Documentation

Collected "traditional" statistics:

• Base composition near 5' end and near 3' end. Each consists of five vectors of counts of A,C,G,T, and everything else. basecompo5 begins with context bases to the left of the 5' end, basecompo3 ends with context bases to the right of the 3' end.
• Substitutions. Counted from the reconstructed alignment, once around the 5' end and once around the 3' end. For a total of 2*4*4 different substitutions. Positions where the query has a gap are skipped.
• Substitutions at CpG motifs. Also counted from the reconstructed alignment, and a CpG site is simply the sequence CG in the reference. Gaps may confuse that definition, so that CpHpG still counts as CpG, because the H is gapped. That might actually be desirable.
• Conditional substitutions. The 5' and 3' ends count as damaged if the very last position has a C-to-T substitution. With that in mind, substs5d5, substs5d3, substs5dd are like substs5, but counting only reads where the 5' end is damaged, where the 3' end is damaged, and where both ends are damaged, respectively.

Common logic for statistics. The function get_ref_and_aln reconstructs reference sequence and alignment from a Bam record. It is expected to construct the alignment with respect to the forwards strand of the reference; we reverse-complement it if necessary.

Enumeratee (almost) that computes some statistics from plain BAM with a valid MD field. The Alignment is also reconstructed and passed downstream. The result of any downstream processing is available in the stats_more field of the result.

• Reconstruct the alignment from CIGAR, SEQ, and MD.
• Filter the alignment to get the reference sequence, accumulate it.
• Accumulate everything over the alignment.

The argument is the amount of context inside desired.

For Complete fragments, we cut the read in the middle, so the 5' and 3' plots stay clean from each other's influence. Leading and Trailing fragments count completely towards the appropriate end.

Enumeratee (almost) that computes some statistics from plain BAM (no MD field needed) and a 2bit file. The Alignment is also reconstructed and passed downstream. The result of any downstream processing is available in the stats_more field of the result.

• Get the reference sequence including both contexts once. If this includes invalid sequence (negative coordinate), pad suitably.
• Accumulate counts for the valid parts around 5' and 3' ends as appropriate from flags and config.
• Combine the part that was aligned to (so no context) with the read to reconstruct the alignment.

Arguments are the table of reference names, the 2bit file with the reference, the amount of context outside the alignment desired, and the amount of context inside desired.

For Complete fragments, we cut the read in the middle, so the 5' and 3' plots stay clean from each other's influence. Leading and Trailing fragments count completely towards the appropriate end.

Reconstructs the alignment from query, cigar, and md. Only positions where the query is not gapped are produced.

Parameters for the universal damage model.

We assume the correct model is either no damage, or single strand damage, or double strand damage. Each of them comes with a probability. It turns out that blending them into one is simply accomplished by multiplying these probabilities onto the deamination probabilities.

For single stranded library prep, only one kind of damage occurs (C frequency (ssd_sigma) in single stranded parts, and the overhang length is distributed exponentially with parameter ssd_lambda at the 5' end and ssd_kappa at the 3' end. (Without UDG treatment, those will be equal. With UDG, those are much smaller and in fact don't literally represent overhangs.)

For double stranded library prep, we get C->T damage at the 5' end and G->A at the 3' end with rate dsd_sigma and both in the interior with rate dsd_delta. Everything is symmetric, and therefore the orientation of the aligned read doesn't matter either. Both overhangs follow a distribution with parameter dsd_lambda.

A DamageModel is a function that gives substitution matrices for each position in a read. The DamageModel can depend on whether the alignment is reversed, the length of the read and the position. (In practice, we should probably memoize precomputed damage models somehow.)

Convenience function to access a substitution matrix that has a mnemonic reading.

Alignment record. The reference sequence is filled with Ns if missing.

Compact storage of a pair of ambiguous Nucleotides. Used to represent alignments in a way that is accessible even to assembly code. The first and sencond field are stored in the low and high nybble, respectively. See fst_np, snd_np, npair.

DamageModel for undamaged DNA. The likelihoods follow directly from the quality score. This needs elaboration to see what to do with amibiguity codes (even though those haven't actually been observed in the wild).

We represent substitution matrices by the type Mat44D. Internally, this is a vector of packed vectors. Conveniently, each of the packed vectors represents all transitions into the given nucleotide.

Adds the two matrices of a mutable substitution model (one for each strand) appropriately, normalizes the result (to make probabilities from pseudo-counts), and freezes that into one immutable matrix. We add a single count everywhere to avoid getting NaNs from bizarre data.

Number of mismatches allowed by BWA. bwa_cal_maxdiff thresh len returns the number of mismatches bwa aln -n $tresh would allow in a read of length len. For reference, here is the code from BWA that computes it (we assume err = 0.02, just like BWA):

int bwa_cal_maxdiff(int l, double err, double thres)
  {
     double elambda = exp(-l * err);
     double sum, y = 1.0;
     int k, x = 1;
     for (k = 1, sum = elambda; k < 1000; ++k) {
         y *= l * err;
         x *= k;
         sum += elambda * y / x;
         if (1.0 - sum < thres) return k;
     }
     return 2;
  }