# Source code for spynnaker.pyNN.models.neuron.plasticity.stdp.common

# Copyright (c) 2017-2019 The University of Manchester
#
# This program is free software: you can redistribute it and/or modify
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

import math
import numpy

# Default value of fixed-point one for STDP
STDP_FIXED_POINT_ONE = (1 << 11)

[docs]def float_to_fixed(value):
"""
:param float value:
:rtype: int
"""
return int(round(float(value) * STDP_FIXED_POINT_ONE))

[docs]def get_exp_lut_array(time_step, time_constant, shift=0):
"""
:param int time_step:
:param float time_constant:
:param int shift:
:rtype: ~numpy.ndarray
"""
# Compute the actual exponential decay parameter
# NB: lambda is a reserved word in Python
l_ambda = time_step / float(time_constant)

# Compute the size of the array, which must be a multiple of 2
size = math.log(STDP_FIXED_POINT_ONE) / l_ambda
size, extra = divmod(size / (1 << shift), 2)
size = ((int(size) + (extra > 0)) * 2)

# Fill out the values in the array
a = numpy.exp((numpy.arange(size) << shift) * -l_ambda)
a = numpy.floor(a * STDP_FIXED_POINT_ONE)