Source code for pennylane.measurements.shots

# Copyright 2018-2023 Xanadu Quantum Technologies Inc.
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#     http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""This module contains the Shots class to hold shot-related information."""
from collections.abc import Sequence

# pylint:disable=inconsistent-return-statements
from typing import NamedTuple, Union


class ShotCopies(NamedTuple):
    """A namedtuple that represents a shot quantity being repeated some number of times.
    For example, ``ShotCopies(10 shots x 2)`` indicates two executions with 10 shots each for 20 shots total.
    """

    shots: int
    copies: int

    def __str__(self):
        """The string representation of the class"""
        return f"{self.shots} shots{' x '+str(self.copies) if self.copies > 1 else ''}"

    def __repr__(self):
        """The representation of the class"""
        return f"ShotCopies({self.shots} shots x {self.copies})"


def valid_int(s):
    """Returns True if s is a positive integer."""
    return isinstance(s, int) and s > 0


def valid_tuple(s):
    """Returns True if s is a tuple of the form (shots, copies)."""
    return isinstance(s, Sequence) and len(s) == 2 and valid_int(s[0]) and valid_int(s[1])


class Shots:
    """
    A data class that stores shot information.

    Args:
        shots (Union[None, int, Sequence[int, Tuple[int, int]]]): Raw shot information

    Defining shots enables users to specify circuit executions, and the Shots class standardizes
    the internal representation of shots. There are three ways to specify shot values:

    * The value ``None``
    * A positive integer
    * A sequence consisting of either positive integers or a tuple-pair of positive integers of the form ``(shots, copies)``

    The tuple-pair of the form ``(shots, copies)`` is represented internally by a NamedTuple called
    :class:`~ShotCopies`. The first value is the number of shots to execute, and the second value is the
    number of times to repeat a circuit with that number of shots.

    The ``Shots`` class exposes two properties:

    * ``total_shots``, the total number of shots to be executed
    * ``shot_vector``, the tuple of :class:`~ShotCopies` to be executed

    Instances of this class are static. If an instance is passed to the constructor, that same
    instance is returned. If an instance is constructed with a ``None`` value, ``total_shots``
    will be ``None``.  This indicates analytic execution. A ``Shots`` object created with a
    ``None`` value is Falsy, while any other value results in a Truthy object:

    >>> bool(Shots(None)), bool(Shots(1))
    (False, True)

    **Examples**

    Example constructing a Shots instance with ``None``:

    >>> shots = Shots(None)
    >>> shots.total_shots, shots.shot_vector
    (None, ())

    Example constructing a Shots instance with an int:

    >>> shots = Shots(100)
    >>> shots.total_shots, shots.shot_vector
    (100, (ShotCopies(100 shots),))

    Example constructing a Shots instance with another instance:

    >>> shots = Shots(100)
    >>> Shots(shots) is shots
    True

    Example constructing a Shots instance with a sequence of ints:

    >>> shots = Shots([100, 200])
    >>> shots.total_shots, shots.shot_vector
    (300, (ShotCopies(100 shots x 1), ShotCopies(200 shots x 1)))

    Example constructing a Shots instance with a sequence of tuple-pairs:

    >>> shots = Shots(((100, 3), (200, 4),))
    >>> shots.total_shots, shots.shot_vector
    (1100, (ShotCopies(100 shots x 3), ShotCopies(200 shots x 4)))

    Example constructing a Shots instance with a sequence of both ints and tuple-pairs.
    Note that the first stand-alone ``100`` gets absorbed into the subsequent tuple because the
    shot value matches:

    >>> shots = Shots((10, 100, (100, 3), (200, 4),))
    >>> shots.total_shots, shots.shot_vector
    (1210, (ShotCopies(10 shots x 1), ShotCopies(100 shots x 4), ShotCopies(200 shots x 4)))

    Example constructing a Shots instance by multiplying an existing one by an int or float:

    >>> Shots(100) * 2
    Shots(total_shots=200, shot_vector=(ShotCopies(200 shots x 1),))
    >>> Shots([7, (100, 2)]) * 1.5
    Shots(total_shots=310, shot_vector=(ShotCopies(10 shots x 1), ShotCopies(150 shots x 2)))

    Example constructing a Shots instance by adding two existing instances together:

    >>> Shots(100) + Shots(((10,2),))
    Shots(total_shots=120, shot_vector=(ShotCopies(100 shots x 1), ShotCopies(10 shots x 2)))

    One should also note that specifying a single tuple of length 2 is considered two different
    shot values, and *not* a tuple-pair representing shots and copies to avoid special behaviour
    depending on the iterable type:

    >>> shots = Shots((100, 2))
    >>> shots.total_shots, shots.shot_vector
    (102, (ShotCopies(100 shots x 1), ShotCopies(2 shots x 1)))

    >>> shots = Shots(((100, 2),))
    >>> shots.total_shots, shots.shot_vector
    (200, (ShotCopies(100 shots x 2),))
    """

    total_shots: int = None
    """The total number of shots to be executed."""

    shot_vector: tuple[ShotCopies] = None
    """The tuple of :class:`~ShotCopies` to be executed. Each element is of the form ``(shots, copies)``."""

