Bernhard Grünewaldt a écrit : > Hi Luc, > > I took a look at nabla and I am impressed. > There are a lot of this Transformer classes that provide the bytecode for > the internal work. > > I don't really understand the full internals, but I think I have an overview.
If you feel the documentation is awkward or hard to understand, please tell me so. The ideas behind this project are not widespread. You can probably also have a lookt at http://www.autodiff.org/. > > I want to write a Parser for the Mathematical Standard Notation (msn). > The first step would be to parse the msn to Java Code. Nabla goal for now is focused on transforming bytecode only. We do not start from source. The reason for this is that we want to be able to differentiate any kind of expression, even corresponding to a complete program with thousands of lines and loop, conditionals, calls ... The use case I have in mind is the following one. I write a program that compute the trajectory of a satellite taking into account many different effects (central term of the gravitational attraction of course, but also perturbations like other gravitational terms from the spherical harmonics development, drag, solar radiation pressure, luni-solar attraction, maneuvers ...). This will be a complex program with lots of subroutines I wrote, but also algorithms from libraries already written like for example an ODE solver in commons-math. Then, I want to use this program as the heart of an optimizer that will try to find the best set of maneuvers to reach some target orbit. I need the derivatives but the program is : 1) already written 2) split in several separate source files and 3) too complex to be differentiated by hand. The program was difficult to develop and validate, so I do not want to rewrite it in another language (msn or other), I want to keep it as is and not modify it at all, to prevent introducing an error. The compiler already has a powerful parser and has already analyzed the program and translated it in a "simple" representation: bytecode. So this is what I consider to be my source. > Then use a on the fly compiler like asm or bcel to create a class and > instantiate it. We use asm in nabla. It seemed more powerful than bcel. > > Unless you give me some specific task I will try to write such a parser. There are two things I have in mind. The first one would be to complete the existing implementation to be able to handle appropriately loops (it almost works), use of intermediate variables, and calls to other functions. This is clearly missing now and we can support only very basic constructs, the ones that are in the unit tests. The second thing would be to add a completely new implementation using reverse mode. This is an impressive mode allowing to compute gradients very cheaply, even if the function to be differentiated has thousands of input parameters. I am currently reading a book referenced in the autodiff site http://www.autodiff.org/?module=Publications&submenu=list%20publications&id=Griewank2008EDP and really want to support this mode in Nabla. The existing AutomaticDifferentiator would probably be renamed ForwardModeAutomaticDifferentiator and a new ReverseModeAutomaticDifferentiator also implementing the UnivariateDifferentiator interface would be needed. What we need now is thinking rather than coding. We should also design an interface for using a differentiator to produce gradients efficiently, and also look at the fact that sometimes one does not always want a complete jacobian J but a gradient in a specific direction J.dX, which according to the book is much cheaper to compute directly than to compute J and then multiplying by dX. Does this makes sense to you ? Luc > > cu > - Bernhard > > > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: dev-unsubscr...@commons.apache.org > For additional commands, e-mail: dev-h...@commons.apache.org > > --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@commons.apache.org For additional commands, e-mail: dev-h...@commons.apache.org
