acdream/src/AcDream.Core/Physics/PhysicsObjUpdate.cs

using System.Numerics;

namespace AcDream.Core.Physics;

/// <summary>
/// Verbatim port of the collision-response tail of retail
/// <c>CPhysicsObj::UpdateObjectInternal</c>: the velocity decision that
/// <c>SetPositionInternal</c> (0x00515330) drives through
/// <c>handle_all_collisions</c> (0x00514780, pc:282647). Kept as a pure function over a
/// <see cref="PhysicsBody"/> + the resolve outcome so the whole decision is unit-testable
/// in Core, independent of the App per-frame loop. The transition INTERNALS
/// (<c>ResolveWithTransition</c> and below) are untouched.
/// </summary>
public static class PhysicsObjUpdate
{
    /// <summary>
    /// Retail <c>SetPositionInternal</c> walkability comparison at
    /// <c>0x00515465-0x0051548E</c>. Equality with FloorZ is walkable.
    /// </summary>
    public static bool IsWalkableContact(bool inContact, Vector3 contactNormal)
        => inContact && contactNormal.Z >= PhysicsGlobals.FloorZ;

    /// <summary>
    /// Applies the two independent transient facts written by retail
    /// <c>CPhysicsObj::SetPositionInternal</c> at <c>0x00515430-0x0051549F</c>.
    /// Contact comes from contact-plane validity; OnWalkable additionally
    /// requires a walkable plane normal. A steep contact therefore remains in
    /// contact without becoming grounded.
    /// </summary>
    public static void ApplySetPositionContact(
        PhysicsBody body,
        bool inContact,
        bool onWalkable)
    {
        if (inContact)
            body.TransientState |= TransientStateFlags.Contact;
        else
            body.TransientState &= ~TransientStateFlags.Contact;

        if (inContact && onWalkable)
            body.TransientState |= TransientStateFlags.OnWalkable;
        else
            body.TransientState &= ~TransientStateFlags.OnWalkable;

        // AP-10 (Campaign P Slice P4, 2026-07-30): retail SetPositionInternal
        // (0x005153e5-0051545f) writes WATER_CONTACT_TS in the same statement
        // block as CONTACT_TS, immediately after. body.ContactPlaneIsWater is
        // already current by the time this runs (every caller sets it, or it
        // carries over from the prior tick, before calling here).
        if (body.ContactPlaneIsWater)
            body.TransientState |= TransientStateFlags.WaterContact;
        else
            body.TransientState &= ~TransientStateFlags.WaterContact;

        body.calc_acceleration();
    }

    /// <summary>
    /// Commits the contact/walkable/collision-response tail of retail
    /// <c>CPhysicsObj::SetPositionInternal</c> (<c>0x00515330</c>) in its
    /// original order. The pre-transition flags are explicit because a
    /// deferred placement may temporarily park the body without contact while
    /// still needing the source edge when its destination cell becomes ready.
    /// </summary>
    /// <remarks>
    /// Retail writes Contact and recalculates acceleration, calls
    /// <c>set_on_walkable</c> (which invokes HitGround/LeaveGround), then calls
    /// <c>handle_all_collisions</c> at <c>0x005154FE</c>. Collision response
    /// must therefore observe any velocity change made by the movement
    /// callback; moving that callback after reflection changes the result.
    /// </remarks>
    public static bool CommitSetPositionTransition(
        PhysicsBody body,
        bool inContact,
        bool onWalkable,
        bool collisionNormalValid,
        Vector3 collisionNormal,
        bool previousContact,
        bool previousOnWalkable,
        Action? hitGround = null,
        Action? leaveGround = null,
        Func<bool>? isCurrent = null,
        Func<bool>? isVelocityCurrent = null)
    {
        ArgumentNullException.ThrowIfNull(body);

        // SetPositionInternal replaces Contact first but retains the source
        // OnWalkable bit through its first calc_acceleration call. A deferred
        // teleport may have parked the live body with both bits cleared, so
        // restore the captured source bit explicitly before reproducing that
        // ordering.
        if (previousOnWalkable)
            body.TransientState |= TransientStateFlags.OnWalkable;
        else
            body.TransientState &= ~TransientStateFlags.OnWalkable;

        if (inContact)
            body.TransientState |= TransientStateFlags.Contact;
        else
            body.TransientState &= ~TransientStateFlags.Contact;
        body.calc_acceleration();

        bool finalOnWalkable = inContact && onWalkable;
        if (finalOnWalkable)
            body.TransientState |= TransientStateFlags.OnWalkable;
        else
            body.TransientState &= ~TransientStateFlags.OnWalkable;

