Function layline_rays 

pub fn layline_rays(
    vessel: Position,
    mark: Position,
    true_wind_direction_deg: f64,
    beat_angle_deg: f64,
) -> Option<(Position, Position, Position)>

The two laylines through mark, as ray endpoints from mark itself – (mark, starboard_end, port_end) – or None when mark does not need a beat at all.

Layline definition

A layline is a line through a mark, marking the boundary where tacking fetches the mark on a single close-hauled board. It requires a mark to exist, and returns None when the mark is already fetchable on one tack or lies dead downwind – the same wind-triangle judgement tack_legs already makes. Each layline is the reciprocal of its own close-hauled heading, extended out from the mark: crossing it means the mark is fetchable on that tack.