angle_to was producing the angle from +x to the difference between a,b
which is unexpected. Instead, it should produce the smallest absolute
angle between the two vectors and be signed to indicate the direction of
rotation.
By using the old angle_to implementation and modifying equal() to print
out the failures, you can see the numbers that it was producing before
didn't make much sense:
right:angle_to(down) = 45.0
right:angle_to(left) = 0.0
right:angle_to(up) = -45.0
down:angle_to(right) = -135.0
down:angle_to(left) = -45.0
down:angle_to(up) = -90.0
left:angle_to(down) = 135.0
left:angle_to(up) = -135.0
up:angle_to(right) = 135.0
up:angle_to(down) = 90.0
up:angle_to(left) = 45.0
Now it produces numbers you'd expect:
right:angle_to(down) = -90.0
right:angle_to(left) = 180.0
right:angle_to(up) = 90.0
down:angle_to(right) = 90.0
down:angle_to(left) = -90.0
down:angle_to(up) = 180.0
left:angle_to(down) = 90.0
left:angle_to(up) = -90.0
up:angle_to(right) = -90.0
up:angle_to(down) = 180.0
up:angle_to(left) = 90.0
See also https://stackoverflow.com/questions/21483999/using-atan2-to-find-angle-between-two-vectors
Also added tests for angle_between.