        // AP-10 (Campaign P Slice P4, 2026-07-30): mirror WATER_CONTACT_TS
        // alongside CONTACT_TS/ON_WALKABLE_TS, same as ApplySetPositionContact.
        // Callers (e.g. RemoteTeleportPlacement) already set body.ContactPlaneIsWater
        // before invoking this commit.
        if (body.ContactPlaneIsWater)
            body.TransientState |= TransientStateFlags.WaterContact;
        else
            body.TransientState &= ~TransientStateFlags.WaterContact;

        if (!previousOnWalkable && finalOnWalkable)
        {
            hitGround?.Invoke();
            if (isCurrent?.Invoke() == false)
                return false;
        }
        else if (previousOnWalkable && !finalOnWalkable)
        {
            leaveGround?.Invoke();
            if (isCurrent?.Invoke() == false)
                return false;
        }
        body.calc_acceleration();

        // Position, Vector, and Movement are independently timestamped but
        // can all install m_velocityVector. If a later one arrived from a
        // callback above, retain its vector and finish the non-overlapping
        // contact/pose commit without applying this older collision response.
        if (isVelocityCurrent?.Invoke() == false)
            return isCurrent?.Invoke() ?? true;

        HandleAllCollisions(
            body,
            collisionNormalValid,
            collisionNormal,
            previousContact,
            previousOnWalkable,
            finalOnWalkable);
        return isCurrent?.Invoke() ?? true;
    }

    /// <summary>
    /// retail <c>handle_all_collisions</c> (0x00514780). Reflects or zeros the body's
    /// <see cref="PhysicsBody.Velocity"/> (retail m_velocityVector) based on
    /// <see cref="PhysicsBody.FramesStationaryFall"/>:
    /// <list type="bullet">
    ///   <item>fsf ≤ 1 → reflect the into-surface component
    ///     (<c>v += -(v·n)(elasticity+1)·n</c>, pc:282712) when we should reflect and a
    ///     collision normal is valid; an INELASTIC mover zeros instead (pc:282720).</item>
    ///   <item>fsf &gt; 1 → <c>v = 0</c> entirely (pc:282729) — the "bleed on block" that
    ///     lets gravity resume so a blocked jump falls/glides off (fixes the #182
    ///     airborne-stuck wedge).</item>
    /// </list>
    /// The Stationary* transient-bit round-trip (pc:282737-758) is owned by the Core resolve
    /// writeback (<c>PhysicsEngine.ResolveWithTransition</c>), not re-encoded here.
    /// </summary>
    /// <param name="body">The mover.</param>
    /// <param name="collisionNormalValid">Was a wall/creature collision normal recorded this resolve.</param>
    /// <param name="collisionNormal">Outward collision normal (points away from the surface).</param>
    /// <param name="prevContact">Whether the body had Contact BEFORE this resolve committed
    ///   (retail arg3 — reserved for environment-collision reporting; unused today).</param>
    /// <param name="prevOnWalkable">Whether the body was OnWalkable before this resolve (retail arg4).</param>
    /// <param name="nowOnWalkable">Whether the body is OnWalkable after this resolve.</param>
    public static void HandleAllCollisions(
        PhysicsBody body,
        bool collisionNormalValid, Vector3 collisionNormal,
        bool prevContact, bool prevOnWalkable, bool nowOnWalkable)
    {
        // var_10_1 (pc:282653-282657): reflect UNLESS the mover stays on walkable ground
        // (and is not sledding). This restores retail's broader rule — the AD-25 airborne-only
        // suppression is retired: the landing-snap fragility it guarded is gone (landing state
        // is now owned by the SetPositionInternal-derived contact flags, not a Velocity.Z<=0
        // gate). A grounded corridor wall-slide keeps its tangential velocity (should_reflect
        // false), exactly as retail.
        bool sledding = body.State.HasFlag(PhysicsStateFlags.Sledding);
        bool shouldReflect = !(prevOnWalkable && nowOnWalkable && !sledding);

        if (body.FramesStationaryFall <= 1)
        {
            if (shouldReflect && collisionNormalValid)
            {
                if (body.State.HasFlag(PhysicsStateFlags.Inelastic))
                {
                    body.Velocity = Vector3.Zero;                          // pc:282720-282722
                }
                else
                {
                    float dot = Vector3.Dot(body.Velocity, collisionNormal);
                    if (dot < 0f)                                          // moving INTO the surface
                    {
                        float k = -(dot * (body.Elasticity + 1f));         // pc:282712
                        body.Velocity += collisionNormal * k;
                    }
                }
            }
        }
        else
        {
            body.Velocity = Vector3.Zero;                                  // fsf>1 → THE BLEED (pc:282729)
        }

        _ = prevContact;  // retail report_environment_collision(arg3) — weenie collision events, later.
    }
}