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```
```use na::{DVector, RealField};
use std::ops::Range;

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

/// A constraint that removes all relative linear motion between two body parts.
pub struct BallConstraint<N: RealField, Handle: BodyHandle> {
b1: BodyPartHandle<Handle>,
b2: BodyPartHandle<Handle>,
anchor1: Point<N>,
anchor2: Point<N>,
impulses: Vector<N>,
break_force_squared: N,
broken: bool,
bilateral_ground_rng: Range<usize>,
bilateral_rng: Range<usize>,
}

impl<N: RealField, Handle: BodyHandle> BallConstraint<N, Handle> {
/// Creates a ball constraint between two body parts.
///
/// This will ensure the two points identified by `anchor1` and `anchor2` will coincide.
/// Both are given in the local-space of their corresponding body part.
pub fn new(
b1: BodyPartHandle<Handle>,
b2: BodyPartHandle<Handle>,
anchor1: Point<N>,
anchor2: Point<N>,
) -> Self {
BallConstraint {
b1,
b2,
anchor1,
anchor2,
impulses: Vector::zeros(),
break_force_squared: N::max_value(),
broken: false,
bilateral_ground_rng: 0..0,
bilateral_rng: 0..0,
}
}

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

/// The maximum force this joint can absorb before breaking.
pub fn set_break_force(&mut self, break_force: N) {
self.break_force_squared = break_force * break_force;
}
}

impl<N: RealField, Handle: BodyHandle> JointConstraint<N, Handle> for BallConstraint<N, Handle> {
fn is_broken(&self) -> bool {
self.broken
}

fn num_velocity_constraints(&self) -> usize {
DIM
}

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

fn velocity_constraints(
&mut self,
_: &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 first_bilateral_ground = constraints.bilateral_ground.len();
let first_bilateral = constraints.bilateral.len();

helper::cancel_relative_linear_velocity(
body1,
part1,
self.b1,
body2,
part2,
self.b2,
assembly_id1,
assembly_id2,
&anchor1,
&anchor2,
ext_vels,
&self.impulses,
0,
ground_j_id,
j_id,
jacobians,
constraints,
);

self.bilateral_ground_rng = first_bilateral_ground..constraints.bilateral_ground.len();
self.bilateral_rng = first_bilateral..constraints.bilateral.len();
}

fn cache_impulses(&mut self, constraints: &LinearConstraints<N, usize>, inv_dt: N) {
for c in &constraints.bilateral_ground[self.bilateral_ground_rng.clone()] {
self.impulses[c.impulse_id] = c.impulse;
}

for c in &constraints.bilateral[self.bilateral_rng.clone()] {
self.impulses[c.impulse_id] = c.impulse;
}

if self.impulses.norm_squared() * inv_dt * inv_dt > self.break_force_squared {
self.broken = true;
}
}
}

impl<N: RealField, Handle: BodyHandle> NonlinearConstraintGenerator<N, Handle>
for BallConstraint<N, Handle>
{
fn num_position_constraints(&self, bodies: &dyn BodySet<N, Handle = Handle>) -> usize {
// FIXME: calling this at each iteration of the non-linear resolution is costly.
if self.is_active(bodies) {
1
} else {
0
}
}

fn position_constraint(
&self,
parameters: &IntegrationParameters<N>,
_: usize,
bodies: &mut dyn BodySet<N, Handle = Handle>,
jacobians: &mut [N],
) -> Option<GenericNonlinearConstraint<N, Handle>> {
let body1 = bodies.get(self.b1.0)?;
let body2 = bodies.get(self.b2.0)?;
let part1 = body1.part(self.b1.1)?;
let part2 = body2.part(self.b2.1)?;

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

helper::cancel_relative_translation(
parameters, body1, part1, self.b1, body2, part2, self.b2, &anchor1, &anchor2, jacobians,
)
}
}
```