import nengo
import numpy as np
from nengo_spa.algebras.base import AbstractAlgebra, ElementSidedness
from nengo_spa.networks.tvtb import TVTB
[docs]class TvtbAlgebra(AbstractAlgebra):
r"""Transposed Vector-derived Transformation Binding (TVTB) algebra.
TVTB uses elementwise addition for superposition. The binding operation
:math:`\mathcal{B}(x, y)` is defined as
.. math::
\mathcal{B}(x, y) := V_y^T x = \left[\begin{array}{ccc}
V_y'^T & 0 & 0 \\
0 & V_y'^T & 0 \\
0 & 0 & V_y'^T
\end{array}\right] x
with
.. math::
V_y' = d^{\frac{1}{4}} \left[\begin{array}{cccc}
y_1 & y_2 & \dots & y_{d'} \\
y_{d' + 1} & y_{d' + 2} & \dots & y_{2d'} \\
\vdots & \vdots & \ddots & \vdots \\
y_{d - d' + 1} & y_{d - d' + 2} & \dots & y_d
\end{array}\right]
and
.. math:: d'^2 = d.
The approximate inverse :math:`y^+` for :math:`y` is permuting the elements
such that :math:`V_{y^+} = V_y^T`.
Note that TVTB requires the vector dimensionality to be square.
The TVTB binding operation is neither associative nor commutative.
In contrast to VTB, however, TVTB has two-sided identities and inverses.
Other properties are equivalent to VTB.
.. seealso::
`.VtbAlgebra`
"""
_instance = None
def __new__(cls):
if type(cls._instance) is not cls:
cls._instance = super(TvtbAlgebra, cls).__new__(cls)
return cls._instance
[docs] def is_valid_dimensionality(self, d):
"""Checks whether *d* is a valid vector dimensionality.
For TVTB all square numbers are valid dimensionalities.
Parameters
----------
d : int
Dimensionality
Returns
-------
bool
*True*, if *d* is a valid vector dimensionality for the use with
the algebra.
"""
if d < 1:
return False
sub_d = np.sqrt(d)
return sub_d * sub_d == d
def _get_sub_d(self, d):
sub_d = int(np.sqrt(d))
if sub_d * sub_d != d:
raise ValueError("Vector dimensionality must be a square number.")
return sub_d
[docs] def make_unitary(self, v):
sub_d = self._get_sub_d(len(v))
m = np.array(v.reshape((sub_d, sub_d)))
for i in range(1, sub_d):
y = -np.dot(m[:i, i:], m[i, i:])
A = m[:i, :i]
m[i, :i] = np.linalg.solve(A, y)
m /= np.linalg.norm(m, axis=1)[:, None]
m /= np.sqrt(sub_d)
return m.flatten()
[docs] def superpose(self, a, b):
return a + b
[docs] def bind(self, a, b):
d = len(a)
if len(b) != d:
raise ValueError("Inputs must have same length.")
m = self.get_binding_matrix(b)
return np.dot(m, a)
[docs] def invert(self, v, sidedness=ElementSidedness.TWO_SIDED):
"""Invert vector *v*.
A vector bound to its inverse will result in the identity vector.
Parameters
----------
v : (d,) ndarray
Vector to invert.
sidedness : ElementSidedness
This argument has no effect because the inverse of the TVTB algebra
is two-sided.
Returns
-------
(d,) ndarray
Inverse of vector.
"""
sub_d = self._get_sub_d(len(v))
return v.reshape((sub_d, sub_d)).T.flatten()
[docs] def get_binding_matrix(self, v, swap_inputs=False):
sub_d = self._get_sub_d(len(v))
m = np.sqrt(sub_d) * np.kron(np.eye(sub_d), v.reshape((sub_d, sub_d)).T)
if swap_inputs:
inv_mat = self.get_inversion_matrix(len(v))
m = np.dot(np.dot(inv_mat, m.T), inv_mat)
return m
[docs] def get_inversion_matrix(self, d, sidedness=ElementSidedness.TWO_SIDED):
"""Returns the transformation matrix for inverting a vector.
Parameters
----------
d : int
Vector dimensionality.
sidedness : ElementSidedness
This argument has no effect because the inverse of the TVTB algebra
is two-sided.
Returns
-------
(d, d) ndarray
Transformation matrix to invert a vector.
"""
sub_d = self._get_sub_d(d)
return np.eye(d).reshape(d, sub_d, sub_d).T.reshape(d, d)
[docs] def implement_superposition(self, n_neurons_per_d, d, n):
node = nengo.Node(size_in=d)
return node, n * (node,), node
[docs] def implement_binding(self, n_neurons_per_d, d, unbind_left, unbind_right):
net = TVTB(n_neurons_per_d, d, unbind_left, unbind_right)
return net, (net.input_left, net.input_right), net.output
[docs] def absorbing_element(self, d, sidedness=ElementSidedness.TWO_SIDED):
"""TVTB has no absorbing element except the zero vector.
Always raises a `NotImplementedError`.
"""
raise NotImplementedError("VtbAlgebra does not have any absorbing elements.")
[docs] def identity_element(self, d, sidedness=ElementSidedness.TWO_SIDED):
"""Return the identity element of dimensionality *d*.
Parameters
----------
d : int
Vector dimensionality.
sidedness : ElementSidedness
This argument has no effect because the identity of the TVTB algebra
is two-sided.
Returns
-------
(d,) ndarray
Identity element.
"""
sub_d = self._get_sub_d(d)
return (np.eye(sub_d) / d ** 0.25).flatten()
[docs] def zero_element(self, d, sidedness=ElementSidedness.TWO_SIDED):
"""Return the zero element of dimensionality *d*.
The zero element produces itself when bound to a different vector.
For VTB this is the zero vector.
Parameters
----------
d : int
Vector dimensionality.
sidedness : ElementSidedness, optional
This argument has no effect because the zero element of the VTB
algebra is two-sided.
Returns
-------
(d,) ndarray
Zero element.
"""
return np.zeros(d)
