uniform.py

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# Copyright (C) IBM Corporation 2019
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"""
The uniform mechanism in differential privacy.
"""
from numbers import Real

from diffprivlib.mechanisms.base import DPMechanism
from diffprivlib.mechanisms.laplace import Laplace
from diffprivlib.utils import copy_docstring


class Uniform(DPMechanism):
    r"""
    The Uniform mechanism in differential privacy.

    This emerges as a special case of the :class:`.LaplaceBoundedNoise` mechanism when epsilon = 0.
    Paper link: https://arxiv.org/pdf/1810.00877.pdf

    Parameters
    ----------
    delta : float
        Privacy parameter :math:`\delta` for the mechanism.  Must be in (0, 0.5].

    sensitivity : float
        The sensitivity of the mechanism.  Must be in [0, ∞).

    random_state : int or RandomState, optional
        Controls the randomness of the mechanism.  To obtain a deterministic behaviour during randomisation,
        ``random_state`` has to be fixed to an integer.

    """
    def __init__(self, *, delta, sensitivity, random_state=None):
        super().__init__(epsilon=0.0, delta=delta, random_state=random_state)
        self.sensitivity = self._check_sensitivity(sensitivity)

    @classmethod
    def _check_epsilon_delta(cls, epsilon, delta):
        if not epsilon == 0:
            raise ValueError("Epsilon must be strictly zero.")

        if not 0 < delta <= 0.5:
            raise ValueError("Delta must be in the half-open interval (0, 0.5]")

        return super()._check_epsilon_delta(epsilon, delta)

    @classmethod
    def _check_sensitivity(cls, sensitivity):
        if not isinstance(sensitivity, Real):
            raise TypeError("Sensitivity must be numeric")

        if sensitivity < 0:
            raise ValueError("Sensitivity must be non-negative")

        return float(sensitivity)

    @copy_docstring(Laplace.bias)
    def bias(self, value):
        return 0.0

    @copy_docstring(Laplace.variance)
    def variance(self, value):
        self._check_all(value)

        return (self.sensitivity / self.delta) ** 2 / 12

    def _check_all(self, value):
        super()._check_all(value)
        self._check_sensitivity(self.sensitivity)

        if not isinstance(value, Real):
            raise TypeError("Value to be randomised must be a number")

        return True

    @copy_docstring(Laplace.randomise)
    def randomise(self, value):
        self._check_all(value)

        unif_rv = 2 * self._rng.random() - 1
        unif_rv *= self.sensitivity / self.delta / 2

        return value + unif_rv