-----------------------------------------
Computing Tensor Network State Amplitudes
-----------------------------------------

The following code example illustrates how to define a tensor network state
and then compute a slice of amplitudes of the tensor network state.
The full code can be found in the `NVIDIA/cuQuantum <https://github.com/NVIDIA/cuQuantum>`_ repository (`here <https://github.com/NVIDIA/cuQuantum/blob/main/samples/cutensornet/high_level/amplitudes_example.cu>`_).

--------------------------
Headers and error handling
--------------------------

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #1
   :end-before: Sphinx: Amplitudes #2

-------------------------------------------------------------------------
Define the tensor network state and the desired slice of state amplitudes
-------------------------------------------------------------------------

Let's define a tensor network state corresponding to a 6-qubit quantum circuit
and request a slice of state amplitudes where qubits 0 and 1 are fixed at value 1.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #2
   :end-before: Sphinx: Amplitudes #3

-----------------------------------------
Initialize the cuTensorNet library handle
-----------------------------------------

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #3
   :end-before: Sphinx: Amplitudes #4

---------------------------
Define quantum gates on GPU
---------------------------

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #4
   :end-before: Sphinx: Amplitudes #5

-------------------------------------------
Allocate the amplitudes slice tensor on GPU
-------------------------------------------

Here we allocate GPU memory for the requested amplitudes slice tensor.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #5
   :end-before: Sphinx: Amplitudes #6

----------------------------------
Allocate the scratch buffer on GPU
----------------------------------

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #6
   :end-before: Sphinx: Amplitudes #7

----------------------------------
Create a pure tensor network state
----------------------------------

Now let's create a pure tensor network state for a 6-qubit quantum circuit.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #7
   :end-before: Sphinx: Amplitudes #8

-------------------
Apply quantum gates
-------------------

Let's construct the GHZ quantum circuit by applying the corresponding quantum gates.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #8
   :end-before: Sphinx: Amplitudes #9

------------------------------------
Create the state amplitudes accessor
------------------------------------

Once the quantum circuit has been constructed, let's create the amplitudes accessor object
that will compute the requested slice of state amplitudes.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #9
   :end-before: Sphinx: Amplitudes #10

---------------------------------------
Configure the state amplitudes accessor
---------------------------------------

Now we can configure the state amplitudes accessor object by setting the number
of hyper-samples to be used by the tensor network contraction path finder.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #10
   :end-before: Sphinx: Amplitudes #11

------------------------------------------------------
Prepare the computation of the amplitudes slice tensor
------------------------------------------------------

Let's create a workspace descriptor and prepare the computation of the
amplitudes slice tensor.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #11
   :end-before: Sphinx: Amplitudes #12

--------------------
Set up the workspace
--------------------

Now we can set up the required workspace buffer.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #12
   :end-before: Sphinx: Amplitudes #13

-----------------------------------------------
Compute the specified slice of state amplitudes
-----------------------------------------------

Once everything has been set up, we compute the requested slice of state amplitudes,
copy it back to Host memory, and print it.

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #13
   :end-before: Sphinx: Amplitudes #14

--------------
Free resources
--------------

.. literalinclude:: ../../../../tensor_network/samples/high_level/amplitudes_example.cu
   :language: c++
   :linenos:
   :lineno-match:
   :start-after: Sphinx: Amplitudes #14