acdream/src/AcDream.App/Rendering/Walk/WalkBuilding.cs

using System.Numerics;

namespace AcDream.App.Rendering.Walk;

/// <summary>Retail <c>CBldPortal</c>: one building portal — the authored
/// side gate, the destination interior cell, its reciprocal portal index,
/// and the stab list of cells whose view stacks are pushed around the
/// look-in (<c>PView::add_views</c>/<c>remove_views</c>).</summary>
public struct WalkBldPortal
{
    public int PortalSide;
    public uint OtherCellId;
    public int OtherPortalId;
    public bool ExactMatch;
    public uint[] StabList;
}

/// <summary>One <c>CPortalPoly</c> reference on a PORT BSP node: the index
/// into the building's portal list plus the portal polygon (building-local).</summary>
public struct WalkPortalRef
{
    public int PortalIndex;
    public WalkPolygon Polygon;
}

/// <summary>A node of the building's drawing BSP as the portal-only walk
/// sees it (retail tags: 'FAIL' leaf = stop, 'PORT' = portal node with
/// in_portals, anything else = plain splitting node).</summary>
public sealed class WalkBspNode
{
    public WalkPlane SplittingPlane;
    public WalkBspNode? PosNode;
    public WalkBspNode? NegNode;
    public bool IsFail;
    public WalkPortalRef[]? InPortals;   // non-null = PORT node

    public bool IsPortal => InPortals is not null;
}

/// <summary>One degrade-ladder level: the drawing BSP of that level's
/// GfxObj (portal-only view) and the level's far distance bound.</summary>
public readonly record struct WalkBuildingDegradeLevel(float MaxDist, WalkBspNode? DrawingBsp);

/// <summary>The walk's building model (retail <c>CBuildingObj</c> +
/// <c>BuildInfo</c> as the frame walk consumes them).</summary>
public sealed class WalkBuilding
{
    /// <summary><c>Position.objcell_id</c> — the id the oracle's BLD events
    /// carry (CBuildingObj+0x4C).</summary>
    public uint PositionCellId;

    public WalkBldPortal[] Portals = [];

    /// <summary>The drawing BSP of part 0's BASE GfxObj (portal-only view).
    /// Used directly when the model has no degrade ladder.</summary>
    public WalkBspNode? DrawingBsp;

    /// <summary>The degrade ladder (near→far). Retail walks the CURRENT
    /// level's drawing BSP (<c>part->gfxobj[deg_level]</c>) — and building
    /// degrade models beyond the first band carry NO portal nodes, which is
    /// what limits look-in punches to nearby buildings (probed 2026-08-30:
    /// every Holtburg building has ports at level 0 only).</summary>
    public WalkBuildingDegradeLevel[] DegradeLevels = [];

    /// <summary><c>part->gfxobj[deg_level] != 0</c> — a degraded-out slot
    /// skips the whole building AFTER publishing the portal list.</summary>
    public bool HasGeometry = true;

    /// <summary>Level selection by viewer distance. Simplified band pick
    /// (first level whose MaxDist covers the distance) — the faithful
    /// hysteresis of <c>CPhysicsPart::UpdateViewerDistance</c> is a port
    /// TODO; still fixtures are insensitive to hysteresis.</summary>
    public WalkBspNode? SelectDrawingBsp(float viewerDistance)
    {
        if (DegradeLevels.Length == 0) return DrawingBsp;
        foreach (WalkBuildingDegradeLevel level in DegradeLevels)
        {
            if (viewerDistance <= level.MaxDist)
                return level.DrawingBsp;
        }
        return null;   // degraded out entirely
    }
}

