# This code is part of a Qiskit project.
#
# (C) Copyright IBM 2020, 2023.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# 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.
"""The converter to map integer variables in a quadratic program to binary variables."""
import copy
from typing import Dict, List, Optional, Tuple, Union
import numpy as np
from .quadratic_program_converter import QuadraticProgramConverter
from ..exceptions import QiskitOptimizationError
from ..problems.quadratic_objective import QuadraticObjective
from ..problems.quadratic_program import QuadraticProgram
from ..problems.variable import Variable
[docs]class IntegerToBinary(QuadraticProgramConverter):
"""Convert a :class:`~qiskit_optimization.problems.QuadraticProgram` into new one by encoding
integer with binary variables.
This bounded-coefficient encoding used in this converted is proposed in [1], Eq. (5).
Examples:
>>> from qiskit_optimization.problems import QuadraticProgram
>>> from qiskit_optimization.converters import IntegerToBinary
>>> problem = QuadraticProgram()
>>> var = problem.integer_var(name='x', lowerbound=0, upperbound=10)
>>> conv = IntegerToBinary()
>>> problem2 = conv.convert(problem)
References:
[1]: Sahar Karimi, Pooya Ronagh (2017), Practical Integer-to-Binary Mapping for Quantum
Annealers. arxiv.org:1706.01945.
"""
_delimiter = "@" # users are supposed not to use this character in variable names
def __init__(self) -> None:
self._src: Optional[QuadraticProgram] = None
self._dst: Optional[QuadraticProgram] = None
self._conv: Dict[Variable, List[Tuple[str, int]]] = {}
# e.g., self._conv = {x: [('x@1', 1), ('x@2', 2)]}
[docs] def convert(self, problem: QuadraticProgram) -> QuadraticProgram:
"""Convert an integer problem into a new problem with binary variables.
Args:
problem: The problem to be solved, that may contain integer variables.
Returns:
The converted problem, that contains no integer variables.
Raises:
QiskitOptimizationError: if variable or constraint type is not supported.
"""
# Copy original QP as reference.
self._src = copy.deepcopy(problem)
if self._src.get_num_integer_vars() > 0:
# Initialize new QP
self._dst = QuadraticProgram(name=problem.name)
# Declare variables
for x in self._src.variables:
if x.vartype == Variable.Type.INTEGER:
new_vars = self._convert_var(x.name, x.lowerbound, x.upperbound)
self._conv[x] = new_vars
for (var_name, _) in new_vars:
self._dst.binary_var(var_name)
else:
if x.vartype == Variable.Type.CONTINUOUS:
self._dst.continuous_var(x.lowerbound, x.upperbound, x.name)
elif x.vartype == Variable.Type.BINARY:
self._dst.binary_var(x.name)
else:
raise QiskitOptimizationError(f"Unsupported variable type {x.vartype}")
self._substitute_int_var()
else:
# just copy the problem if no integer variables exist
self._dst = copy.deepcopy(problem)
return self._dst
def _convert_var(
self, name: str, lowerbound: float, upperbound: float
) -> List[Tuple[str, int]]:
var_range = upperbound - lowerbound
power = int(np.log2(var_range)) if var_range > 0 else 0
bounded_coef = var_range - (2**power - 1)
coeffs = [2**i for i in range(power)] + [bounded_coef]
return [(name + self._delimiter + str(i), coef) for i, coef in enumerate(coeffs)]
def _convert_linear_coefficients_dict(
self, coefficients: Dict[str, float]
) -> Tuple[Dict[str, float], float]:
constant = 0.0
linear: Dict[str, float] = {}
for name, v in coefficients.items():
x = self._src.get_variable(name)
if x in self._conv:
for y, coeff in self._conv[x]:
linear[y] = v * coeff
constant += v * x.lowerbound
else:
linear[x.name] = v
return linear, constant
def _convert_quadratic_coefficients_dict(
self, coefficients: Dict[Tuple[str, str], float]
) -> Tuple[Dict[Tuple[str, str], float], Dict[str, float], float]:
constant = 0.0
linear: Dict[str, float] = {}
quadratic = {}
for (name_i, name_j), v in coefficients.items():
x = self._src.get_variable(name_i)
y = self._src.get_variable(name_j)
if x in self._conv and y not in self._conv:
for z_x, coeff_x in self._conv[x]:
quadratic[z_x, y.name] = v * coeff_x
linear[y.name] = linear.get(y.name, 0.0) + v * x.lowerbound
elif x not in self._conv and y in self._conv:
for z_y, coeff_y in self._conv[y]:
quadratic[x.name, z_y] = v * coeff_y
linear[x.name] = linear.get(x.name, 0.0) + v * y.lowerbound
elif x in self._conv and y in self._conv:
for z_x, coeff_x in self._conv[x]:
for z_y, coeff_y in self._conv[y]:
quadratic[z_x, z_y] = v * coeff_x * coeff_y
for z_x, coeff_x in self._conv[x]:
linear[z_x] = linear.get(z_x, 0.0) + v * coeff_x * y.lowerbound
for z_y, coeff_y in self._conv[y]:
linear[z_y] = linear.get(z_y, 0.0) + v * coeff_y * x.lowerbound
constant += v * x.lowerbound * y.lowerbound
else:
quadratic[x.name, y.name] = v
return quadratic, linear, constant
def _substitute_int_var(self):
# set objective
linear, linear_constant = self._convert_linear_coefficients_dict(
self._src.objective.linear.to_dict(use_name=True)
)
quadratic, q_linear, q_constant, = self._convert_quadratic_coefficients_dict(
self._src.objective.quadratic.to_dict(use_name=True)
)
constant = self._src.objective.constant + linear_constant + q_constant
for i, v in q_linear.items():
linear[i] = linear.get(i, 0) + v
if self._src.objective.sense == QuadraticObjective.Sense.MINIMIZE:
self._dst.minimize(constant, linear, quadratic)
else:
self._dst.maximize(constant, linear, quadratic)
# set linear constraints
for constraint in self._src.linear_constraints:
linear, constant = self._convert_linear_coefficients_dict(
constraint.linear.to_dict(use_name=True)
)
self._dst.linear_constraint(
linear, constraint.sense, constraint.rhs - constant, constraint.name
)
# set quadratic constraints
for constraint in self._src.quadratic_constraints:
linear, linear_constant = self._convert_linear_coefficients_dict(
constraint.linear.to_dict(use_name=True)
)
quadratic, q_linear, q_constant = self._convert_quadratic_coefficients_dict(
constraint.quadratic.to_dict(use_name=True)
)
constant = linear_constant + q_constant
for i, v in q_linear.items():
linear[i] = linear.get(i, 0) + v
self._dst.quadratic_constraint(
linear,
quadratic,
constraint.sense,
constraint.rhs - constant,
constraint.name,
)
[docs] def interpret(self, x: Union[np.ndarray, List[float]]) -> np.ndarray:
"""Convert back the converted problem (binary variables)
to the original (integer variables).
Args:
x: The result of the converted problem or the given result in case of FAILURE.
Returns:
The result of the original problem.
"""
# interpret integer values
sol = {var.name: x[i] for i, var in enumerate(self._dst.variables)}
new_x = np.zeros(self._src.get_num_vars())
for i, var in enumerate(self._src.variables):
if var in self._conv:
new_x[i] = sum(sol[aux] * coef for aux, coef in self._conv[var]) + var.lowerbound
else:
new_x[i] = sol[var.name]
return np.array(new_x)