/* ---------------------------------------------------------------------------

   Gmsh C tutorial 1

   Elementary entities (points, curves, surfaces), physical groups

   --------------------------------------------------------------------------- */

/* The Gmsh C API is entirely defined in the `gmshc.h' header (which contains
   the full documentation of all the functions in the API): */
#include <gmshc.h>

int main(int argc, char **argv)
{
  int ierr, p4;
  double lc = 1e-2;
  int cl1[] = {4, 1, -2, 3}, s1[] = {1};
  int g5[] = {1, 2, 4}, g6[] = {1}, ps;

  /* Before using any functions in the C API, Gmsh must be initialized. In the C
     API the last argument of all functions returns the error code, if any. */
  gmshInitialize(argc, argv, 1, 0, &ierr);

  /* We now add a new model, named "t1". If gmsh::model::add() is not called, a
     new default (unnamed) model will be created on the fly, if necessary. */
  gmshModelAdd("t1", &ierr);

  /* The C API provides direct access to each supported geometry (CAD)
     kernel. The built-in kernel is used in this first tutorial: the
     corresponding API functions have the "gmshModelGeo" prefix.

     The first type of `elementary entity' in Gmsh is a `Point'. To create a
     point with the built-in CAD kernel, the C API function is
     gmshModelGeoAddPoint():
     - the first 3 arguments are the point coordinates (x, y, z)
     - the next argument is the target mesh size close to the point
     - the next argument is the point tag (a stricly positive integer that
       uniquely identifies the point); if the tag is set to -1, the function
       will return a new tag */
  gmshModelGeoAddPoint(0, 0, 0, lc, 1, &ierr);

  /* The distribution of the mesh element sizes will be obtained by
     interpolation of these mesh sizes throughout the geometry. Another method
     to specify mesh sizes is to use general mesh size Fields. A particular case
     is the use of a background mesh.

     If no target mesh size of provided, a default uniform coarse size will be
     used for the model, based on the overall model size.

     We can then define some additional points. All points should have different
     tags: */
  gmshModelGeoAddPoint(.1, 0,  0, lc, 2, &ierr);
  gmshModelGeoAddPoint(.1, .3, 0, lc, 3, &ierr);

  /* If the tag is not provided explicitly (by passing a negative value), a new
     tag is automatically created, and returned by the function: */
  p4 = gmshModelGeoAddPoint(0,  .3, 0, lc, -1, &ierr);

  /* Curves are Gmsh's second type of elementery entities, and, amongst curves,
     straight lines are the simplest. The API to create straight line segments
     with the built-in kernel follows the same conventions: the first 2
     arguments are point tags (the start and end points of the line), and the
     next one is the line tag.

     In the commands below, for example, the line 1 starts at point 1 and ends
     at point 2.

     Note that curve tags are separate from point tags - hence we can reuse tag
     `1' for our first curve. And as a general rule, elementary entity tags in
     Gmsh have to be unique per geometrical dimension. */
  gmshModelGeoAddLine(1, 2, 1, &ierr);
  gmshModelGeoAddLine(3, 2, 2, &ierr);
  gmshModelGeoAddLine(3, p4, 3, &ierr);
  gmshModelGeoAddLine(p4, 1, 4, &ierr);

  /* The third elementary entity is the surface. In order to define a simple
     rectangular surface from the four curves defined above, a curve loop has
     first to be defined. A curve loop is defined by an ordered list of
     connected curves, a sign being associated with each curve (depending on the
     orientation of the curve to form a loop). The API function to create curve
     loops takes a pointer to an array of integers as first argument, the number
     of elements in the array as the second argument, and the curve loop tag
     (which must be unique amongst curve loops) as the third argument: */
  gmshModelGeoAddCurveLoop(cl1, sizeof(cl1)/sizeof(cl1[0]), 1, 0, &ierr);

  /* We can then define the surface as a list of curve loops (only one here,
     representing the external contour, since there are no holes): */
  gmshModelGeoAddPlaneSurface(s1, sizeof(s1)/sizeof(s1[0]), 1, &ierr);