/// <summary>
/// Campaign FW1 — the building look-in machinery, ported from the first
/// decomp appendix report 3 (docs/research/2026-08-30-fw-walk-pseudocode-appendix.md):
/// <c>BSPTREE/BSPNODE::build_draw_portals_only</c> @0x00539860/@0x0053c100,
/// <c>BSPPORTAL::portal_draw_portals_only</c> @0x0053d870,
/// <c>PView::DrawPortal</c> @0x005a5ab0, and the CBldPortal
/// <c>PView::ConstructView</c> overload @0x005a59a0.
///
/// The invisible-panel primitive (<c>DrawPortalPolyInternal</c> @0x0059bc90
/// — punch far-Z / seal own-depth) is a GPU submission and belongs to FW2's
/// ordered stream; here it surfaces as the <see cref="IWalkPortalPassSink"/>
/// punch callback so FW1 conformance can observe when retail would draw it.
/// </summary>
public static class WalkBuildingPortals
{
    /// <summary>What the portal passes report outward: punches (pass 1) and
    /// look-in cell floods (pass 2, the oracle's <c>DC ov=…</c> events).</summary>
    public interface IWalkPortalPassSink
    {
        /// <summary>Pass 1 drew the portal polygon as a far-Z punch
        /// (<c>DrawPortalPolyInternal(poly, 1)</c>).</summary>
        void OnPunch(WalkPolygon polygon);

        /// <summary>Pass 2 completed a look-in flood and retail called
        /// <c>PView::DrawCells</c> — the DC event with the flood's list.</summary>
        void OnDrawCells(WalkPView pview);
    }

    /// <summary>
    /// <c>BSPTREE::build_draw_portals_only</c> @0x00539860 + the node/portal
    /// walkers: dispatch the root, then walk plane-side ordered — the child
    /// OPPOSITE the viewer first, so portals emit far-to-near. PORT nodes
    /// emit every in_portal on the POSITIVE and NEGATIVE arms; the IN_PLANE
    /// arm (|d| ≤ ε) visits the positive child and emits NOTHING.
    /// </summary>
    public static void BuildDrawPortalsOnly(
        WalkBspNode? root, int pass, Vector3 viewpointInBuilding,
        Action<WalkPortalRef, int> emitPortal)
    {
        if (root is null || root.IsFail) return;
        Walk(root, pass, viewpointInBuilding, emitPortal);
    }

    private static void Walk(
        WalkBspNode node, int pass, Vector3 viewpoint,
        Action<WalkPortalRef, int> emitPortal)
    {
        while (true)
        {
            float d = Vector3.Dot(node.SplittingPlane.Normal, viewpoint) + node.SplittingPlane.D;
            int side = d > WalkVisibilityMath.Epsilon ? 0
                : d < -WalkVisibilityMath.Epsilon ? 1 : 2;

            WalkBspNode? next;
            if (node.IsPortal)
            {
                if (side == 0)
                {
                    Visit(node.NegNode, pass, viewpoint, emitPortal);
                    foreach (WalkPortalRef portal in node.InPortals!)
                        emitPortal(portal, pass);
                    next = node.PosNode;
                }
                else if (side == 1)
                {
                    Visit(node.PosNode, pass, viewpoint, emitPortal);
                    foreach (WalkPortalRef portal in node.InPortals!)
                        emitPortal(portal, pass);
                    next = node.NegNode;
                }
                else
                {
                    Visit(node.PosNode, pass, viewpoint, emitPortal);
                    next = node.NegNode;
                }
            }
            else
            {
                if (side == 0)
                {
                    Visit(node.NegNode, pass, viewpoint, emitPortal);
                    next = node.PosNode;
                }
                else
                {
                    Visit(node.PosNode, pass, viewpoint, emitPortal);
                    next = node.NegNode;
                }
            }

            if (next is null || next.IsFail) return;
            node = next;   // retail's tail-continue
        }
    }

    private static void Visit(
        WalkBspNode? child, int pass, Vector3 viewpoint,
        Action<WalkPortalRef, int> emitPortal)
    {
        if (child is null || child.IsFail) return;
        Walk(child, pass, viewpoint, emitPortal);
    }

