acdream/src/AcDream.App/Streaming/GraphicalRemotePlacementServiceWindow.cs

using AcDream.Runtime.Session;

namespace AcDream.App.Streaming;

/// <summary>
/// C4 route 4b-1: the graphical host's
/// <see cref="IRuntimeRemotePlacementServiceWindow"/> — headless already had
/// one (<c>HeadlessCollisionNeighborhood.IsWithinServiceWindow</c>,
/// <c>HeadlessSessionWorldProjection.cs:27</c>/<c>:245-257</c>); the
/// graphical host had none.
/// </summary>
/// <remarks>
/// <para>
/// <b>Predicate choice — <see cref="GpuWorldState.IsNearTier"/>, not
/// <see cref="GpuWorldState.IsNearTierOrPending"/>.</b> The contract asks
/// whether near-tier residency is exactly co-extensive with collision
/// publication and, if not, to propose the correct predicate rather than
/// ship the assumption. It is not — for the "pending" half specifically:
/// <c>IsNearTierOrPending</c> is true for a landblock that has only been
/// PARKED as a pending near-tier entity bucket
/// (<c>GpuWorldState.CommitEntitiesToExistingLandblockSpatialCore</c>'s
/// <c>parkIfMissing</c> branch, <c>:1919-1952</c>) — i.e. queued, not yet
/// collision-published. Using it here would let this guard say "go" for a
/// destination whose collision generation has not committed, reopening
/// exactly the DeferredCell park this route exists to prevent.
/// <see cref="GpuWorldState.IsNearTier"/> alone is the correct predicate:
/// </para>
/// <para>
/// <b>Promotion direction (verified by reading, not assumed):</b> the only
/// two writers of <c>_tierByLandblock[...] = LandblockStreamTier.Near</c> are
/// <c>GpuWorldState.CommitLandblockSpatialCore</c> (:969) and
/// <c>CommitEntitiesToExistingLandblockSpatialCore</c> (:1972), both called
/// from <c>LandblockPresentationPipeline.Advance</c>'s
/// <c>SpatialPresentationCommitted</c> stage — which runs strictly AFTER the
/// preceding <c>PresentationCommitted</c> stage, the one that drives
/// <c>LandblockPhysicsPublisher</c>'s staged collision/EnvCell advance to
/// completion (<c>LandblockPresentationPipeline.cs</c> ~826-935). A
/// landblock's tier cannot read Near before its collision has committed.
/// </para>
/// <para>
/// <b>Retirement direction (verified by reading, not assumed):</b>
/// <c>GpuWorldState.DetachNearLayer</c> (:1755-1834) flips the tier to Far as
/// the FIRST, synchronous step of a "detach-first" landblock retirement —
/// <c>LandblockRetirementCoordinator.AdoptDetachedFull</c>'s own doc comment
/// states spatial detachment has already committed by the time its retirement
/// ticket is constructed, and that ticket's <c>LandblockRetirementStage.Physics</c>
/// step (<c>_physics.AdvanceRemoval</c> → <c>RuntimeSetPositionState
/// .WithdrawCollision</c> → the collision-side retirement
/// <c>ParkCollisionResidents</c> can reach) runs strictly AFTER. A
/// landblock's tier cannot still read Near once its collision has begun
/// retiring.
/// </para>
/// <para>
/// <b>Residual, not closed:</b> a landblock-prefix collision MUTATION that is
/// not a full retirement (a live in-place quiescence/refresh while the tier
/// stays Near) is not ruled out by this reading and is not exercised by any
/// gate this route adds. <c>RuntimeSetPositionState</c>'s private
/// <c>TryGetBlockingQuiescence</c> (:3820-3863) is Core's own check for
/// exactly this case — an active <c>CollisionPrefixQuiescence</c> entry for
/// the destination's landblock prefix, independent of tier/residency — and
/// it is what a placement can still hit even after this guard passes,
/// producing the narrow <c>DeferredCell</c> residual
/// <see cref="AcDream.Runtime.Session.RuntimeRemotePlacementDriveController"/>'s
/// class doc describes.
/// </para>
/// </remarks>
internal sealed class GraphicalRemotePlacementServiceWindow
    : IRuntimeRemotePlacementServiceWindow
{
    private readonly GpuWorldState _state;

    internal GraphicalRemotePlacementServiceWindow(GpuWorldState state)
    {
        _state = state ?? throw new ArgumentNullException(nameof(state));
    }

    /// <summary>
    /// <paramref name="landblockId"/> is a full ACE cell id (landblock high
    /// word + cell low word — <c>CreateObject.ServerPosition.LandblockId</c>
    /// carries the full id despite its name; see
    /// <c>RuntimeAcceptedPositionDriveControllerTests</c>'s own
    /// <c>SourceCell = SourceLandblock | 0x0001u</c> fixture shape).
    /// Canonicalized to <c>0xAAAAFFFF</c> the same way every
    /// <see cref="GpuWorldState"/> tier writer/reader does
    /// (<c>DetachNearLayer</c>, <c>CommitLandblockSpatialCore</c>) before the
    /// dictionary lookup — <see cref="GpuWorldState.IsNearTier"/> itself does
    /// not canonicalize its argument.
    /// </summary>
    public bool IsWithinServiceWindow(uint landblockId) =>
        _state.IsNearTier((landblockId & 0xFFFF0000u) | 0xFFFFu);
}