Add utility function for merging directory names with filenames, and fix issue in mj_addfileVFS where directory names without trailing filesystem separators won't append correctly.

PiperOrigin-RevId: 545450158
Change-Id: I550394f11ac2837d35f3d869f8718a119a9eac38

src/engine/engine_util_misc.c

@@ -1177,6 +1177,51 @@ char* mju_strncpy(char *dst, const char *src, int n) {
 
 
 
+// assemble full filename from directory and filename, return 0 on success
+int mju_makefullname(char* full, size_t nfull, const char* dir, const char* file) {
+  int dirlen = (!dir) ? 0 : strlen(dir);
+  int filelen = (!file) ? 0 : strlen(file);
+  char* filepos = full + dirlen;
+
+  // missing filename
+  if (!filelen) {
+    return -1;
+  }
+
+  // no directory then just copy filename over
+  if (!dirlen) {
+    // make sure full has space
+    if (filelen >= nfull) {
+      return -1;
+    }
+    strcpy(full, file);
+    return 0;
+  }
+
+  // make sure full has space
+  if (dirlen + filelen >= nfull) {
+    return -1;
+  }
+
+  // dir doesn't end with a slash
+  if (dir[dirlen - 1] != '\\' && dir[dirlen - 1] != '/') {
+    // need extra space for forward slash
+    if ((dirlen + filelen + 1) >= nfull) {
+      return -1;
+    }
+
+    // add forward slash
+    *filepos++ = '/';
+  }
+
+  // copy directory and file over
+  memcpy(full, dir, sizeof(char) * dirlen);
+  strcpy(filepos, file);
+  return 0;
+}
+
+
+
 // sigmoid function over 0<=x<=1 using quintic polynomial
 mjtNum mju_sigmoid(mjtNum x) {
   // fast return