using System.Numerics;

namespace AcDream.Core.Physics;

/// <summary>
/// Performs the compressed first-gravity-frame settle used to establish
/// retail Contact/OnWalkable state for a newly placed body.
///
/// Retail gains spawn contact from the FIRST GRAVITY FRAME, not the
/// placement itself: <c>CPhysicsObj::enter_world</c> (0x00516170) runs
/// <c>SetPosition</c> (find_placement validates the spot but records no
/// touch) and every retail CPhysicsObj then simulates, falls the few
/// centimetres onto the floor, and the transition's touch grants the
/// contact plane + CONTACT/ON_WALKABLE. Bodies that do not run that first
/// ordinary frame at placement (stationary remotes — #270 — and the local
/// player's Runtime first-entry activation — C3c-F5) compress the settle
/// here: a short downward sweep from the placed position whose touch
/// handler produces exactly the state retail's first frame would. A sweep
/// that finds no floor (true airborne spawn) leaves the body airborne,
/// exactly like retail's fall.
/// </summary>
public static class SpawnPlacementSettler
{
    public const float SettleDistance = 0.5f;

    public static bool TrySettle(
        PhysicsEngine physicsEngine,
        PhysicsBody body,
        Vector3 worldPosition,
        uint cellId,
        float sphereRadius,
        float sphereHeight,
        ObjectInfoState moverFlags,
        uint movingEntityId,
        Action hitGround,
        Action leaveGround)
    {
        ArgumentNullException.ThrowIfNull(physicsEngine);
        ArgumentNullException.ThrowIfNull(body);
        ArgumentNullException.ThrowIfNull(hitGround);
        ArgumentNullException.ThrowIfNull(leaveGround);

        if (cellId == 0)
            return false;

        ResolveResult settle = physicsEngine.ResolveWithTransition(
            worldPosition,
            worldPosition - new Vector3(0f, 0f, SettleDistance),
            cellId,
            sphereRadius,
            sphereHeight,
            stepUpHeight: 0.4f,
            stepDownHeight: 0.4f,
            isOnGround: false,
            body,
            moverFlags,
            movingEntityId);
        if (!settle.Ok || !settle.InContact)
            return false;

        // #276: adopt the settle's RESOLVED cell, not just its position.
        // Retail's CPhysicsObj::SetPositionInternal(CTransition const*)
        // @0x00515330 commits both sphere_path.curr_pos.objcell_id and its
        // frame, including EnvCells.
        //
        // A bare `body.Position = settle.Position` is sufficient only while
        // the body is OUTDOOR: that setter mirrors the world delta into the
        // landblock-local frame and LandDefs.AdjustToOutside then recomputes
        // the 24 m cell index from it. It cannot do so for an EnvCell — an
        // EnvCell id is not derivable from a position, so the mirror
        // deliberately preserves it (PhysicsBody.cs, the `not (>= 1 and
        // <= 0x40)` arm). settle.CellId is therefore the ONLY carrier of a
        // cell identity across an indoor seam, and dropping it stranded the
        // body in the placement cell until some later resolve corrected it.
        //
        // Identical guarded shape to the per-tick resolve writeback
        // (RuntimeOrdinaryPhysicsUpdater): a transition reporting no cell
        // falls back to the source cell rather than zeroing residency.
        uint resolvedCellId = settle.CellId != 0
            ? settle.CellId
            : cellId;
        body.CommitTransitionPosition(resolvedCellId, settle.Position);
        PhysicsObjUpdate.CommitSetPositionTransition(
            body,
            settle.InContact,
            settle.OnWalkable,
            settle.CollisionNormalValid,
            settle.CollisionNormal,
            previousContact: false,
            previousOnWalkable: false,
            hitGround,
            leaveGround);
        return true;
    }
}