    _SHOT_ERROR = ValueError(
        "Shots must be a single positive integer, a tuple pair of the form (shots, copies), or a sequence of these types."
    )

    _frozen = False

    def __new__(cls, shots=None):
        return shots if isinstance(shots, cls) else object.__new__(cls)

    def __setattr__(self, name, value):
        if self._frozen:
            raise AttributeError(
                "Shots is an immutable class. Consider creating a new instance if you need different shot values."
            )
        return super().__setattr__(name, value)

    def __init__(self, shots=None):
        if shots is None:
            self.total_shots = None
            self.shot_vector = ()
        elif isinstance(shots, int):
            if shots < 1:
                raise self._SHOT_ERROR
            self.total_shots = shots
            self.shot_vector = (ShotCopies(shots, 1),)
        elif isinstance(shots, Sequence):
            if not all(valid_int(s) or valid_tuple(s) for s in shots):
                raise self._SHOT_ERROR
            self.__all_tuple_init__([s if isinstance(s, Sequence) else (s, 1) for s in shots])
        elif isinstance(shots, self.__class__):
            return  # self already _is_ shots as defined by __new__
        else:
            raise self._SHOT_ERROR

        self._frozen = True

    def __copy__(self):
        return self

    def __deepcopy__(self, memo):
        memo[id(self)] = self
        return self

    def __str__(self):
        """The string representation of the class"""
        if not self.has_partitioned_shots:
            return f"Shots(total={self.total_shots})"

        shot_copy_str = ", ".join([str(sc) for sc in self.shot_vector]) or None
        return f"Shots(total={self.total_shots}, vector=[{shot_copy_str}])"

    def __repr__(self):
        """The representation of the class"""
        return f"Shots(total_shots={self.total_shots}, shot_vector={self.shot_vector})"

    def __eq__(self, other):
        """Equality between Shot instances."""
        return (
            isinstance(other, Shots)
            and self.total_shots == other.total_shots
            and self.shot_vector == other.shot_vector
        )

    def __hash__(self):
        """Hash for a given Shot instance."""
        return hash(self.shot_vector)

    def __iter__(self):
        for shot_copy in self.shot_vector:
            for _ in range(shot_copy.copies):
                yield shot_copy.shots

    def __all_tuple_init__(self, shots: Sequence[tuple]):
        res = []
        total_shots = 0
        current_shots, current_copies = shots[0]
        for s in shots[1:]:
            if s[0] == current_shots:
                current_copies += s[1]
            else:
                res.append(ShotCopies(current_shots, current_copies))
                total_shots += current_shots * current_copies
                current_shots, current_copies = s
        self.shot_vector = tuple(res + [ShotCopies(current_shots, current_copies)])
        self.total_shots = total_shots + current_shots * current_copies

    def __bool__(self):
        return self.total_shots is not None

    def __add__(self, other):
        return add_shots(self, other)

    def __mul__(self, scalar):
        if not isinstance(scalar, (int, float)):
            raise TypeError("Can't multiply Shots with non-integer or float type.")
        if self.total_shots is None:
            return self

        scaled_shot_vector = tuple(
            ShotCopies(int(i.shots * scalar), i.copies) for i in self.shot_vector
        )

        return self.__class__(scaled_shot_vector)

    def __rmul__(self, scalar):
        return self.__mul__(scalar)

    @property
    def has_partitioned_shots(self):
        """
        Evaluates to True if this instance represents either multiple shot
        quantities, or the same shot quantity repeated multiple times.

        Returns:
            bool: whether shots are partitioned
        """
        if not self:
            return False
        return len(self.shot_vector) > 1 or self.shot_vector[0].copies > 1

    @property
    def num_copies(self):
        """The total number of copies of any shot quantity."""
        return sum(s.copies for s in self.shot_vector)

    def bins(self):
        """
        Yields:
            tuple: A tuple containing the lower and upper bounds for each shot quantity in shot_vector.

        Example:
            >>> shots = Shots((1, 1, 2, 3))
            >>> list(shots.bins())
            [(0,1), (1,2), (2,4), (4,7)]
        """
        lower_bound = 0
        for sc in self.shot_vector:
            for _ in range(sc.copies):
                upper_bound = lower_bound + sc.shots
                yield lower_bound, upper_bound
                lower_bound = upper_bound


ShotsLike = Union[Shots, None, int, Sequence[Union[int, tuple[int, int]]]]


def add_shots(s1: Shots, s2: Shots) -> Shots:
    """Add two :class:`~.Shots` objects by concatenating their shot vectors.

    Args:
        s1 (Shots): a Shots object to add
        s2 (Shots): a Shots object to add

    Returns:
        Shots: a :class:`~.Shots` object built by concatenating the shot vectors of ``s1`` and ``s2``

    Example:
        >>> s1 = Shots((5, (10, 2)))
        >>> s2 = Shots((3, 2, (10, 3)))
        >>> print(qml.measurements.add_shots(s1, s2))
        Shots(total=60, vector=[5 shots, 10 shots x 2, 3 shots, 2 shots, 10 shots x 3])
    """
    if s1.total_shots is None:
        return s2

    if s2.total_shots is None:
        return s1

    shot_vector = []
    for shot in s1.shot_vector + s2.shot_vector:
        shot_vector.append((shot.shots, shot.copies))

    return Shots(shots=shot_vector)

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