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# Code source de qiskit.circuit.library.arithmetic.integer_comparator

# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020.
#
# obtain a copy of this license in the LICENSE.txt file in the root directory
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""Integer Comparator."""

from __future__ import annotations
import warnings
import numpy as np

from qiskit.circuit import QuantumCircuit, QuantumRegister, AncillaRegister
from qiskit.circuit.exceptions import CircuitError
from ..boolean_logic import OR
from ..blueprintcircuit import BlueprintCircuit

[docs]class IntegerComparator(BlueprintCircuit):
r"""Integer Comparator.

Operator compares basis states :math:|i\rangle_n against a classically given integer
:math:L of fixed value and flips a target qubit if :math:i \geq L
(or :math:< depending on the parameter geq):

.. math::

|i\rangle_n |0\rangle \mapsto |i\rangle_n |i \geq L\rangle

This operation is based on two's complement implementation of binary subtraction but only
uses carry bits and no actual result bits. If the most significant carry bit
(the results bit) is 1, the :math:\geq condition is True otherwise it is False.
"""

def __init__(
self,
num_state_qubits: int | None = None,
value: int | None = None,
geq: bool = True,
name: str = "cmp",
) -> None:
"""Create a new fixed value comparator circuit.

Args:
num_state_qubits: Number of state qubits. If this is set it will determine the number
of qubits required for the circuit.
value: The fixed value to compare with.
geq: If True, evaluate a >= condition, else <.
name: Name of the circuit.
"""
super().__init__(name=name)

self._value = None
self._geq = None
self._num_state_qubits = None

self.value = value
self.geq = geq
self.num_state_qubits = num_state_qubits

@property
def value(self) -> int:
"""The value to compare the qubit register to.

Returns:
The value against which the value of the qubit register is compared.
"""
return self._value

@value.setter
def value(self, value: int) -> None:
if value != self._value:
self._invalidate()
self._value = value

@property
def geq(self) -> bool:
"""Return whether the comparator compares greater or less equal.

Returns:
True, if the comparator compares >=, False if <.
"""
return self._geq

@geq.setter
def geq(self, geq: bool) -> None:
"""Set whether the comparator compares greater or less equal.

Args:
geq: If True, the comparator compares >=, if False <.
"""
if geq != self._geq:
self._invalidate()
self._geq = geq

@property
def num_ancilla_qubits(self):
warnings.warn(
"The IntegerComparator.num_ancilla_qubits property is deprecated "
"as of 0.16.0. It will be removed no earlier than 3 months after the release "
"date. You should use the num_ancillas property instead."
)
return self.num_ancillas

@property
def num_state_qubits(self) -> int:
"""The number of qubits encoding the state for the comparison.

Returns:
The number of state qubits.
"""
return self._num_state_qubits

@num_state_qubits.setter
def num_state_qubits(self, num_state_qubits: int | None) -> None:
"""Set the number of state qubits.

Note that this will change the quantum registers.

Args:
num_state_qubits: The new number of state qubits.
"""
if self._num_state_qubits is None or num_state_qubits != self._num_state_qubits:
self._invalidate()  # reset data
self._num_state_qubits = num_state_qubits

if num_state_qubits is not None:
# set the new qubit registers
qr_state = QuantumRegister(num_state_qubits, name="state")
q_compare = QuantumRegister(1, name="compare")

self.qregs = [qr_state, q_compare]

num_ancillas = num_state_qubits - 1
if num_ancillas > 0:
qr_ancilla = AncillaRegister(num_ancillas)

def _get_twos_complement(self) -> list[int]:
"""Returns the 2's complement of self.value as array.

Returns:
The 2's complement of self.value.
"""
twos_complement = pow(2, self.num_state_qubits) - int(np.ceil(self.value))
twos_complement = f"{twos_complement:b}".rjust(self.num_state_qubits, "0")
twos_complement = [
1 if twos_complement[i] == "1" else 0 for i in reversed(range(len(twos_complement)))
]
return twos_complement

def _check_configuration(self, raise_on_failure: bool = True) -> bool:
"""Check if the current configuration is valid."""
valid = True

if self._num_state_qubits is None:
valid = False
if raise_on_failure:
raise AttributeError("Number of state qubits is not set.")

if self._value is None:
valid = False
if raise_on_failure:
raise AttributeError("No comparison value set.")

required_num_qubits = 2 * self.num_state_qubits
if self.num_qubits != required_num_qubits:
valid = False
if raise_on_failure:
raise CircuitError("Number of qubits does not match required number of qubits.")

return valid

def _build(self) -> None:
"""If not already built, build the circuit."""
if self._is_built:
return

super()._build()

qr_state = self.qubits[: self.num_state_qubits]
q_compare = self.qubits[self.num_state_qubits]
qr_ancilla = self.qubits[self.num_state_qubits + 1 :]

circuit = QuantumCircuit(*self.qregs, name=self.name)

if self.value <= 0:  # condition always satisfied for non-positive values
if self._geq:  # otherwise the condition is never satisfied
circuit.x(q_compare)
# condition never satisfied for values larger than or equal to 2^n
elif self.value < pow(2, self.num_state_qubits):

if self.num_state_qubits > 1:
twos = self._get_twos_complement()
for i in range(self.num_state_qubits):
if i == 0:
if twos[i] == 1:
circuit.cx(qr_state[i], qr_ancilla[i])
elif i < self.num_state_qubits - 1:
if twos[i] == 1:
circuit.compose(
OR(2), [qr_state[i], qr_ancilla[i - 1], qr_ancilla[i]], inplace=True
)
else:
circuit.ccx(qr_state[i], qr_ancilla[i - 1], qr_ancilla[i])
else:
if twos[i] == 1:
# OR needs the result argument as qubit not register, thus
# access the index [0]
circuit.compose(
OR(2), [qr_state[i], qr_ancilla[i - 1], q_compare], inplace=True
)
else:
circuit.ccx(qr_state[i], qr_ancilla[i - 1], q_compare)

# flip result bit if geq flag is false
if not self._geq:
circuit.x(q_compare)

# uncompute ancillas state
for i in reversed(range(self.num_state_qubits - 1)):
if i == 0:
if twos[i] == 1:
circuit.cx(qr_state[i], qr_ancilla[i])
else:
if twos[i] == 1:
circuit.compose(
OR(2), [qr_state[i], qr_ancilla[i - 1], qr_ancilla[i]], inplace=True
)
else:
circuit.ccx(qr_state[i], qr_ancilla[i - 1], qr_ancilla[i])
else:

# num_state_qubits == 1 and value == 1:
circuit.cx(qr_state[0], q_compare)

# flip result bit if geq flag is false
if not self._geq:
circuit.x(q_compare)

else:
if not self._geq:  # otherwise the condition is never satisfied
circuit.x(q_compare)

self.append(circuit.to_gate(), self.qubits)