[SciPy-Dev] Discrete-time additions to scipy.signal

[Jeffrey Armstrong]

On Tue, 26 Apr 2011, Ralf Gommers wrote:

> Hi Jeffrey,
>
> I looked through your changes and at first glance it looks pretty good
> to me. It's not so easy to review however, because it's all in your
> master branch, it's a lot of code, and there is quite some code added
> that is later deleted again. It would be easier if you would create
> separate branches for separate features (not your master branch, keep
> that a clean mirror of the numpy master branch). There are at least
> two, the discrete versions of features already present for the
> continuous domain, and your additions of Riccati, Lyapunov and
> Sylvester solvers. Perhaps the sort kw for decomp_schur is a third.

Ralf,

I apologize for the lack of branches.  I must confess that this is my 
first large-scale experience in a git environment so I'm still getting the 
hang of it.  You've summarized the changes pretty well:

1. Duplicating most of the continuous functionality in the discrete domain 
(dltisys.py, c2d.py)

2. Adding Lyapunov and algebraic Riccati equation solvers (lyap.py, 
dlyap.py, dare.py, care.py, decomp_schur.py, trsyl flapack wrapper).

The sort kw in decomp_schur is necessary to properly solve the algebraic 
Riccati equation.

>
> The current names of modules are not very descriptive. They could be
> changed to something like:
> c2d -> cont2discrete
> care+dare -> riccati
> lyap+dlyap -> lyapunov
> Or maybe put it in a "control" module.

The module names could be more descriptive, I agree.  While I think the 
addition of a "control" module in scipy would perhaps be a good idea, I 
didn't have the audacity to make such a suggestion from the start.

>
> Can you explain the relation to pydare a bit? Is this code all from
> pydare and are you relicensing it as BSD and proposing it for
> inclusion, or is part of this new code?
>

The pydare package (http://code.google.com/p/pydare/) is a small package 
providing discrete algebraic Riccati and Lyapunov equation solvers, and I 
am the sole author.  The code was originally licensed under GPLv3 because 
the discrete Lyapunov equation solver utilized code that had been directly 
translated from some Octave code into Python, which was also GPL.  The 
pydare package can also exploit SLICOT, another GPL'd package.

To contribute the code to SciPy, I do indeed relicense the entirety of my 
code as BSD-licensed, and I'll assign copyright or whatever as necessary. 
The GPL-based code in the discrete Lyapunov equation solver has been 
redacted in its entirety, replaced by original code that relies on the 
solution of a Sylvester equation instead (requiring an inversion, which is 
less than ideal).  All references to SLICOT have also been removed, and 
there is not any code present any longer that is not original.

Jeff Armstrong - jba at sdf.lonestar.org
SDF Public Access UNIX System - http://sdf.lonestar.org