  /* Before they can be meshed (and, more generally, before they can be used by
     API functions outside of the built-in CAD kernel functions), the CAD
     entities must be synchronized with the Gmsh model, which will create the
     relevant Gmsh data structures. This is achieved by the
     gmsh.model.geo.synchronize() API call for the built-in CAD
     kernel. Synchronizations can be called at any time, but they involve a non
     trivial amount of processing; so while you could synchronize the internal
     CAD data after every CAD command, it is usually better to minimize the
     number of synchronization points. */
  gmshModelGeoSynchronize(&ierr);

  /* At this level, Gmsh knows everything to display the rectangular surface 1
     and to mesh it. An optional step is needed if we want to group elementary
     geometrical entities into more meaningful groups, e.g. to define some
     mathematical ("domain", "boundary"), functional ("left wing", "fuselage")
     or material ("steel", "carbon") properties.

     Such groups are called "Physical Groups" in Gmsh. By default, if physical
     groups are defined, Gmsh will export in output files only mesh elements
     that belong to at least one physical group. (To force Gmsh to save all
     elements, whether they belong to physical groups or not, set the
     `Mesh.SaveAll' option to 1.) Physical groups are also identified by tags,
     i.e. stricly positive integers, that should be unique per dimension (0D,
     1D, 2D or 3D). Physical groups can also be given names.

     Here we define a physical curve that groups the left, bottom and right
     curves in a single group (with prescribed tag 5); and a physical surface
     with name "My surface" (with an automatic tag) containing the geometrical
     surface 1: */
  gmshModelAddPhysicalGroup(1, g5, sizeof(g5)/sizeof(g5[0]), 5, &ierr);
  ps = gmshModelAddPhysicalGroup(2, g6, sizeof(g6)/sizeof(g6[0]), -1, &ierr);
  gmshModelSetPhysicalName(2, ps, "My surface", &ierr);

  /* We can then generate a 2D mesh... */
  gmshModelMeshGenerate(2, &ierr);

  /* ... and save it to disk */
  gmshWrite("t1.msh", &ierr);

  /* Remember that by default, if physical groups are defined, Gmsh will export
     in the output mesh file only those elements that belong to at least one
     physical group. To force Gmsh to save all elements, you can use

     gmshOptionSetNumber("Mesh.SaveAll", 1, &ierr);
  */

  /* By default, Gmsh saves meshes in the latest version of the Gmsh mesh file
     format (the `MSH' format). You can save meshes in other mesh formats by
     specifying a filename with a different extension. For example

     gmshWrite("t1.unv", &ierr);

     will save the mesh in the UNV format. You can also save the mesh in older
     versions of the MSH format: simply set

     gmshOptionSetNumber("Mesh.MshFileVersion", x, &ierr);

     for any version number `x'. As an alternative, you can also not specify
     the format explicitly, and just choose a filename with the `.msh2' or
     `.msh4' extension. */

  /* To visualize the model we could run the graphical user interface with:

     gmshFltkRun(&ierr);
  */

  /* Note that starting with Gmsh 3.0, models can be built using other geometry
     kernels than the default "built-in" kernel. To use the OpenCASCADE CAD
     kernel instead of the built-in kernel, you should use the functions with
     the "gmshModelOcc" prefix.

     Different CAD kernels have different features. With OpenCASCADE, instead of
     defining the surface by successively defining 4 points, 4 curves and 1
     curve loop, one can define the rectangular surface directly with

     gmshModelOccAddRectangle(.2, 0, 0, .1, .3, -1, 0, &ierr);

     After synchronization with the Gmsh model with

     gmshModelOccSynchronize(&ierr);

     the underlying curves and points could be accessed with
     gmshModelGetBoundary(&ierr). */

  /* This should be called when you are done using the Gmsh C API: */
  gmshFinalize(&ierr);
  return 0;
}
