From 0778c78068adc94ade265f205c383e360ab981d1 Mon Sep 17 00:00:00 2001
From: Filippo Luca Ferretti <102977828+flferretti@users.noreply.github.com>
Date: Tue, 26 Mar 2024 14:56:37 +0100
Subject: [PATCH] Fix `api.contact.in_contact`

The length of `links_in_contact` was always equal to number of links in the model
---
 src/jaxsim/api/contact.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/jaxsim/api/contact.py b/src/jaxsim/api/contact.py
index 47650cf95..464f56917 100644
--- a/src/jaxsim/api/contact.py
+++ b/src/jaxsim/api/contact.py
@@ -100,7 +100,7 @@ def in_contact(
 
     link_names = link_names if link_names is not None else model.link_names()
 
-    if set(link_names) - set(model.link_names()) != set():
+    if (link_names := set(link_names)) and link_names.difference(model.link_names()):
         raise ValueError("One or more link names are not part of the model")
 
     from jaxsim.physics.algos.soft_contacts import collidable_points_pos_vel
@@ -124,7 +124,7 @@ def in_contact(
             below_terrain,
             jnp.zeros_like(below_terrain, dtype=bool),
         ).any()
-    )(jnp.arange(model.number_of_links()))
+    )(jnp.arange(len(link_names)))
 
     return links_in_contact