Make the mpileup BAM_CREF_SKIP filter optional.

Mpileup removes alignments using the cigar ref skip operator ("N").
This was originally added in 2011 in samtools/samtools#d1643d6 with
the commit message of "fixed a bug in indel calling related to
unmapped and refskip reads".

Unfortunately I don't know what that bug was, but removing the code
shows it still works (at least for some data!).  We need better
understanding of what's going on and why it was added, but this PR
makes it optional, keeping the default as before.  Note there appears
to be no filtering of BAM_CREF_SKIP in indels-2.0 so the option would
be a nop there.

This is an alternative PR to samtools#2281.  I've leave it to the project
maintainer as to what is preferable: removing the (no longer needed?)
filtering, or keeping the default behaviour identical and adding a new
option instead (which is safer, but possibly leads to accidental bad
calls due to not noticing a new option has appeared).

Fixes samtools#2277

bam2bcf.h

@@ -108,7 +108,7 @@ plp_cd_t;
 
 
 typedef struct __bcf_callaux_t {
-    int fmt_flag, ambig_reads;
+    int fmt_flag, ambig_reads, ref_skip_reads;
     int capQ, min_baseQ, max_baseQ, delta_baseQ;
     int openQ, extQ, tandemQ; // for indels
     uint32_t min_support, max_support; // for collecting indel candidates

bam2bcf_edlib.c

@@ -1559,18 +1559,21 @@ int bcf_edlib_gap_prep(int n, int *n_plp, bam_pileup1_t **plp, int pos,
                     }
                 }
 
-                int qbeg, qpos, qend, tbeg, tend, kk;
+                int qbeg, qpos, qend, tbeg, tend;
                 uint8_t *seq = bam_get_seq(p->b);
-                uint32_t *cigar = bam_get_cigar(p->b);
                 if (p->b->core.flag & BAM_FUNMAP) continue;
 
                 // FIXME: the following loop should be better moved outside;
                 // nonetheless, realignment should be much slower anyway.
-                for (kk = 0; kk < p->b->core.n_cigar; ++kk)
-                    if ((cigar[kk]&BAM_CIGAR_MASK) == BAM_CREF_SKIP)
-                        break;
-                if (kk < p->b->core.n_cigar)
-                    continue;
+                if (!bca->ref_skip_reads) {
+                    uint32_t *cigar = bam_get_cigar(p->b);
+                    int kk;
+                    for (kk = 0; kk < p->b->core.n_cigar; ++kk)
+                        if ((cigar[kk]&BAM_CIGAR_MASK) == BAM_CREF_SKIP)
+                            break;
+                    if (kk < p->b->core.n_cigar)
+                        continue;
+                }
 
                 // determine the start and end of sequences for alignment
                 int left2 = left, right2 = right;

bam2bcf_indel.c

@@ -845,18 +845,21 @@ int bcf_call_gap_prep(int n, int *n_plp, bam_pileup1_t **plp, int pos,
                     }
                 }
 
-                int qbeg, qpos, qend, tbeg, tend, kk;
+                int qbeg, qpos, qend, tbeg, tend;
                 uint8_t *seq = bam_get_seq(p->b);
-                uint32_t *cigar = bam_get_cigar(p->b);
                 if (p->b->core.flag & BAM_FUNMAP) continue;
 
                 // FIXME: the following loop should be better moved outside;
                 // nonetheless, realignment should be much slower anyway.
-                for (kk = 0; kk < p->b->core.n_cigar; ++kk)
-                    if ((cigar[kk]&BAM_CIGAR_MASK) == BAM_CREF_SKIP)
-                        break;
-                if (kk < p->b->core.n_cigar)
-                    continue;
+                if (!bca->ref_skip_reads) {
+                    uint32_t *cigar = bam_get_cigar(p->b);
+                    int kk;
+                    for (kk = 0; kk < p->b->core.n_cigar; ++kk)
+                        if ((cigar[kk]&BAM_CIGAR_MASK) == BAM_CREF_SKIP)
+                            break;
+                    if (kk < p->b->core.n_cigar)
+                        continue;
+                }
 
                 // determine the start and end of sequences for alignment
                 // FIXME: loops over CIGAR multiple times

doc/bcftools.txt

@@ -2503,6 +2503,14 @@ INFO/DPR    .. Deprecated in favor of INFO/AD; Number of high-quality bases for
     exclude from calling and increment the first value of the AD counter
     ('incAD0') ['drop']
 
