// <copyright file="OfflineHlcManager.cs" company="StellaOps">
// Copyright (c) StellaOps. Licensed under BUSL-1.1.
// </copyright>

using System.Security.Cryptography;
using System.Text;
using Microsoft.Extensions.Logging;
using StellaOps.AirGap.Sync.Models;
using StellaOps.AirGap.Sync.Stores;
using StellaOps.Canonical.Json;
using StellaOps.Determinism;
using StellaOps.HybridLogicalClock;

namespace StellaOps.AirGap.Sync.Services;

/// <summary>
/// Interface for offline HLC management.
/// </summary>
public interface IOfflineHlcManager
{
    /// <summary>
    /// Enqueues a job locally while offline, maintaining the local chain.
    /// </summary>
    /// <typeparam name="T">The payload type.</typeparam>
    /// <param name="payload">The job payload.</param>
    /// <param name="idempotencyKey">The idempotency key for deterministic job ID.</param>
    /// <param name="partitionKey">Optional partition key.</param>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>The enqueue result.</returns>
    Task<OfflineEnqueueResult> EnqueueOfflineAsync<T>(
        T payload,
        string idempotencyKey,
        string? partitionKey = null,
        CancellationToken cancellationToken = default) where T : notnull;

    /// <summary>
    /// Gets the current node's job log for export.
    /// </summary>
    /// <param name="cancellationToken">Cancellation token.</param>
    /// <returns>The node job log, or null if empty.</returns>
    Task<NodeJobLog?> GetNodeJobLogAsync(CancellationToken cancellationToken = default);

    /// <summary>
    /// Gets the node ID.
    /// </summary>
    string NodeId { get; }
}

/// <summary>
/// Manages HLC operations for offline/air-gap scenarios.
/// </summary>
public sealed class OfflineHlcManager : IOfflineHlcManager
{
    private readonly IHybridLogicalClock _hlc;
    private readonly IOfflineJobLogStore _jobLogStore;
    private readonly IGuidProvider _guidProvider;
    private readonly ILogger<OfflineHlcManager> _logger;

    /// <summary>
    /// Initializes a new instance of the <see cref="OfflineHlcManager"/> class.
    /// </summary>
    public OfflineHlcManager(
        IHybridLogicalClock hlc,
        IOfflineJobLogStore jobLogStore,
        IGuidProvider guidProvider,
        ILogger<OfflineHlcManager> logger)
    {
        _hlc = hlc ?? throw new ArgumentNullException(nameof(hlc));
        _jobLogStore = jobLogStore ?? throw new ArgumentNullException(nameof(jobLogStore));
        _guidProvider = guidProvider ?? throw new ArgumentNullException(nameof(guidProvider));
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
    }

    /// <inheritdoc/>
    public string NodeId => _hlc.NodeId;

    /// <inheritdoc/>
    public async Task<OfflineEnqueueResult> EnqueueOfflineAsync<T>(
        T payload,
        string idempotencyKey,
        string? partitionKey = null,
        CancellationToken cancellationToken = default) where T : notnull
    {
        ArgumentNullException.ThrowIfNull(payload);
        ArgumentException.ThrowIfNullOrWhiteSpace(idempotencyKey);

        // 1. Generate HLC timestamp
        var tHlc = _hlc.Tick();

        // 2. Compute deterministic job ID from idempotency key
        var jobId = ComputeDeterministicJobId(idempotencyKey);

        // 3. Serialize and hash payload
        var payloadJson = CanonJson.Serialize(payload);
        var payloadHash = SHA256.HashData(Encoding.UTF8.GetBytes(payloadJson));

        // 4. Get previous chain link
        var prevLink = await _jobLogStore.GetLastLinkAsync(NodeId, cancellationToken)
            .ConfigureAwait(false);

        // 5. Compute chain link
        var link = ComputeLink(prevLink, jobId, tHlc, payloadHash);

        // 6. Create and store entry
        var entry = new OfflineJobLogEntry
        {
            NodeId = NodeId,
            THlc = tHlc,
            JobId = jobId,
            PartitionKey = partitionKey,
            Payload = payloadJson,
            PayloadHash = payloadHash,
            PrevLink = prevLink,
            Link = link,
            EnqueuedAt = DateTimeOffset.UtcNow
        };

        await _jobLogStore.AppendAsync(entry, cancellationToken).ConfigureAwait(false);

        _logger.LogInformation(
            "Enqueued offline job {JobId} with HLC {THlc} on node {NodeId}",
            jobId, tHlc, NodeId);

        return new OfflineEnqueueResult
        {
            THlc = tHlc,
            JobId = jobId,
            Link = link,
            NodeId = NodeId
        };
    }

    /// <inheritdoc/>
    public Task<NodeJobLog?> GetNodeJobLogAsync(CancellationToken cancellationToken = default)
        => _jobLogStore.GetNodeJobLogAsync(NodeId, cancellationToken);

    /// <summary>
    /// Computes deterministic job ID from idempotency key.
    /// </summary>
    private Guid ComputeDeterministicJobId(string idempotencyKey)
    {
        var hash = SHA256.HashData(Encoding.UTF8.GetBytes(idempotencyKey));
        // Use first 16 bytes of SHA-256 as deterministic GUID
        return new Guid(hash.AsSpan(0, 16));
    }

    /// <summary>
    /// Computes chain link: Hash(prev_link || job_id || t_hlc || payload_hash).
    /// </summary>
    internal static byte[] ComputeLink(
        byte[]? prevLink,
        Guid jobId,
        HlcTimestamp tHlc,
        byte[] payloadHash)
    {
        using var hasher = IncrementalHash.CreateHash(HashAlgorithmName.SHA256);

        // Previous link (or 32 zero bytes for first entry)
        hasher.AppendData(prevLink ?? new byte[32]);

        // Job ID as bytes
        hasher.AppendData(jobId.ToByteArray());

        // HLC timestamp as UTF-8 bytes
        hasher.AppendData(Encoding.UTF8.GetBytes(tHlc.ToSortableString()));

        // Payload hash
        hasher.AppendData(payloadHash);

        return hasher.GetHashAndReset();
    }
}