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From: Matthias Troyer (troyer_at_[hidden])
Date: 2006-06-09 12:26:44

Hi, unfortunately I did not find as much time as I wanted to, due to
other commitments but I will still give my impressions so far.

> - What is your evaluation of the design?

While potentially a very useful library, there are some problems with
the current design that make it, as already mentioned by a few
others, unsuitable for serious physical computations. Below I will
mention that issues that I found in a quick reading:

* The name t1_quantity is not well chosen, but that was already
discussed before.

* The conversion factor between units only allow for 6 digits
accuracy. This is a serious problem when using non-SI units, e.g.
when measuring the energy in eV. Any conversion can incur an error of
about 1e-6, which is larger than the accuracy of some simulations.
The accuracy has to be increased. An arbitrary precision rational
number would be needed.

* When using float and units common in quantum simulations, where
hbar is set to 1, implying a conversion factor of about 1e-34, one
gets a serious underflow when looking at common terms like hbar^3 or
hbar^4. Hence there is a need for supporting other unit systems than
SI as the base.

* The problem of BOOST_PQS_INT32 instead of using int32_t was already
mentioned, and I will not go into details.

* Similarly, dimensioned quantities should behave just as plain
floating point types for all arithmetic operations - at least when no
unit conversions take place. This, and the problem of
BOOST_PQS_COMPARISON_EPSILON was already mentioned by Peter
Henderson. I will thus again not repeat the details.

* Finally the extension mechanism to define new units is awkward and
will soon lead to serious conflicts. My two main concerns are:
  - can't you provide a simple macro implementing the extension?
  - is the Abstract_quantity_id really necessary? Can't it just be
replaced by the scale factor except for those cases where the scale
factor for two units is the same and one needs to distinguish them.
The immediate problem with the current design that I can see is that
you recommend programmers to use the first available id for their new
unit. If now two programmers define a new unit, these will have the
same abstract id, and cannot be used together without changing the
code. Using the scale factor as part of the abstract id would make
such clashes much less likely.

> - What is your evaluation of the implementation?

I have not studied the implementation.

> - What is your evaluation of the documentation?

While not being optimal, one can learn what is needed. The one
section that completely confused me and needs serious improvements is
the concept documentation and the "Defintiion of Terms", hat Dave
Abrahams already wrote about in detail - I will thus not repeat my
concerns here but support his view.

> - What is your evaluation of the potential usefulness of the library?

An improved library, solving the issues above, would be very useful.
In the current form it is usable only for toy problems, and student
homework assignments, but not for serious calculations.

> - Did you try to use the library? With what compiler? Did you have
> any problems?


> - How much effort did you put into your evaluation? A glance? A quick
> reading? In-depth study?

About 4 hours, a quick reading.

> - Are you knowledgeable about the problem domain?

Having worked for 16 years in the domain of high performance
simulations of physical systems, I believe I can answer this with yes.

> - Do you think the library should be accepted as a Boost library?

Not in the current version which is not suitable for high-accuracy
scientific computations.

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