The changes after r317 aim to improve the performance and accuracy for very
long query alignment. The short-read alignment should not be affected. The
changes include:
1) Z-dropoff. This is a variant of blast's X-dropoff. I orginally thought this
heuristic only improves speed, but now I realize it also reduces poor
alignment with long good flanking alignments. The difference from blast's
X-dropoff is that Z-dropoff allows big gaps, but X-dropoff does not.
2) Band width doubling. When band width is too small, we will get a poor
alignment in the middle. Sometimes such alignments cannot be fully excluded
with Z-dropoff. Band width doubling is an alternative heuristic. It is based
on the observation that the existing of close-to-boundary high score
possibly implies inadequate band width. When we see such a signal, we double
the band width.
While we can compute smems very efficiently, there is still a long way to get
the alignment. On simulated data, this smem-based algorithm is 4X faster than
bwasw and twice as fast as bowtie2, but the accuracy is far lower than bwasw
and even lower than bowtie2 in the high-mapQ range. I am kind of sure that if
we continue to increase the mapping accuracy, the speed will approach to bwasw,
if not slower.
Smem-based mapping algorithm is still interesting, but given that I am short of
time, I will not explore it further.
Heng Li
Heng Li