/// </summary>
public sealed class PolicyDecisionAttestationOptions
{
    /// <summary>
    /// Default TTL for policy decisions in days.
    /// </summary>
    public int DefaultDecisionTtlDays { get; set; } = 30;

    /// <summary>
    /// Whether to enable DSSE signing when Attestor is available.
    /// </summary>
    public bool EnableSigning { get; set; } = true;

    /// <summary>
    /// Key profile to use for signing attestations.
    /// </summary>
    public string SigningKeyProfile { get; set; } = "Reasoning";
}