ResourcesEstimator instance shows wrong estimates when used more than once

[msoeken]

Describe the bug

ResourcesEstimator does not reset internal state and a single instance cannot be used to simulate two operations.

To Reproduce

Operation.qs

namespace Namespace {
  open Microsoft.Quantum.Intrinsic;

  operation Operation1() : Unit {
    using ((a, b) = (Qubit(), Qubit())) {
      CNOT(a, b);
      CNOT(a, b);
      CNOT(a, b);
    }
  }

  operation Operation2() : Unit {
    using ((a, b) = (Qubit(), Qubit())) {
      CNOT(a, b);
      CNOT(a, b);
      CNOT(a, b);
      CNOT(a, b);
      CNOT(a, b);
    }
  }
}

Driver.cs

using System;
using Microsoft.Quantum.Simulation.Simulators;

namespace Namespace {
  public class Program {
    public static void Main(string[] args) {
      var sim = new ResourcesEstimator();

      Operation1.Run(sim).Wait();
      Console.WriteLine(sim.ToTSV());

      Operation2.Run(sim).Wait();
      Console.WriteLine(sim.ToTSV());
    }
  }
}

Expected behavior

Output should be to accumulate the estimates:

Metric          Sum            
CNOT            3
QubitClifford   0
R               0
Measure         0
T               0
Depth           0
Width           2
BorrowedWidth   0
Metric          Sum            
CNOT            8
QubitClifford   0
R               0
Measure         0
T               0
Depth           0
Width           2
BorrowedWidth   0

or to reset the estimates:

Metric          Sum            
CNOT            3
QubitClifford   0
R               0
Measure         0
T               0
Depth           0
Width           2
BorrowedWidth   0
Metric          Sum            
CNOT            5
QubitClifford   0
R               0
Measure         0
T               0
Depth           0
Width           2
BorrowedWidth   0

Actual behavior

Output currently shows the estimates for the first operation:

Metric          Sum            
CNOT            3
QubitClifford   0
R               0
Measure         0
T               0
Depth           0
Width           2
BorrowedWidth   0
Metric          Sum            
CNOT            3
QubitClifford   0
R               0
Measure         0
T               0
Depth           0
Width           2
BorrowedWidth   0

System information

• Q# Version: 0.11.2004.2825
• .NET Core Version: 3.1.201

[bettinaheim]

Thanks for filing!
My suggestion would be that the counts after the second operation should be the combined counts for everything that has been executed on that resources estimator instance. Does that make sense?

[msoeken]

That is true. I have added both interpretations into the description.

[msoeken]

I just saw that in the documentation a note that says that the expected behaviour is to aggregate.

[IrinaYatsenko]

In Debug build hit Debug.Assert(results.rows.Count() == 1) in src\Simulation\Simulators\ResourcesEstimator\ResourcesEstimator.cs:124