/* js/src/d2/pred/sinsign.js 
Computes the cross product of vectors _ab_ and _ac_.
Can be interpreted as the sinus sign in a "right-handed"
coordinate system (i.e. clockwise angle values).
sinsign(a, b, c) > 0 iff point c 'lies to the left' of segment _ab_.

It also computes the area of the parallelogram defined by a, b
and c. It can for example be used in the Quick Hull algorithm to find the
farthest points from the pivot line.

<p>
Originally implemented as

a[1] * (c[0] - b[0]) + b[1] * (a[0] - c[0]) + c[1] * (b[0] - a[0])

with
- 2 add
- 3 sub
- 3 mul

Reduced to

(b[0] - a[0]) * (c[1] - a[1]) - (b[1] - a[1]) * (c[0] - a[0])

with
- 5 sub
- 2 mul
function sinsign(a, b, c) {

			return (b[0] - a[0]) * (c[1] - a[1]) - (b[1] - a[1]) * (c[0] - a[0]);
		}