+*--ref-skip-reads*::
+    Include reads with CIGAR 'N' (ref-skip) operators in their alignment.
+    By default these are filtered out.  Enabling this option may be
+    necessary for calling on some RNASeq pipelines.  This works with
+    the default caller and --indels-cns.  The experimental -indels-2.0
+    option does not filter out alignments with ref-skips so this option
+    is unnecessary there.
+
 *-e, --ext-prob* 'INT'::
     Phred-scaled gap extension sequencing error probability. Reducing 'INT'
     leads to longer indels [20]

mpileup.c

@@ -68,7 +68,7 @@ typedef struct _mplp_pileup_t mplp_pileup_t;
 // Data shared by all bam files
 typedef struct {
     int min_mq, flag, min_baseQ, max_baseQ, delta_baseQ, capQ_thres, max_depth,
-        max_indel_depth, max_read_len, ambig_reads;
+        max_indel_depth, max_read_len, ambig_reads, ref_skip_reads;
     uint32_t fmt_flag;
     int rflag_skip_any_unset, rflag_skip_all_unset, rflag_skip_any_set, rflag_skip_all_set, output_type;
     int openQ, extQ, tandemQ, min_support, indel_win_size; // for indels
@@ -882,6 +882,7 @@ static int mpileup(mplp_conf_t *conf)
     conf->bca->per_sample_flt = conf->flag & MPLP_PER_SAMPLE;
     conf->bca->fmt_flag = conf->fmt_flag;
     conf->bca->ambig_reads = conf->ambig_reads;
+    conf->bca->ref_skip_reads = conf->ref_skip_reads;
     conf->bca->indel_win_size = conf->indel_win_size;
     conf->bca->indels_v20 = conf->indels_v20;
     conf->bca->edlib = conf->edlib;
@@ -1291,6 +1292,8 @@ static void print_usage(FILE *fp, const mplp_conf_t *mplp)
         "  -p, --per-sample-mF     Apply -m and -F per-sample for increased sensitivity\n"
         "  -P, --platforms STR     Comma separated list of platforms for indels [all]\n"
         "  --ar, --ambig-reads STR   What to do with ambiguous indel reads: drop,incAD,incAD0 [drop]\n");
+    fprintf(fp,
+        "      --ref-skip-reads    Use reads with N CIGAR op [discard by default]\n");
     fprintf(fp,
         "      --indel-bias FLOAT  Raise to favour recall over precision [%.2f]\n", mplp->indel_bias);
     fprintf(fp,
@@ -1381,6 +1384,7 @@ int main_mpileup(int argc, char *argv[])
     mplp.fmt_flag = B2B_INFO_BQBZ|B2B_INFO_IDV|B2B_INFO_IMF|B2B_INFO_MQ0F|B2B_INFO_MQBZ|B2B_INFO_MQSBZ|B2B_INFO_RPBZ|B2B_INFO_SCBZ|B2B_INFO_SGB|B2B_INFO_VDB|B2B_FMT_AD;
     mplp.max_read_len = 500;
     mplp.ambig_reads = B2B_DROP;
+    mplp.ref_skip_reads = 0;
     mplp.indel_win_size = 110;
     mplp.poly_mqual = 0;
     mplp.seqQ_offset = 120;
@@ -1458,6 +1462,7 @@ int main_mpileup(int argc, char *argv[])
         {"seed", required_argument, NULL, 13},
         {"ambig-reads", required_argument, NULL, 14},
         {"ar", required_argument, NULL, 14},
+        {"ref-skip-reads", no_argument, NULL, 29},
         {"write-index",optional_argument,NULL,'W'},
         {"del-bias", required_argument, NULL, 23},
         {"poly-mqual", no_argument, NULL, 24},
@@ -1620,6 +1625,9 @@ int main_mpileup(int argc, char *argv[])
             if (mplp.seqQ_offset > 200)
                 mplp.seqQ_offset = 200;
             break;
+        case 29:
+            mplp.ref_skip_reads = 1;
+            break;
         case  23: mplp.del_bias = atof(optarg); break;
         case  24: mplp.poly_mqual = 1; break;
         case  26: mplp.poly_mqual = 0; break;