    /// <summary>
    /// <c>PView::DrawPortal</c> @0x005a5ab0 for one emitted portal polygon:
    /// resolve the CBldPortal, push view slots on its stab cells
    /// (<c>add_views</c>), run the CBldPortal ConstructView, on pass 2
    /// success run the look-in DrawCells, then pop the stab views. The
    /// GPU-state backup/restore and the building-frame re-push are
    /// submission concerns (FW2); the CPU state here is complete.
    /// </summary>
    public static bool DrawPortal(
        WalkPView pview, WalkBuilding building, in WalkPortalRef portalRef,
        int pass, IWalkBuildingFrameContext ctx, IWalkPortalPassSink sink)
    {
        ref readonly WalkBldPortal bldPortal = ref building.Portals[portalRef.PortalIndex];
        AddViews(bldPortal.StabList, ctx);
        bool ok = ConstructBuildingView(
            pview, building, in bldPortal, portalRef.Polygon, pass, ctx, sink);
        if (ok && pass != 1)
            sink.OnDrawCells(pview);
        RemoveViews(bldPortal.StabList, ctx);
        return ok;
    }

    /// <summary>
    /// The CBldPortal <c>PView::ConstructView</c> overload @0x005a59a0:
    /// the viewer's side of the portal plane must EQUAL the authored
    /// portal_side (IN_PLANE within ±ε fails both gates); the polygon must
    /// survive the active-view clip with ≥3 points; the destination cell
    /// must be Visible; the clipped view is appended to its top slot. Pass 1
    /// punches the polygon; pass ≠ 1 recurses into the interior flood.
    /// </summary>
    public static bool ConstructBuildingView(
        WalkPView pview, WalkBuilding building, in WalkBldPortal bldPortal,
        WalkPolygon polygon, int pass, IWalkBuildingFrameContext ctx,
        IWalkPortalPassSink sink)
    {
        Vector3 viewpoint = ctx.ViewpointInBuilding(building);
        float d = Vector3.Dot(polygon.Plane.Normal, viewpoint) + polygon.Plane.D;
        int side = d > WalkVisibilityMath.Epsilon ? 0
            : d < -WalkVisibilityMath.Epsilon ? 1 : 2;
        if (bldPortal.PortalSide != 0)
        {
            if (side != 1) return false;
        }
        else if (side != 0)
        {
            return false;
        }

        Span<WalkScreenPoint> clipped = stackalloc WalkScreenPoint[64];
        int n = ctx.ClipBuildingPolygon(building, polygon, side, clipped);
        if (n == 0) return false;

        WalkCell? cell = ctx.GetVisible(bldPortal.OtherCellId);
        if (cell is null) return false;
        if (!WalkCopyView.Append(
                cell.TopView, clipped[..n], ctx.Rays, ctx.WorldViewpoint))
            return false;

        if (pass != 2)
            sink.OnPunch(polygon);   // DrawPortalPolyInternal(poly, pass == 1)
        if (pass != 1)
            pview.ConstructView(cell, ToEntryIndex(bldPortal.OtherPortalId), ctx.CellContext);
        return true;
    }

    private static int ToEntryIndex(int otherPortalId)
        => otherPortalId < 0 ? 0xFFFF : otherPortalId;

    private static void AddViews(uint[] stabList, IWalkBuildingFrameContext ctx)
    {
        foreach (uint id in stabList)
            ctx.GetVisible(id)?.PushView();
    }

    private static void RemoveViews(uint[] stabList, IWalkBuildingFrameContext ctx)
    {
        foreach (uint id in stabList)
            ctx.GetVisible(id)?.PopView();
    }
}

/// <summary>The building half of the frame context: building-local
/// viewpoint and projection (retail pushes the building's object frame
/// before the portal pass), plus the shared cell context.</summary>
public interface IWalkBuildingFrameContext
{
    Vector3 ViewpointInBuilding(WalkBuilding building);

    /// <summary>The viewer's distance to the building
    /// (<c>CPhysicsPart::UpdateViewerDistance</c>'s input) — selects the
    /// degrade level whose drawing BSP the portal pass walks.</summary>
    float ViewerDistanceTo(WalkBuilding building);

    /// <summary>Project + clip one building-local portal polygon against
    /// the ACTIVE view (retail: GetClip with do_clip=1 in the building
    /// frame). Returns the surviving count.</summary>
    int ClipBuildingPolygon(
        WalkBuilding building, WalkPolygon polygon, int side, Span<WalkScreenPoint> output);

    WalkCell? GetVisible(uint cellId);
    IWalkRayCaster Rays { get; }
    Vector3 WorldViewpoint { get; }
    IWalkFrameContext CellContext { get; }
}