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

using System.Numerics;

namespace AcDream.App.Rendering.Walk;

/// <summary>
/// Campaign FW1 — retail's CPU visibility primitives, ported from the named
/// 2013 decomp (extractions:
/// docs/research/2026-08-30-fw-flood-pseudocode-appendix.md, report 4; all
/// x87-ambiguous branches Ghidra-arbitrated). Two families:
///
/// 1. The landscape interval test — a grid corner's vertical column is
///    classified against each active clip plane as a signed height
///    (<c>Render::get_pt_limit</c> @0x0054b840), and a landblock's z-slab
///    is tested against its four corner columns plane by plane
///    (<c>corner_plane_check</c> @0x0054b930, <c>block_plane_check</c>
///    @0x0054d060, <c>block_check</c> @0x0054dc50).
/// 2. The sphere-vs-view test (<c>Render::viewconeCheck</c> @0x0054c250).
///
/// Both consume the same installed state: the CY near plane (built by
/// <c>Render::update_viewpoint</c>) plus the active view's edge planes
/// (installed by <c>Render::set_view</c>).
/// </summary>
public static class WalkVisibilityMath
{
    /// <summary>Retail F_EPSILON (raw float 0x3951B717).</summary>
    public const float Epsilon = 0.000199999995f;

    /// <summary>get_pt_limit sky cap: a clip height at or above this means
    /// the column holds nothing (up-normal arm) / everything (down-normal
    /// arm).</summary>
    public const float SkyHeight = 1000f;

    /// <summary>Sentinel: the column is wholly inside the plane's positive
    /// half-space.</summary>
    public const float InsideColumn = 0f;

    /// <summary>Sentinel: the column is wholly outside (static init 1000+1).</summary>
    public const float OutsideColumn = 1001f;

    /// <summary>
    /// <c>Render::get_pt_limit</c> @0x0054b840: classify the vertical column
    /// at (x, y) against one plane. Returns <see cref="InsideColumn"/> /
    /// <see cref="OutsideColumn"/>, a positive h (inside only BELOW h —
    /// down-pointing normal), or a negative −h (inside only ABOVE h —
    /// up-pointing normal). Vertical planes (|N.z| ≤ ε) collapse to
    /// all-in/all-out via the plane side of (x, y, 0).
    /// </summary>
    public static float GetPointLimit(float x, float y, in WalkPlane plane)
    {
        Vector3 n = plane.Normal;
        if (n.Z > Epsilon)
        {
            float h = -((x * n.X + y * n.Y + plane.D) / n.Z);
            if (h >= SkyHeight) return OutsideColumn;
            return h > 0f ? -h : InsideColumn;
        }
        if (n.Z < -Epsilon)
        {
            float h = -((x * n.X + y * n.Y + plane.D) / n.Z);
            if (h <= 0f) return OutsideColumn;
            return h >= SkyHeight ? InsideColumn : h;
        }
        // Vertical plane: no z dependence; test point (x, y, 0), retail
        // Plane::which_side semantics (NEGATIVE = strictly below -epsilon).
        float d = x * n.X + y * n.Y + plane.D;
        return d < -Epsilon ? OutsideColumn : InsideColumn;
    }

    /// <summary>
    /// <c>Render::get_clip_height</c> @0x0054cff0: fill one corner's
    /// interval vector — bounds[0] vs the CY plane, bounds[1..edgeCount] vs
    /// each active edge plane (the loop is INCLUSIVE of edgeCount entries:
    /// edgeCount + 1 floats are written).
    /// </summary>
    public static void FillClipHeights(
        float x, float y, in WalkPlane cyPlane, ReadOnlySpan<WalkPlane> edgePlanes,
        Span<float> bounds)
    {
        bounds[0] = GetPointLimit(x, y, cyPlane);
        for (int i = 0; i < edgePlanes.Length; i++)
            bounds[i + 1] = GetPointLimit(x, y, edgePlanes[i]);
    }

    /// <summary>
    /// <c>Render::corner_plane_check</c> @0x0054b930: one corner's encoding
    /// vs the block z-slab [minZ, maxZ]. Boundary semantics are retail's:
    /// touching the clip height on the OUT side is Outside; on the IN side
    /// is EntirelyInside.
    /// </summary>
    public static WalkBoundingType CornerPlaneCheck(float bound, float minZ, float maxZ)
    {
        if (bound == OutsideColumn) return WalkBoundingType.Outside;
        if (bound != InsideColumn)
        {
            if (bound <= 0f)
            {
                // inside is z >= h, h = -bound
                float h = -bound;
                if (h > minZ)
                {
                    if (maxZ <= h) return WalkBoundingType.Outside;
                    return WalkBoundingType.PartiallyInside;
                }
            }
            else if (bound < maxZ)
            {
                // inside is z <= h, h = bound; the block top pokes above h
                if (bound <= minZ) return WalkBoundingType.Outside;
                return WalkBoundingType.PartiallyInside;
            }
        }
        return WalkBoundingType.EntirelyInside;
    }

