PPR = 1440 * 4   # decoder position steps per revolution

# reduce x modulo n, but giving result in range(base,base+n) rather than range(0,n).
# default output range is -n/2..n/2-1 (if n even) or -(n-1)/2..(n-1)/2 (if n odd).
def mod( x, n, base=-(n//2) ):
    return ( x - base ) % n + base

# update position from shmem.position, unwrap from (mod 2**32) to integer
position += mod( shmem.position - position, 2**32 )

angle = position * 360 / PPR