Source code for qrisp.jasp.program_control.sampling

"""
\********************************************************************************
* Copyright (c) 2025 the Qrisp authors
*
* This program and the accompanying materials are made available under the
* terms of the Eclipse Public License 2.0 which is available at
* http://www.eclipse.org/legal/epl-2.0.
*
* This Source Code may also be made available under the following Secondary
* Licenses when the conditions for such availability set forth in the Eclipse
* Public License, v. 2.0 are satisfied: GNU General Public License, version 2
* with the GNU Classpath Exception which is
* available at https://www.gnu.org/software/classpath/license.html.
*
* SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
********************************************************************************/
"""

import jax
import jax.numpy as jnp

from qrisp.jasp.tracing_logic import quantum_kernel, check_for_tracing_mode

# The following function implements the sample feature.

# The basic functionality would be relatively straightforward to implement,
# however there are some complications. The reason for that is that the resulting
# jaxpr should be "readable" by the terminal sampling interpreter.
# Terminal sampling means that instead of performing the simulations "shots"-times
# it is performed once and the shots are then sampled from that distribution.
# Naturally this implies a massive performance increase, which is why a lot
# of effort is spent to realize a smooth implementation.

# The underlying idea to make the feature easily "readable" by the terminal
# sampling interpreter is to structure one iteration of sampling into three
# steps.

# 1. Evaluating the user function, which generates the distribution.
# 2. Sampling from that distribution via the "measure" function.
# 3. Decoding and postprocessing the measurement results.

# For the final two steps we deploy some custom logic to realize the terminal
# sampling behavior. To simplify the automatic processing of these steps,
# we capture each into individual pjit calls.

# The terminal sampling interpreter then identifies each steps via the
# eqn.params["name"] attribute and executes the custom logic.



[docs]
def sample(state_prep = None, shots = 0, post_processor = None):
    r"""
    The ``sample`` function allows to take samples from a state that is specified
    by a preparation procedure. This preparation procedure can be supplied via
    a Python function that returns one or more :ref:`QuantumVariables <QuantumVariable>`.
    
    The samples are returned in the form of a 
    `Jax Array <https://jax.readthedocs.io/en/latest/_autosummary/jax.Array.html>`_
    which is shaped according to the ``shots`` parameter. Because of this, shots
    can only be a **static integer** (no dynamic values!). If you want to sample
    with a dynamic shot amount, look into :ref:`expectation_value`.
    
    
    Parameters
    ----------
    state_prep : callable
        A function returning one or more :ref:`QuantumVariables <QuantumVariable>`. 
        The state from this QuantumVariables will be sampled.
        The state preparation function can only take classical values as arguments.
        This is because a quantum value would need to be copied for each sampling 
        iteration, which is prohibited by the no-cloning theorem.
    shots : int
        The amounts of samples to take.
    post_processor : callable, optional
        A function to apply to the samples directly after measuring. By default no post processing is applied.

    Raises
    ------
    Exception
        Tried to sample with dynamic shots value (static integer required)
    Exception
        Tried to sample from state preparation function taking a quantum value

    Returns
    -------
    callable
        A classical, Jax traceable function returning a jax array containing 
        the measurement results of each shot.

    Examples
    --------
    
    We prepare the state
    
    .. math::
        
        \ket{\psi} = \frac{1}{\sqrt{2}} \left(\ket{0}\ket{0}\ket{\text{True}} + \ket{k}\ket{k}\ket{\text{True}})\right)
    
    ::
        
        from qrisp import *
        from qrisp.jasp import *


        def state_prep(k):
            a = QuantumFloat(4)
            b = QuantumFloat(4)
            
            qbl = QuantumBool()
            h(qbl)
            
            with control(qbl[0]):
                a[:] = k
                
            cx(a, b)
            
            return a, b

    And subsequently sample from the QuantumFloats:
        
    ::
        
        @jaspify
        def main(k):
            
            sampling_function = sample(state_prep, 
                                       shots = 10)
            
            return sampling_function(k)

        print(main(3))
        
        # Yields
        # [[3. 3.]
        #  [0. 0.]
        #  [0. 0.]
        #  [3. 3.]
        #  [0. 0.]
        #  [0. 0.]
        #  [3. 3.]
        #  [3. 3.]
        #  [0. 0.]
        #  [0. 0.]]

