When aligning long reads, we would prefer to align through low-quality
regions. This requires a large Z-drop threshold. However, to find small
inversions, we need to use a small Z-drop. This commit address this
conflict with two Z-drop thresholds. When Z-drop exceeds the smaller
threshold, we perform a local alignment to check if there is a potential
inversion. If there is one, we break the alignment; otherwise we break
the alignment only if Z-drop excess the larger threshold.
This commit also fixes a bug that reported wrong coordinates when the
inversion is on the forward strand (#112).
When a chain is split during base-level alignment, its chaining score is
reduced. However, the chaining score of its suboptimal chain remains the same.
This leads to underestimated mapping quality.
[PMID:18688272] shows that the base following GT tends to be A or G (i.e. R) in
both human and yeast, and that the base preceeding AG tends to be C or T (i.e.
Y). In the new model, we pay no cost to GTr..yAG, but we pay half of the cost
if there is no r or y. This improves the junction accuracy when mapping to
human and mouse and decreases the accuacy when mapping to SIRV. My guess is
that SIRV does not honor this trend. Need to investigate in future.
Also in this commit, --cost-non-gt-ag is aliased to -C. The default is changed
to 9 instead of 5. I also added --splice-flank to enable the above model. This
may become the default once I confirm my hypothesis on SIRV.
though it might not work in an extremely rare case: the end of a sequence ends
at X*16384 and it is the last sequence in a batch. This can be resolved by
never letting the kstream_t buffer empty.
In the default splice mode, minimap2 applies two rounds of spliced alignment:
first assuming GT-AG to be the splice signal across all splicing sites and then
assuming CT-AC to be the signal. This is the idea strategy.
In the MM_F_SPLICE_BOTH mode, minimap2 applies one round of spliced alignment,
assuming GT-AG and CT-AC to be the splice signals AT THE SAME TIME. This will
be faster but less accurate. I don't think anyone would like to run minimap2 in
this mode, so I am removing it for clarity.
Heng Li
mvdbeek