# The Resources Estimator Target Machine

As the name implies, the `ResourcesEstimator`

estimates the resources
required to run a given instance of a Q# operation on a quantum computer.
It accomplishes this by executing the quantum operation without actually
simulating the state of a quantum computer; for this reason,
it can estimate resources for Q# operations that use thousands of qubits, if the classical part of the code can be run in a reasonable time.

## Usage

The `ResourcesEstimator`

is just another type of target machine, thus
it can be used to run any Q# operation.

As other target machines, to use it on a C# host program create an instance and pass it
as the first parameter of the operation's `Run`

method:

```
using Microsoft.Quantum.Simulation.Core;
using Microsoft.Quantum.Simulation.Simulators;
namespace Quantum.MyProgram
{
class Driver
{
static void Main(string[] args)
{
ResourcesEstimator estimator = new ResourcesEstimator();
MyQuantumProgram.Run(estimator).Wait();
Console.WriteLine(estimator.ToTSV());
}
}
}
```

As the example shows, the `ResourcesEstimator`

provides a `ToTSV()`

method to generate
a table with tab-separated-values (TSV) that can be saved into a file
or written to the console for analysis. The output of the above program should look something like this:

```
Metric Sum
CNOT 1000
QubitClifford 1000
R 0
Measure 4002
T 0
Depth 0
Width 2
BorrowedWidth 0
```

Note

The `ResourcesEstimator`

does not reset its calculations on every run, if the same instance is
used to execute another operation it will keep aggregating counts on top of existing results.
If you need to reset calculations between runs, create a new instance for every execution.

## Programmatically Retrieving the Estimated Data

In addition to a TSV table, the resources estimated can be retrieved programmatically
via the `ResourcesEstimator`

's `Data`

property. `Data`

provides a `System.DataTable`

instance with two columns: `Metric`

and `Sum`

, indexed by the metrics names.

The following code shows how to retrieve and print the total number of `QubitClifford`

, `T`

and `CNOT`

gates used by a Q# operation:

```
using Microsoft.Quantum.Simulation.Core;
using Microsoft.Quantum.Simulation.Simulators;
namespace Quantum.MyProgram
{
class Driver
{
static void Main(string[] args)
{
ResourcesEstimator estimator = new ResourcesEstimator();
MyQuantumProgram.Run(estimator).Wait();
var data = estimator.Data;
Console.WriteLine($"QubitCliffords: {data.Rows.Find("QubitClifford")["Sum"]}");
Console.WriteLine($"Ts: {data.Rows.Find("T")["Sum"]}");
Console.WriteLine($"CNOTs: {data.Rows.Find("CNOT")["Sum"]}");
}
}
}
```

## Metrics Reported

The following is the list of metrics estimated by the `ResourcesEstimator`

:

**CNOT**: The count of CNOT (also known as the Controlled Pauli X gate) gates executed.**QubitClifford**: The count of any single qubit Clifford and Pauli gates executed.**Measure**: The count of any measurements executed.**R**: The count of any single qubit rotations executed, excluding T, Clifford and Pauli gates.**T**: The count of T gates and their conjugates, including the T gate, T_x = H.T.H, and T_y = Hy.T.Hy, executed.**Depth**: The lower bound for the depth of the quantum circuit executed by the Q# operation. By default, only T gates are counted in the depth, see depth counter for details.**Width**: The lower bound for the maximum number of qubits allocated during the execution of the Q# operation. It might not be possible to achieve both**Depth**and**Width**lower bounds simultaneously.**BorrowedWidth**: Maximum number of qubits borrowed inside the Q# operation.

## Providing the Probability of Measurement Outcomes

AssertProb from the Microsoft.Quantum.Intrinsic namespace can be used to provide information about the expected probability of a measurement to help drive the execution of the Q# program. The following example illustrates this:

```
operation Teleport(source : Qubit, target : Qubit) : Unit {
using (qubit = Qubit()) {
H(q);
CNOT(qubit, target);
CNOT(source, qubit);
H(source);
AssertProb([PauliZ], [source], Zero, 0.5, "Outcomes must be equally likely", 1e-5);
AssertProb([PauliZ], [qubit], Zero, 0.5, "Outcomes must be equally likely", 1e-5);
if (M(source) == One) { Z(target); X(source); }
if (M(qubit) == One) { X(target); X(qubit); }
}
}
```

When the `ResourcesEstimator`

encounters `AssertProb`

it will record that measuring
`PauliZ`

on `source`

and `q`

should be given an outcome of `Zero`

with probability
0.5. When it executes `M`

later, it will find the recorded values of
the outcome probabilities and `M`

will return `Zero`

or `One`

with probability
0.5.

## See also

The `ResourcesEstimator`

is built on top of the quantum computer trace simulator, which provides a richer set of metrics,
the ability to report metrics on the full call-graph, and features like distinct inputs checker to help find bugs on Q# programs.
Please refer to the trace simulator
documentation for more information.