Monday, 28 July 2014

Week 10

This week, I removed the redundant store_trans parameter from the context object. Now, the capturing matrix is always updated unless the user passes NULL as the value. Also, in the special case that the user inputs a d x d identity and the number of columns of the input basis is greater than the number of rows (i.e. the embedding dimension is greater than the lattice dimension), I avoid updating the vectors during reduction itself and instead do a matrix multiplication at the end.

Also, as the mpf and wrapper functions of the LLL subroutines are supposed to guarantee that the output is indeed LLL-reduced, we need to check this in the most efficient way. This means that a fp test for reducedness should be used prior to using the exact arithmetic version (which is slower). Thus, I've added is_reduced functions in the module which first test using doubles, then mpfs if the matrix is certified non-reduced in the first test and finally, fmpq.

An initial implementation of the ULLL function was also added. It is different from the one in flint-1.6 because it does not perform any adjoining operations to the matrix. Instead, the option to store the unimodular transformations is utilised here. Also, the original did not use recursion on the truncated data, but the current code does. However, it needs to be tested.

This week, I plan to add test code and documentation for the ULLL function and document the functions for checking LLL-reducedness.

Tuesday, 22 July 2014

Week 9

This week, I updated the removals code to use a heuristic lower bound, mentioned by Curtis on flint-devel, on the final GS norm while removing the last vector during the execution of the reduction algorithm (after a failed Lovasz test involving kappa = d - 1). This was required because the proof of a theorem in the L^2 paper assumes that the final vector is LLL-reduced while deriving the error bound on the accuracy of the norm. This, of course, doesn't matter in the case of standard LLL but matters here because we need to be sure about the accuracy, lest we may remove something useful.

Besides this, I also unified the code for the cases where the exact Gram matrix is computed or input as mentioned in the todo section of my previous post. This required factoring out the row exponents from the Gram matrix rather than the basis itself, because the latter is not always be available (i.e. fl->rt == GRAM).

Also, I changed the code dealing with the matrix capturing the unimodular transformations to not make any assumptions regarding its column dimension. Earlier, I was assuming U to be a d x d matrix which was to be updated to satisfy the relation B* = UB where B* is the basis obtained by LLL-reducing B, i.e. U was the change-of-basis matrix. However, now U can be any matrix with the same number of rows as B.

I think the main implementation of LLL and LLL with removals is completed now, modulo a few (hopefully minor) changes. So, I plan to at least start working on the ULLL function this week.

Monday, 14 July 2014

Week 8

The eighth week of GSoC involved a lot of debugging to find the reasons for the bad behaviour of the removals function. In theory, the last vector can be removed from the basis if its squared GS length becomes greater than the bound at any point during the execution of the algorithm. However, it is not so straightforward in practice if the GSO is known only approximately. I think this may be a reason why the version in flint-1.6 removed vectors at the end as the algorithm seems to show much better behaviour in this case, i.e. the norms were more accurate.

I added the wrapper function for  LLL with removals optimised for knapsack lattices. Test code for this was also written.  Knapsack LLL differs from the textbook version in the fact that it performs early size reductions on the input basis occasionally which speeds up things in the knapsack case. Speaking of LLL with removals, the code was modified to remove the numerical inaccuracies plaguing it. Earlier, due to a floating point approximation of the Gram Schmidt orthogonalisation being used, the norm was incorrectly flagged as being greater than the bound which led to the removal of some useful vectors. The documentation was also updated and is now up to date with the module. Thus, the prerequisites for ULLL have now been completed.

This week I plan to unify the code for performing LLL on the input matrix in the cases that it is a lattice basis and an exact Gram matrix is to be used for computing the GSO or it is the Gram matrix itself.

Tuesday, 8 July 2014

Week 7

This week, I added support for LLL with removals on Gram matrix. This finds application in vector rational number reconstruction. Also, the inaccuracy checks (incorrect Lovasz tests and too many Babai loops) were improved to be similar to those used in fpLLL.

Also, the LLL function specialized to knapsack-type lattices, for which the Babai functions were implemented last week was added. It performs early size reductions which tend to make things faster for knapsack problem type lattice bases. It is also a prerequisite for the ULLL with removals function. Another feature implemented was early removal of vectors in LLL with removals. Now, the vectors whose squared GS lengths are greater than the input bound are removed from the basis during the reduction algorithm itself to avoid unnecessary overhead involved in keeping them updated during the computations.

This week, I plan to write the wrapper function for knapsack LLL with removals and fix any loose ends which may be remaining. The plan is to get the existing module to ready before I start working on the actual ULLL algorithm itself.

Thursday, 3 July 2014

Week 6

Well, this week's update is late. Sorry for that. I'll try to summarize the preceding week here. I added the heuristic version of LLL with removals, along with it's test code and documentation. In the MPFR version in flint-1.6, the squared GS length is divided by 8 instead of 2 as mentioned in the comments. I don't know if this an overlook or extra precaution. If the latter, I see no reason for this though. As I mentioned in my previous post, however, I avoid division altogether. LLL with removals was completed when its arbitrary precision variant was added and the wrapper function written. Its return value is the new dimension of the basis to be considered for further computation.

This brings us to the third major and perhaps, the most important part of this project: implementing ULLL with removals. This is because of its property of sub-quadratic time complexity in the size of the entries. Also, it is numerically more stable. It isn't mentioned in literature and hence, Bill graciously offered to write a paper on this for reference. Before I start work on the actual ULLL function, however, I need to implement an LLL with removals optimised for knapsack type lattices as it is used in ULLL. This requires a few Babai-like functions as well. These procedures will only reduce the kappa'th vector against the vectors upto cur_kappa (which is an index before which the basis is assumed to be LLL reduced) and not kappa - 1. The Babai functions added differ in their way of computing the dot products like the former versions

Along with the progress reported on the mailing list, one important thing to do now is to update the documentation which is lagging behind. I hope I'll be able to find time to do this task this week.

"The documentation needs documentation."   -- a Bellevue Linux Users Group member, 2005