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>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.
    }
}