using ZB.MOM.WW.ScadaBridge.Commons.Entities.Kpi;

namespace ZB.MOM.WW.ScadaBridge.KpiHistory;

/// <summary>
/// Root-provider-owned admission for source collections. Actor replacement must not
/// start another call while an earlier cancellation-ignoring collection still owns its scope.
/// Registration indices are stable because the root provider's service collection is immutable.
/// </summary>
internal sealed class KpiSourceAdmission
{
    private readonly object _gate = new();
    private readonly Dictionary<int, Task<IReadOnlyList<KpiSample>>> _tasks = new();

    /// <summary>
    /// Starts a collection for the given registration unless an earlier one is still running, in which case the running one is returned.
    /// </summary>
    /// <param name="registrationIndex">Stable index of the source registration.</param>
    /// <param name="collect">Delegate that performs the collection when a new one is started.</param>
    /// <param name="collection">The new collection task, or the still-running earlier one.</param>
    /// <returns><see langword="true"/> if a new collection was started; <see langword="false"/> if an earlier one is still running.</returns>
    internal bool TryStart(
        int registrationIndex,
        Func<Task<IReadOnlyList<KpiSample>>> collect,
        out Task<IReadOnlyList<KpiSample>> collection)
    {
        lock (_gate)
        {
            if (_tasks.TryGetValue(registrationIndex, out var existing) && !existing.IsCompleted)
            {
                collection = existing;
                return false;
            }

            collection = Task.Run(collect);
            _tasks[registrationIndex] = collection;
            return true;
        }
    }
}