using Google.Protobuf;
using Google.Protobuf.Collections;
using Grpc.Core;

namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;

/// <summary>Bounds audit batches using complete protobuf row sizes, length/tag prefixes, and gRPC framing.</summary>
public static class AuditWireBudget
{
    /// <summary>Five-byte gRPC frame plus two bytes reserved for a true MoreAvailable field.</summary>
    public const int InitialBytes = 7;

    /// <summary>
    /// Appends one row to a repeated-message collection if it fits. A row larger than the
    /// normal budget travels alone, up to the hard channel ceiling. Oversize rows are retained
    /// locally and fail explicitly with their identity; they are never acknowledged or truncated.
    /// </summary>
    /// <typeparam name="T">The protobuf message type of the rows.</typeparam>
    /// <param name="rows">The repeated field to append to.</param>
    /// <param name="row">The row to append.</param>
    /// <param name="wireBytes">The running wire size of the batch; increased by the row's size when appended.</param>
    /// <param name="rowIdentity">Identity of the row, used in the error raised when it exceeds the hard ceiling.</param>
    /// <returns><c>true</c> when the row was appended; <c>false</c> when it does not fit in the current batch.</returns>
    public static bool TryAppend<T>(RepeatedField<T> rows, T row, ref int wireBytes, string rowIdentity)
        where T : IMessage<T>
    {
        var rowBytes = 1L + CodedOutputStream.ComputeMessageSize(row);
        if (rows.Count > 0 && wireBytes + rowBytes > GrpcMessageLimits.AuditBatchBytes) return false;
        if (rowBytes + InitialBytes > GrpcMessageLimits.MaxMessageBytes)
            throw new RpcException(new Status(StatusCode.ResourceExhausted,
                $"Audit row {rowIdentity} requires {rowBytes + InitialBytes} wire bytes, exceeding {GrpcMessageLimits.MaxMessageBytes}; retained locally. Reduce its payload or raise coordinated peer limits before retrying."));
        rows.Add(row);
        wireBytes += (int)rowBytes;
        return true;
    }
}