using AcDream.Runtime.Entities;

namespace AcDream.Runtime.Physics;

/// <summary>
/// Public host boundary for Runtime-owned SetPosition projection receipts.
/// The channel owns no placement state: observation, retry, and exact-token
/// acknowledgement delegate to the canonical entity lifetime's event stream
/// and SetPosition owner.
/// </summary>
public sealed class RuntimePlacementProjectionChannel
{
    private readonly RuntimeEntityObjectEventStream _events;
    private readonly RuntimeSetPositionState _setPosition;
    private readonly RuntimeInitialCreateContinuationExecutor _initialCreateExecution;
    private Func<RuntimeGenerationToken> _generation = static () => default;
    private bool _generationBound;

    internal RuntimePlacementProjectionChannel(
        RuntimeEntityObjectEventStream events,
        RuntimeSetPositionState setPosition,
        RuntimeInitialCreateContinuationExecutor initialCreateExecution)
    {
        _events = events ?? throw new ArgumentNullException(nameof(events));
        _setPosition = setPosition
            ?? throw new ArgumentNullException(nameof(setPosition));
        _initialCreateExecution = initialCreateExecution
            ?? throw new ArgumentNullException(nameof(initialCreateExecution));
    }

    /// <summary>
    /// Observes ordered immutable projection receipts on the Runtime commit
    /// thread. A host must acknowledge only after its projection succeeds.
    /// </summary>
    public IDisposable Subscribe(IRuntimePlacementObserver observer) =>
        _events.SubscribePlacement(observer);

    internal void BindGeneration(Func<RuntimeGenerationToken> generation)
    {
        ArgumentNullException.ThrowIfNull(generation);
        if (_generationBound)
        {
            throw new InvalidOperationException(
                "The Runtime placement generation source is already bound.");
        }

        _generation = generation;
        _generationBound = true;
    }

    /// <summary>
    /// Acknowledges only the exact oldest outstanding receipt. Stale,
    /// reordered, superseded, or already acknowledged tokens are rejected.
    /// </summary>
    public bool Acknowledge(
        RuntimeGenerationToken expectedGeneration,
        in RuntimePlacementProjectionToken token) =>
        IsCurrent(expectedGeneration)
        && _setPosition.AcknowledgeProjection(token);

    /// <summary>
    /// Republishes every still-pending immutable receipt in canonical order.
    /// Runtime authority is never replayed or recommitted by a retry.
    /// </summary>
    public bool RetryPending(RuntimeGenerationToken expectedGeneration)
    {
        if (!IsCurrent(expectedGeneration))
            return false;
        _setPosition.RetryPendingProjections();
        return true;
    }

    /// <summary>
    /// Returns the exact oldest outstanding receipt without consuming it.
    /// </summary>
    public bool TryPeek(
        RuntimeGenerationToken expectedGeneration,
        out RuntimePlacementProjectionSnapshot projection)
    {
        if (IsCurrent(expectedGeneration))
            return _setPosition.TryPeekProjection(out projection);
        projection = default;
        return false;
    }

    public int PendingCount => _setPosition.PendingProjectionCount;

    /// <summary>
    /// C3-1: the public host consumption shape for an initial-Create
    /// continuation-executor drain's completion. Reached with the exact
    /// <see cref="RuntimePlacementProjectionToken"/> carried by a
    /// <see cref="RuntimePlacementProjectionKind.ExecutorCompleted"/> receipt
    /// observed through <see cref="Subscribe"/> - the same correlation
    /// identity (Entity/Sequence) every other placement Kind uses. Exposes
    /// exactly the facts a cutover host needs to bind presentation off an
    /// initial placement (the teleport-hook phase, the drained Position
    /// continuations' route facts for constrain/interpolation binding, and
    /// the replayed-deferred-child count) without widening any internal
    /// Runtime type's accessibility. Returns false for a generation
    /// mismatch or a stale/superseded/unknown token, mirroring every other
    /// generation-gated method on this channel.
    /// </summary>
    public bool TryGetInitialCreateCompletion(
        RuntimeGenerationToken expectedGeneration,
        in RuntimePlacementProjectionToken token,
        out RuntimeInitialCreatePlacementCompletion completion)
    {
        if (IsCurrent(expectedGeneration))
            return _initialCreateExecution.TryGetCompletion(token, out completion);
        completion = default;
        return false;
    }

    private bool IsCurrent(RuntimeGenerationToken expectedGeneration) =>
        _generationBound
        && expectedGeneration.Value != 0UL
        && expectedGeneration == _generation();
}