# Google Summer of Code 2015

Both Sage and the Lmonade project were selected for Google’s Summer of Code 2015. If you are an eligible student, you should consider applying. If you need ideas what to work on, there are many fine projects/project ideas on either the Lmonade or the Sage GSoC pages. In particular, here are the fplll project ideas, for which I could be one of the two mentors.

## Add intermediate floating-point arithmetic(s), between double precision and multi-precision

fplll internally relies on floating-point arithmetic for the underlying Gram-Schmidt orthogonalisation. It tries to perform the computations in machine floating-point numbers: doubles, with 53 bits of precision, or long doubles which sometimes enjoy 64 bits of precision. If that does not prove sufficient, it switches to multi-precision arithmetic (MPFR), and increases the precision progressively, until it seems numerically stable for that precision. A drawback of this approach is that MPFR is much slower than machine floating-point numbers, creating a huge run-time gap between instances that can be run on machine precision, and instances that require larger precision.

The project is to add annother floating-point arithmetic type, with intermediate precision (such as double-double precision) and intermediate efficiency. This should then be combined with dpe and the so-called “fast” Gram-Schmidt approach, which aim at extending the range of exponents. And these arithmetics should finally be incorporated in the fplll wrapper.

## Implement a test-suite

Most of the code of fplll is not automatically tested. Sometimes, simple modifications result lead to subtle bugs in cases that are not tried by the main developers, and that are found out by users much later.

The project would be to add unit-tests to fplll to give more assurance that a code change doesn’t break too much of the code. The tests should be devised so that the different arithmetics, the different wrapper scenarios, and also the different main functions (LLL, BKZ, SVP) are tested. This could be extended to adding some standard-ish benchmarks which make it easier to decide if a code modification improves or degrades run-times (for “typical” inputs.)

## Implement transformation matrix recovery and LLL on Gram matrices

At the moment, fplll’s LLL routines take as input a basis B of a lattice and return a reduced basis C of the same lattice. For some applications, it is interesting to also have access to the transformation matrix U such that C = B*U. One way is to compute U on the fly during the computations, another way is to compute U a posterio, from B and C. In theory, no method is superior to the other: in some contexts the first approach should be better, in others it should not.

In other applications, the input basis is only given implicitely via its Gram matrix G = B^T * B. The LLL algorithm can still be run on such inputs, to get the gram matrix C^T * C of the reduced basis C. Such a variant, taking as input a Gram matrix and returning another Gram matrix, is currently unavailable.

The project is to implement these extensions of the LLL algorithm.

Get in touch on fplll-devel if any of these interest you.