Module ring 

Ring (closed polygon) utility functions.

A ring is a Vec<[i64; 2]> of vertices in order (first != last, closing edge implied). All operations use exact integer arithmetic — no epsilon, no floating point.

Input contract for decompose():

• Rings must be CCW (counter-clockwise)
• Rings must be simple (no self-intersections)
• Collinear vertices should be removed for cleaner decomposition

Functions

ensure_ccw

Ensure the ring is CCW. Reverses in place if CW.

is_ccw

True if the ring is counter-clockwise (positive signed area).

is_simple

True if the ring has no self-intersections (is simple). O(n²) check — all non-adjacent edge pairs. A ring with n < 3 vertices is not simple.

normalize_ring

Normalize a ring: remove collinear vertices, ensure CCW winding. Does NOT check for simplicity (caller must ensure input is simple). Returns None if the result has fewer than 3 vertices (degenerate).

remove_collinear

Remove collinear vertices from a ring. A vertex is collinear if cross(prev, curr, next) == 0 (exact integer test). Returns a new ring with collinear vertices removed. The result may have fewer vertices, but preserves the polygon shape exactly.

rotate_ring

Rotate ring so that index start becomes index 0. Preserves winding.

signed_area_2x

Twice the signed area of the ring (positive = CCW, negative = CW). Uses the shoelace formula with i128 intermediate arithmetic. NOT the same as twice_area_fp2 in area.rs — this preserves sign for orientation detection.