## Calculating the Constraint System

The main task of C3D Solver is to satisfy the sketch constraint system,
it is implemented using the

```cpp
GCE_result GCE_Evaluate(GCE_system gSys) 
```

The function returns the **gSys** constraint system calculation. The
function does not execute any calculation operations if all constraints
are satisfied at the time when it is called.

Returned values:

-   **GCE_RESULT_Ok** means that the constraint system is successfully
    resolved, and the solver assigned a new state meeting all given
    system constraints to all geometric objects.

-   **GCE_RESULT_Overconstrained** means that the function has
    discovered contradictory constraints during calculation, i.e. the
    constraint system has a subset, in which the constraints cant be
    satisfied at the same time. Geometry state remains unchanged.

-   **GCE_RESULT_Not_Satisfied** means that the function was not able to
    find a solution that meets all specified constraints. Geometry state
    remains unchanged. This diagnostic code is returned in the following
    situations:

    -   a dimension is out of boundaries of the solution existence
        domain;

    -   positions of fixed or frozen points are out of boundaries of
        solution existence domain;

    -   in rare cases it is caused by poor initial approximation.
        Meeting the constraints requires considerable changes in
        parametric coordinates;

    -   high mutual interference. For example, for such conditions if an
        angle is changed by one degree, then the object would offset to
        a considerable distance;

    -   in rare cases, this results from lack of numeric robustness
        caused by dramatic difference between the dimensions of objects
        in one drawing, for example, when the smallest and the largest
        sketch objects differ by more than 7 orders of magnitude (as
        when one ellipse is 1 mm long and the other one is 10 km long);

    -   in general case, it can be caused by other situations when the
        solution doesn't exist or can't be calculated.

-   **GCE_RESULT_InvalidGeometry**. The found solution leads to
    degradation of geometry objects (for example, a segment/arc has zero
    length, a circle/ellipse has zero radius). Geometry state remains
    unchanged. Occurs if a geometric object (circle/line segment/bounded
    curve) is prohibited from degeneration by means of the
    **GCE_allow_zero_radius**/**GCE_allow_zero_length** callback
    functions. And also, in general, without using a structure with
    callback functions GCE_callback_table or with this structure set by
    default.

-   **GCE_RESULT_InvalidGeometry**. The found solution leads to
    degeneration of geometric objects, for example, a segment or arc
    becomes zero length, and a circle or ellipse becomes zero radius.
    The geometry state remains unchanged. This situation occurs if the
    callback functions **GCE_allow_zero_radius** or
    **GCE_allow_zero_length** prohibit a geometric object (circle,
    segment, or bounded curve) from taking a zero radius or length. It
    can also occur generally if the callback function structure
    `GCE_callback_table` is used with default settings (without override
    via the GCE_Register call).