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# Author: Eric Larson
# 2014
from __future__ import absolute_import
import numpy as np
cimport numpy as np
cimport cython
# Fast inner loop of max_len_seq.
@cython.cdivision(True) # faster modulo
@cython.boundscheck(False) # designed to stay within bounds
@cython.wraparound(False) # we don't use negative indexing
def _max_len_seq_inner(Py_ssize_t[::1] taps,
np.int8_t[::1] state,
Py_ssize_t nbits, Py_ssize_t length,
np.int8_t[::1] seq):
# Here we compute MLS using a shift register, indexed using a ring buffer
# technique (faster than using something like np.roll to shift)
cdef Py_ssize_t n_taps = taps.shape[0]
cdef Py_ssize_t idx = 0
cdef np.int8_t feedback
cdef Py_ssize_t i
for i in range(length):
feedback = state[idx]
seq[i] = feedback
for ti in range(n_taps):
feedback ^= state[(taps[ti] + idx) % nbits]
state[idx] = feedback
idx = (idx + 1) % nbits
# state must be rolled s.t. next run, when idx==0, it's in the right place
return np.roll(state, -idx, axis=0)
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