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```use alga::linear::FiniteDimVectorSpace;
use na::{DVector, RealField, Unit};

use crate::joint::JointConstraint;
use crate::math::{Point, Vector, DIM};
use crate::object::{BodyHandle, BodyPartHandle, BodySet};
use crate::solver::{
helper, BilateralConstraint, BilateralGroundConstraint, ForceDirection, ImpulseLimits,
};
use crate::solver::{
GenericNonlinearConstraint, IntegrationParameters, LinearConstraints,
NonlinearConstraintGenerator,
};

/// A spring-like constraint to be used to drag a body part with the mouse.
pub struct MouseConstraint<N: RealField, Handle: BodyHandle> {
b1: BodyPartHandle<Handle>,
b2: BodyPartHandle<Handle>,
anchor1: Point<N>,
anchor2: Point<N>,
limit: N,
}

impl<N: RealField, Handle: BodyHandle> MouseConstraint<N, Handle> {
/// Initialize a mouse constraint between two bodies.getPartHandle
///
/// Typically, `b1` will be the ground and the anchor the position of the mouse.
/// Both anchors are expressed in the local coordinate frames of the corresponding body parts.
pub fn new(
b1: BodyPartHandle<Handle>,
b2: BodyPartHandle<Handle>,
anchor1: Point<N>,
anchor2: Point<N>,
limit: N,
) -> Self {
MouseConstraint {
b1,
b2,
anchor1,
anchor2,
limit,
}
}

/// Change the first anchor, expressed in the local space of the first body part.
pub fn set_anchor_1(&mut self, anchor1: Point<N>) {
self.anchor1 = anchor1;
}

/// Change the first anchor, expressed in the local space of the second body part.
pub fn set_anchor_2(&mut self, anchor2: Point<N>) {
self.anchor2 = anchor2;
}
}

impl<N: RealField, Handle: BodyHandle> JointConstraint<N, Handle> for MouseConstraint<N, Handle> {
fn num_velocity_constraints(&self) -> usize {
DIM
}

fn anchors(&self) -> (BodyPartHandle<Handle>, BodyPartHandle<Handle>) {
(self.b1, self.b2)
}

fn velocity_constraints(
&mut self,
parameters: &IntegrationParameters<N>,
bodies: &dyn BodySet<N, Handle = Handle>,
ext_vels: &DVector<N>,
ground_j_id: &mut usize,
j_id: &mut usize,
jacobians: &mut [N],
constraints: &mut LinearConstraints<N, usize>,
) {
let body1 = try_ret!(bodies.get(self.b1.0));
let body2 = try_ret!(bodies.get(self.b2.0));
let part1 = try_ret!(body1.part(self.b1.1));
let part2 = try_ret!(body2.part(self.b2.1));

/*
*
* Joint constraints.
*
*/
let anchor1 = body1.world_point_at_material_point(part1, &self.anchor1);
let anchor2 = body2.world_point_at_material_point(part2, &self.anchor2);

let assembly_id1 = body1.companion_id();
let assembly_id2 = body2.companion_id();

let limits = ImpulseLimits::Independent {
min: -self.limit,
max: self.limit,
};

let error = anchor2 - anchor1;
let (ext_vels1, ext_vels2) =
helper::split_ext_vels(body1, body2, assembly_id1, assembly_id2, ext_vels);

let mut i = 0;
Vector::canonical_basis(|dir| {
let fdir = ForceDirection::Linear(Unit::new_unchecked(*dir));
let mut rhs = -error.dot(&*dir) * parameters.erp * parameters.inv_dt();
let geom = helper::constraint_pair_geometry(
body1,
part1,
self.b1,
body2,
part2,
self.b2,
&anchor1,
&anchor2,
&fdir,
ground_j_id,
j_id,
jacobians,
Some(&ext_vels1),
Some(&ext_vels2),
Some(&mut rhs),
);

if geom.ndofs1 == 0 || geom.ndofs2 == 0 {
constraints
.bilateral_ground
.push(BilateralGroundConstraint::new(
geom,
assembly_id1,
assembly_id2,
limits,
rhs,
N::zero(),
0,
));
} else {
constraints.bilateral.push(BilateralConstraint::new(
geom,
assembly_id1,
assembly_id2,
limits,
rhs,
N::zero(),
0,
));
}

i += 1;

true
});
}

fn cache_impulses(&mut self, _: &LinearConstraints<N, usize>, _: N) {}
}

impl<N: RealField, Handle: BodyHandle> NonlinearConstraintGenerator<N, Handle>
for MouseConstraint<N, Handle>
{
fn num_position_constraints(&self, _: &dyn BodySet<N, Handle = Handle>) -> usize {
0
}

fn position_constraint(
&self,
_: &IntegrationParameters<N>,
_: usize,
_: &mut dyn BodySet<N, Handle = Handle>,
_: &mut [N],
) -> Option<GenericNonlinearConstraint<N, Handle>> {
None
}
}
```