    /// <summary>
    /// <c>Render::block_plane_check</c> @0x0054d060: four corners vs one
    /// plane. Outside/EntirelyInside require unanimity; anything mixed is
    /// PartiallyInside.
    /// </summary>
    public static WalkBoundingType BlockPlaneCheck(
        float b1, float b2, float b3, float b4, float minZ, float maxZ)
    {
        WalkBoundingType c1 = CornerPlaneCheck(b1, minZ, maxZ);
        WalkBoundingType c2 = CornerPlaneCheck(b2, minZ, maxZ);
        WalkBoundingType c3 = CornerPlaneCheck(b3, minZ, maxZ);
        WalkBoundingType c4 = CornerPlaneCheck(b4, minZ, maxZ);
        if (c1 == WalkBoundingType.Outside)
        {
            if (c2 == WalkBoundingType.Outside
                && c3 == WalkBoundingType.Outside
                && c4 == WalkBoundingType.Outside)
            {
                return WalkBoundingType.Outside;
            }
        }
        else if (c1 == WalkBoundingType.EntirelyInside
                 && c2 == WalkBoundingType.EntirelyInside
                 && c3 == WalkBoundingType.EntirelyInside
                 && c4 == WalkBoundingType.EntirelyInside)
        {
            return WalkBoundingType.EntirelyInside;
        }
        return WalkBoundingType.PartiallyInside;
    }

    /// <summary>
    /// <c>Render::block_check</c> @0x0054dc50: the block-vs-view test over
    /// four corner interval vectors. Retail's call site passes
    /// (max_zval, min_zval) and an internal double positional swap hands
    /// corner_plane_check (min, max) — this port takes (maxZ, minZ) to
    /// mirror the caller's argument order and performs the same swap, so
    /// call sites read like the decomp. Any plane with all four corners
    /// outside culls; any partial plane demotes stickily; EntirelyInside
    /// requires unanimity on every plane. planeCount = the active view's
    /// edge count (bounds[0] is the CY plane, tested first with early-out).
    /// </summary>
    public static WalkBoundingType BlockCheck(
        ReadOnlySpan<float> corner00, ReadOnlySpan<float> corner01,
        ReadOnlySpan<float> corner10, ReadOnlySpan<float> corner11,
        int planeCount, float maxZ, float minZ)
    {
        WalkBoundingType result = BlockPlaneCheck(
            corner00[0], corner01[0], corner10[0], corner11[0], minZ, maxZ);
        if (result == WalkBoundingType.Outside) return WalkBoundingType.Outside;
        for (int k = 1; k <= planeCount; k++)
        {
            WalkBoundingType r = BlockPlaneCheck(
                corner00[k], corner01[k], corner10[k], corner11[k], minZ, maxZ);
            if (r == WalkBoundingType.Outside) return WalkBoundingType.Outside;
            if (r == WalkBoundingType.PartiallyInside)
                result = WalkBoundingType.PartiallyInside;
        }
        return result;
    }

    /// <summary>
    /// <c>Render::viewconeCheck</c> @0x0054c250 (plane-test half): a sphere
    /// (center already in viewer-block space, radius already scaled) vs the
    /// CY plane plus the active view's edge planes. Cull is STRICT
    /// (d &lt; −r); the partial flag is INCLUSIVE (d ≤ r) — a sphere exactly
    /// tangent from inside counts Partial, not EntirelyInside. The retail
    /// body's side effects (publishing local_object_center/radius) belong
    /// to the walk context, not this math.
    /// </summary>
    public static WalkBoundingType ViewconeCheck(
        Vector3 center, float radius, in WalkPlane cyPlane,
        ReadOnlySpan<WalkPlane> edgePlanes)
    {
        float d = Vector3.Dot(cyPlane.Normal, center) + cyPlane.D;
        if (d < -radius) return WalkBoundingType.Outside;
        bool partial = d <= radius;
        foreach (ref readonly WalkPlane plane in edgePlanes)
        {
            d = Vector3.Dot(plane.Normal, center) + plane.D;
            if (d < -radius) return WalkBoundingType.Outside;
            if (d <= radius) partial = true;
        }
        return partial ? WalkBoundingType.PartiallyInside : WalkBoundingType.EntirelyInside;
    }
}

/// <summary>Retail <c>Plane</c>: dot(N, p) + d, positive side = inside.</summary>
public readonly record struct WalkPlane(Vector3 Normal, float D);

/// <summary>Retail <c>BoundingType</c>, values used raw by the walk.</summary>
public enum WalkBoundingType
{
    Outside = 0,
    PartiallyInside = 1,
    EntirelyInside = 2,
}