/* js/src/d2/pred/cossign.js 
Computes the cosinus sign in a "right-handed"
coordinate system (i.e. clockwise angle values).
cossign(x, y, z) > 0 iff angle _xyz_ is acute.

<p>

http://en.wikipedia.org/wiki/Dot_product#Geometric_definition

A.B = ||A|| ||B|| cos t
A.B = A0 B0 + A1 B1
A = [y[0] - x[0], y[1] - x[1]]
B = [y[0] - z[0], y[1] - z[1]]
A.B = (y[0] - x[0]) * (y[0] - z[0]) + (y[1] - x[1]) * (y[1] - z[1])


# of operations

- 1 add
- 4 sub
- 2 mul


Originally implemented as

return y[0] * (y[0] - x[0] - z[0]) +
y[1] * (y[1] - x[1] - z[1]) +
z[0] * x[0] +
z[1] * x[1];

with
- 3 add
- 4 sub
- 4 mul
function cossign(x, y, z) {
			return (y[0] - x[0]) * (y[0] - z[0]) + (y[1] - x[1]) * (y[1] - z[1]);
		}