DrawRandomInt should work when min and max are equal

[rhpreston]

Describe the bug

DrawRandomInt produces an ExecutionFailExecption when min and max parameters are equal.

To Reproduce

Call Microsoft.Quantum.Random.DrawRandomInt with the same number for min and max. For example, DrawRandomInt(0, 0).

Expected behavior

Random integer functions typically allow a range of 1, for situations where the range may be variable. For example, I would expect to be able to select a random index of an array like this:

// assume "array" is of length > 0
let randomIndex = DrawRandomInt(0, Length(array) - 1);

Actual behavior

The above code will fail if array is of length 1.

System information

N/A

Additional context
To be clear, the current behavior is intended per the documentation. Really the purpose of this issue is to ask if there is a reason why DrawRandomInt was implemented this way, and if the team would consider changing it.

Thanks 😄

[bettinaheim]

Thanks for filing the issue, @rhpreston.
I don't see a reason why it would have to be this way. To support this, the implementation in both the C# runtime and in the QIR runtime will need to be adapted. @cgranade @kuzminrobin Do you see any issues with that?

[cgranade]

No objections here; that would also be very nice for use with MaybeChooseElement.

[bettinaheim]

Given that the QIR runtime actually does already seem to support it (see here), it sounds reasonable to support it in general. @rhpreston Do you by any chance want to make a contribution with that? I believe all that is needed is to edit the C# implementation here.

[kuzminrobin]

I looked at the code of QIR runtime and ran a few tests. The QIR implementation runs as requested - for equal min and max returns min (does not fail). As I understand, this bug (if observed at all) relates to C# Runtime only.

FYI:
In the docs for DrawRandomInt(), DrawRandomDouble() I see:

Fails if max <= min

I expected:

Fails if max < min

Otherwise the docs contradict to themselves (to the fragment "An integer in the inclusive range from min to max with uniform probability").
I added the PR to fix that.