From 49dd6b41933a78da46f54504edba2e0c5e8c7d41 Mon Sep 17 00:00:00 2001
From: Kyle Bayes <kylebayes@google.com>
Date: Mon, 6 Mar 2023 01:58:51 -0800
Subject: [PATCH] Clean up mj_id2name and mj_name2id.

PiperOrigin-RevId: 514340647
Change-Id: Idf895b2d7a4a8833725afbe5c71c777081b42d9e
---
 doc/APIreference/functions.rst |  4 ++--
 include/mujoco/mujoco.h        |  4 ++--
 introspect/functions.py        |  4 ++--
 src/engine/engine_support.c    | 29 +++++++++++++++--------------
 src/engine/engine_support.h    |  4 ++--
 5 files changed, 23 insertions(+), 22 deletions(-)

diff --git a/doc/APIreference/functions.rst b/doc/APIreference/functions.rst
index 59fd8b4e..28040c82 100644
--- a/doc/APIreference/functions.rst
+++ b/doc/APIreference/functions.rst
@@ -960,7 +960,7 @@ mj_name2id
 
 .. mujoco-include:: mj_name2id
 
-Get id of object with specified name, return -1 if not found; type is mjtObj.
+Get id of object with the specified mjtObj type and name, returns -1 if id not found.
 
 .. _mj_id2name:
 
@@ -969,7 +969,7 @@ mj_id2name
 
 .. mujoco-include:: mj_id2name
 
-Get name of object with specified id, return 0 if invalid type or id; type is mjtObj.
+Get name of object with the specified mjtObj type and id, returns ``NULL`` if name not found.
 
 .. _mj_fullM:
 
diff --git a/include/mujoco/mujoco.h b/include/mujoco/mujoco.h
index 8151e363..3e392a80 100644
--- a/include/mujoco/mujoco.h
+++ b/include/mujoco/mujoco.h
@@ -400,10 +400,10 @@ MJAPI void mj_jacSite(const mjModel* m, const mjData* d, mjtNum* jacp, mjtNum* j
 MJAPI void mj_jacPointAxis(const mjModel* m, mjData* d, mjtNum* jacPoint, mjtNum* jacAxis,
                            const mjtNum point[3], const mjtNum axis[3], int body);
 
-// Get id of object with specified name, return -1 if not found; type is mjtObj.
+// Get id of object with the specified mjtObj type and name, returns -1 if id not found.
 MJAPI int mj_name2id(const mjModel* m, int type, const char* name);
 
-// Get name of object with specified id, return 0 if invalid type or id; type is mjtObj.
+// Get name of object with the specified mjtObj type and id, returns NULL if name not found.
 MJAPI const char* mj_id2name(const mjModel* m, int type, int id);
 
 // Convert sparse inertia matrix M into full (i.e. dense) matrix.
diff --git a/introspect/functions.py b/introspect/functions.py
index 8aeb0c90..9ef1f67a 100644
--- a/introspect/functions.py
+++ b/introspect/functions.py
@@ -2184,7 +2184,7 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
                  ),
              ),
          ),
-         doc='Get id of object with specified name, return -1 if not found; type is mjtObj.',  # pylint: disable=line-too-long
+         doc='Get id of object with the specified mjtObj type and name, returns -1 if id not found.',  # pylint: disable=line-too-long
      )),
     ('mj_id2name',
      FunctionDecl(
@@ -2208,7 +2208,7 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
                  type=ValueType(name='int'),
              ),
          ),
-         doc='Get name of object with specified id, return 0 if invalid type or id; type is mjtObj.',  # pylint: disable=line-too-long
+         doc='Get name of object with the specified mjtObj type and id, returns NULL if name not found.',  # pylint: disable=line-too-long
      )),
     ('mj_fullM',
      FunctionDecl(
diff --git a/src/engine/engine_support.c b/src/engine/engine_support.c
index 89289e0c..7860e45e 100644
--- a/src/engine/engine_support.c
+++ b/src/engine/engine_support.c
@@ -636,8 +636,8 @@ uint64_t mj_hashdjb2(const char* s, uint64_t n) {
   return h % n;
 }
 
-
-// get id of object with specified name; -1: not found
+// get id of object with the specified mjtObj type and name,
+// returns -1 if id not found
 int mj_name2id(const mjModel* m, int type, const char* name) {
   int mapadr;
   int* adr = 0;
@@ -652,19 +652,23 @@ int mj_name2id(const mjModel* m, int type, const char* name) {
 
     do {
       int j = m->names_map[mapadr + i];
-      if (j < 0) return -1;
+      if (j<0) {
+        return -1;
+      }
+
       if (!strncmp(name, m->names+adr[j], m->nnames-adr[j])) {
         return j;
       }
-      if (++i == num) i = 0;
-    } while(i != hash);
+      if ((++i)==num) i = 0;
+    } while (i!=hash);
   }
   return -1;
 }
 
 
 
-// get name of object with specified id; 0: invalid type or id, or null name
+// get name of object with the specified mjtObj type and id,
+// returns NULL if name not found
 const char* mj_id2name(const mjModel* m, int type, int id) {
   int mapadr;
   int* adr = 0;
@@ -672,15 +676,12 @@ const char* mj_id2name(const mjModel* m, int type, int id) {
   // get number of objects and name addresses
   int num = _getnumadr(m, type, &adr, &mapadr);
 
-  if (id>=0 && id<num) {
-    if (m->names[adr[id]]) {
-      return m->names+adr[id];
-    } else {
-      return 0;
-    }
-  } else {
-    return 0;
+  // id is in [0, num) and the found name is not the empty string "\0"
+  if (id>=0 && id<num && m->names[adr[id]]) {
+    return m->names+adr[id];
   }
+
+  return NULL;
 }
 
 
diff --git a/src/engine/engine_support.h b/src/engine/engine_support.h
index 18fbaf33..cab2f4bc 100644
--- a/src/engine/engine_support.h
+++ b/src/engine/engine_support.h
@@ -81,10 +81,10 @@ MJAPI int mj_jacDifPair(const mjModel* m, const mjData* d, int* chain,
 // get string hash, see http://www.cse.yorku.ca/~oz/hash.html
 uint64_t mj_hashdjb2(const char* s, uint64_t n);
 
-// get id of object with specified name; -1: not found; type is mjtObj
+// get id of object with the specified mjtObj type and name, returns -1 if id not found
 MJAPI int mj_name2id(const mjModel* m, int type, const char* name);
 
-// get name of object with specified id; 0: invalid type or id; type is mjtObj
+// get name of object with the specified mjtObj type and id, returns NULL if name not found
 MJAPI const char* mj_id2name(const mjModel* m, int type, int id);