When backtracking, bwa-short does not keep the detailed alignment or the exact
start and end positions. To find the boundary and the CIGAR, the old code does
a global alignment with a small end-gap penalty. It then deals with a lot of
special cases to derive the right position and CIGAR, which are actually not
always right. It is a mess.
As the new ksw.{c,h} does not support a different end-gap penalty, the old
strategy does not work. But we get something better. The new code finds the
boundaries with ksw_extend(). It is cleaner and gives more accurate CIGAR in
most cases.
stdaln.{c,h} was written ten years ago. Its local and SW extension code are
actually buggy (though that rarely happens and usually does not affect the
results too much). ksw.{c,h} is more concise, potentially faster, less buggy,
and richer in features.
Heng Li
Heng Li