implemented create_atoms and encode_image_flags and unit tests for them; added IMGMASK and friends to extract_setting and unit tests for them; wrote documentation for them

src/library.cpp

@@ -953,6 +953,7 @@ not recognized, the function returns -1.  The integer sizes functions may
 be called without a valid LAMMPS object handle (it is ignored).
 
 * :ref:`Integer sizes <extract_integer_sizes>`
+* :ref:`Image masks <extract_image_masks>`
 * :ref:`System status <extract_system_status>`
 * :ref:`System sizes <extract_system_sizes>`
 * :ref:`Atom style flags <extract_atom_flags>`
@@ -977,6 +978,28 @@ be called without a valid LAMMPS object handle (it is ignored).
      - size of the ``imageint`` integer type, 4 or 8 bytes.
        Set at :ref:`compile time <size>`.
 
+.. _extract_image_masks:
+
+**Image masks**
+
+These settings are related to how LAMMPS stores and interprets periodic images. The values are used
+internally by the Fortran interface and are not likely to be useful to users.
+
+.. list-table::
+   :header-rows: 1
+   :widths: auto
+
+   * - Keyword
+     - Description / Return value
+   * - IMGMASK
+     - Bit-mask used to convert image flags to a single integer
+   * - IMGMAX
+     - Maximum allowed image number for a particular atom
+   * - IMGBITS
+     - Bits used in image counts
+   * - IMG2BITS
+     - Second bitmask used in image counts
+
 .. _extract_system_status:
 
 **System status**
@@ -1096,6 +1119,11 @@ int lammps_extract_setting(void *handle, const char *keyword)
   if (strcmp(keyword,"tagint") == 0) return sizeof(tagint);
   if (strcmp(keyword,"imageint") == 0) return sizeof(imageint);
 
+  if (strcmp(keyword,"IMGMASK") == 0) return IMGMASK;
+  if (strcmp(keyword,"IMGBITS") == 0) return IMGBITS;
+  if (strcmp(keyword,"IMG2BITS") == 0) return IMG2BITS;
+  if (strcmp(keyword,"IMGMAX") == 0) return IMGMAX;
+
   if (strcmp(keyword,"dimension") == 0) return lmp->domain->dimension;
   if (strcmp(keyword,"box_exist") == 0) return lmp->domain->box_exist;
   if (strcmp(keyword,"newton_bond") == 0) return lmp->force->newton_bond;