This function causes all kinds of problems when the reference genome consists
of many short reads/contigs/chromsomes. Some of the problems are nearly
unfixable at the point where bwa_fix_xref() gets called. This commit attempts
to fix the problem at the root. It disallows chains spanning multiple contigs
and never retrieves sequences bridging two adjacent contigs. Thus all the
chaining, extension, SW and global alignments are confined to on contig only.
This commit brings many changes. I have tested it on a couple examples
including Peter Field's PacBio example. It works well so far.
Ksw uses two rounds of SSE2-SW to find the boundaries of an alignment. If the
second round gives a different score from the first round, it will fail. The
fix checks if this happens, though I have not dig into an example to understand
why this may happen in the first place.
This detects desynchronised input files, which occasionally happens
due to user error or system failure.
Checking the names just after printing them has no real performance
implications because the strings are already in cache. (It might
be better to check while reading the input, but that would be more
complicated in the two-input-files case.)
Remove xmalloc, xcalloc, xrealloc and xstrdup from utils.h and revert calls
to the normal malloc, calloc, realloc, strdup. Add new files malloc_wrap.[ch]
with the wrapper functions. malloc_wrap.h #defines malloc etc. to the
wrapper, but only if USE_MALLOC_WRAPPERS has been defined.
Put #include "malloc_wrap.h" in any file that uses *alloc or strdup. This
is also in a #ifdef USE_MALLOC_WRAPPERS ... #endif block to make using the
wrappers optional. Add -DUSE_MALLOC_WRAPPERS into the makefile so they
should normally get added.
This is an improvement on the previous method as we now don't need to
worry about stray function calls that were not changed to the wrapped version
and the code will still work even if the wrapping is disabled.
Other possible methods of doing this are using malloc_hook (glibc-specific),
adding -include malloc_wrap.h to the gcc command-line (somewhat
gcc-specific) or making our own malloc function and using dlopen (scary).
This way is probably the most portable.
If one end has a low quality tail that happens to have a score-20 hit,
the pair won't be flagged as properly paired because bwa-mem thought it has
multiple hits. By filtering with -T, we won't have this problem.
I really hate this: complex and twisted logic for a nasty scenario that almost
never happens to short reads - but it may become serious when the reference
genome consists of many contigs.
On toy examples, the code seems to work. Don't know if it really works...
This is important for bwa-mem which does local alignment. A short exact match
is worse than a long inexact match. Also fixed a bug in approximating mapping
quality.
Nils Homer
Heng Li
John Marshall
Rob Davies