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using System.Threading;
using System.Threading.Tasks;

namespace Opc.Ua.Di.Server.Transfer
{
    /// <summary>
    /// Application-facing facade for OPC 10000-100 §10.4
    /// <c>TransferServicesType</c>. Implementations provide
    /// import/export of the parameter set of a topology element and
    /// hold the chunked transfer state between
    /// <c>TransferToDevice</c> / <c>TransferFromDevice</c> and
    /// <c>FetchTransferResultData</c> calls.
    /// </summary>
    /// <remarks>
    /// <para>
    /// The two transfer-direction methods kick off an asynchronous
    /// transfer and return an integer transfer ID.
    /// Clients then poll <see cref="FetchAsync"/> with that ID and a
    /// monotonically-increasing sequence number to drain the chunks
    /// of the result.
    /// </para>
    /// <para>
    /// Default implementation:
    /// <see cref="DefaultTransferService"/> — in-memory.
    /// </para>
    /// </remarks>
    public interface ITransferService
    {
        /// <summary>
        /// Starts a transfer of parameters TO the device. The server
        /// records the <paramref name="parameters"/> internally and
        /// returns a transfer ID the client can poll with
        /// <see cref="FetchAsync"/> to retrieve the per-parameter
        /// status codes (mostly <see cref="Opc.Ua.StatusCodes.Good"/>
        /// on success).
        /// </summary>
        ValueTask<int> TransferToDeviceAsync(
            ISystemContext context,
            NodeId elementId,
            ParameterSet parameters,
            CancellationToken ct = default);

        /// <summary>
        /// Starts a transfer of parameters FROM the device — the
        /// server snapshots the current parameter set and queues it
        /// for chunked fetch by <see cref="FetchAsync"/>.
        /// </summary>
        ValueTask<int> TransferFromDeviceAsync(
            ISystemContext context,
            NodeId elementId,
            CancellationToken ct = default);

        /// <summary>
        /// Fetches the next chunk of a previously-started transfer.
        /// </summary>
        ValueTask<FetchResult> FetchAsync(
            ISystemContext context,
            int transferId,
            int sequenceNumber,
            int maxResults,
            bool omitGoodResults,
            CancellationToken ct = default);
    }

    /// <summary>
    /// One chunk of a transfer-result stream returned by
    /// <see cref="ITransferService.FetchAsync"/>.
    /// </summary>
    public sealed record FetchResult(
        int SequenceNumber,
        bool EndOfResults,
        ParameterEntry[] Entries,
        StatusCode TransferError);
}