behavior of GEOSCoordSeq_isCCW() is unclear when input is not a simple closed curve

[sirsigurd]

Here's GEOSCoordSeq_isCCW() doc from C API header:

/*
 * Check orientation of a CoordinateSequence and set 'is_ccw' to 1
 * if it has counter-clockwise orientation, 0 otherwise.
 * Return 0 on exception, 1 on success.
 */
extern int GEOS_DLL GEOSCoordSeq_isCCW_r(GEOSContextHandle_t handle,
                                         const GEOSCoordSequence* s,
                                         char* is_ccw);

As I understand from ​Wikipedia, 'orientation' is defined only for simple closed curves (hereafter referred to as SCC).

So from my perspective GEOSCoordSeq_isCCW() could follow one these approaches for input which is not SCC:

1. Orientation is defined only for SCC, so if input is not SCC, it can't be CCW-oriented curve (is_ccw = 0).

2. Orientation is defined only for SCC, so if input is not SCC, it's invalid (error code is returned)

Currently GEOSCoordSeq_isCCW() doesn't follow any of these approaches consistently. If coordinate sequence has less than 4 points error code is returned. At later stage if it turns out that there are less than 3 distinctive points is_ccw is set to 0.

[pramsey]

No, checking this back against JTS, apparently JTS always returns false on invalid input. So we could ape that, which would be a slight change.

[Paul Ramsey <pramsey@…>]

In f4d05b2/git:

Document fun behaviour of GEOSCoordSeq_isCCW, closes #1000