    To demonstrate the post processing feature, we write a simple post 
    processing function:
        
    ::
        
        def post_processor(x, y):
            return 2*x + y//2
        
        @jaspify
        def main(k):
            
            sampling_function = sample(state_prep, 
                                       shots = 10,
                                       post_processor = post_processor)
            
            return sampling_function(k)

        print(main(4))
        # Yields
        # [10. 10.  0.  0.  0.  0.  0.  0. 10. 10.]

    """
    
    from qrisp.jasp import qache
    from qrisp.core import QuantumVariable, measure
    
    if isinstance(state_prep, int):
        shots = state_prep
        state_prep = None
    
    if state_prep is None:
        return lambda x : sample(x, shots, post_processor = post_processor)
    
    if post_processor is None:
        def identity(*args):
            if len(args) == 1:
                return args[0]
            return args
        
        post_processor = identity
    
    if isinstance(shots, jax.core.Tracer):
        raise Exception("Tried to sample with dynamic shots value (static integer required)")
    elif not isinstance(shots, int):
        raise Exception(f"Tried to sample with shots value of non-integer type {type(shots)}")
    
    # Qache the user function
    @qache
    def user_func(*args):
        return state_prep(*args)

    # This function evaluates the sampling process
    @jax.jit
    def sampling_eval_function(tracerized_shots, *args):
        
        for arg in args:
            if isinstance(arg, QuantumVariable):
                raise Exception("Tried to sample from state preparation function taking a quantum value")
        
        # We now construct a loop to collect the samples by 
        # inserting the postprocessed measurement result into an array.
        # The following function is the loop body, which is kernelized.
        @quantum_kernel
        def sampling_body_func(i, args):
            
            acc = args[0]
            
            # Evaluate the user function
            qv_tuple = user_func(*args[1:])
            
            if not isinstance(qv_tuple, tuple):
                qv_tuple = (qv_tuple,)
            
            for qv in qv_tuple:
                if not isinstance(qv, QuantumVariable):
                    raise Exception("Tried to sample from function not returning a QuantumVariable")
            
            # Trace the DynamicQubitArray measurements
            # Since we execute the measurements on the .reg attribute, no decoding
            # is applied. The decoding happens in sampling_helper_2
            @qache
            def sampling_helper_1(*args):
                res_list = []
                for reg in args:
                    res_list.append(measure(reg))
                return tuple(res_list)
            
            measurement_ints = sampling_helper_1(*[qv.reg for qv in qv_tuple])
            
            # Trace the decoding
            @jax.jit
            def sampling_helper_2(acc, i, *meas_ints):
                decoded_values = []
                for j in range(len(qv_tuple)):
                    decoded_values.append(qv_tuple[j].jdecoder(meas_ints[j]))
            
                if len(qv_tuple) > 1:
                    decoded_values = post_processor(*decoded_values)
                else:
                    decoded_values = post_processor(*decoded_values)
                
                if isinstance(decoded_values, tuple):
                    # Save the return amount (for more details check the comment of the)
                    # initialization command of return_amount
                    return_amount.append(len(decoded_values))
                    if len(acc.shape) == 1:
                        raise AuxException()
                    
                # Insert into the accumulating array
                acc = acc.at[i].set(decoded_values)
                
                return acc
            
            acc = sampling_helper_2(acc, i, *measurement_ints)
            
            return (acc, *args[1:])
        
        # This list captures the amount of return values. The strategy here is
        # to initially assume only one QuantumVariable is returned, which is then
        # added to the expectation value accumulator. If more than one is returned,
        # the amount is saved in this list and an exception is raised, which
        # subsequently causes another call but this time with the correct accumulator
        # dimension.
        
        return_amount = []
        
        try:
            loop_res = jax.lax.fori_loop(0, tracerized_shots, sampling_body_func, (jnp.zeros(shots), *args))
            return loop_res[0]
        except AuxException:
            loop_res = jax.lax.fori_loop(0, tracerized_shots, sampling_body_func, (jnp.zeros((shots, return_amount[0])), *args))
            return loop_res[0]
    
    from qrisp.jasp import terminal_sampling
    def return_function(*args):
        
        if check_for_tracing_mode():
            return sampling_eval_function(shots, *args)
        else:
            return terminal_sampling(state_prep, shots)(*args)
    
    return return_function


class AuxException(Exception):
    pass