using ZB.MOM.WW.MxGateway.Contracts.Proto.Galaxy;

namespace ZB.MOM.WW.MxGateway.Client;

/// <summary>
///     One node in a lazy-loaded Galaxy browse tree. Holds the underlying
///     <see cref="GalaxyObject"/> and exposes <see cref="ExpandAsync"/> to fetch
///     its direct children on demand. Expansion is one-shot: a second call is a
///     no-op. Pagination of large sibling sets is handled internally.
/// </summary>
public sealed class LazyBrowseNode
{
    private readonly GalaxyRepositoryClient _client;
    private readonly BrowseChildrenOptions _options;
    private readonly List<LazyBrowseNode> _children = [];
    private bool _isExpanded;

    internal LazyBrowseNode(
        GalaxyRepositoryClient client,
        GalaxyObject @object,
        bool hasChildrenHint,
        BrowseChildrenOptions options)
    {
        _client = client;
        Object = @object;
        HasChildrenHint = hasChildrenHint;
        _options = options;
    }

    /// <summary>The underlying Galaxy object for this node.</summary>
    public GalaxyObject Object { get; }

    /// <summary>True when the server reports this node has at least one matching descendant.</summary>
    public bool HasChildrenHint { get; }

    /// <summary>Direct children loaded by <see cref="ExpandAsync"/>; empty until then.</summary>
    public IReadOnlyList<LazyBrowseNode> Children => _children;

    /// <summary>True after the first <see cref="ExpandAsync"/> call completes.</summary>
    public bool IsExpanded => _isExpanded;

    /// <summary>
    ///     Fetches direct children from the gateway and populates <see cref="Children"/>.
    ///     Idempotent: subsequent calls are no-ops.
    /// </summary>
    /// <param name="cancellationToken">Token to observe for cancellation.</param>
    public async Task ExpandAsync(CancellationToken cancellationToken = default)
    {
        if (_isExpanded)
        {
            return;
        }

        string pageToken = string.Empty;
        HashSet<string> seenPageTokens = new(StringComparer.Ordinal);
        do
        {
            BrowseChildrenRequest request = GalaxyRepositoryClient.BuildBrowseChildrenRequest(_options);
            request.ParentGobjectId = Object.GobjectId;
            request.PageToken = pageToken;

            BrowseChildrenReply reply = await _client
                .BrowseChildrenRawAsync(request, cancellationToken)
                .ConfigureAwait(false);

            for (int i = 0; i < reply.Children.Count; i++)
            {
                bool hint = i < reply.ChildHasChildren.Count && reply.ChildHasChildren[i];
                _children.Add(new LazyBrowseNode(_client, reply.Children[i], hint, _options));
            }

            pageToken = reply.NextPageToken;
            if (!string.IsNullOrWhiteSpace(pageToken) && !seenPageTokens.Add(pageToken))
            {
                throw new MxGatewayException(
                    $"Galaxy BrowseChildren returned a repeated page token '{pageToken}'.");
            }
        }
        while (!string.IsNullOrWhiteSpace(pageToken));

        _isExpanded = true;
    }
}