code: def _CO_f1ecac(X_ac):
      # First 1/e crossing of autocorrelation function.
      threshold = 0.36787944117144233  # 1 / np.exp(1)
      for i in range(1, len(X_ac)):
          if (X_ac[i - 1] - threshold) * (X_ac[i] - threshold) < 0:
              return i
      return len(X_ac)
  
imports: import math
  import numpy
  from sktime.utils.numba.njit import njit
label_desc: Use image node_id 20 for calling a global function with example usage: _CO_f1ecac(X_ac) and returns: len, i