implemented seekn(); not tested

hts.c

@@ -897,6 +897,13 @@ int hts_itr_next(BGZF *fp, hts_itr_t *iter, void *r, hts_readrec_f readrec, void
 	return ret;
 }
 
+int hts_idx_seekn_aux(BGZF *fp, const hts_idx_t *idx, int64_t r)
+{
+	if (idx->ridx.n == 0 || r >= idx->n_rec) return -1;
+	bgzf_seek(fp, idx->ridx.offset[r>>idx->rec_shift], SEEK_SET);
+	return r & (((1<<idx->rec_shift) - 1));
+}
+
 /**********************
  *** Retrieve index ***
  **********************/

hts.h

@@ -115,6 +115,7 @@ extern "C" {
 	typedef int (*hts_readrec_f)(BGZF*, void*, void*, int*, int*, int*);
 	typedef int (*hts_name2id_f)(void*, const char*);
 
+	int hts_idx_seekn_aux(BGZF *fp, const hts_idx_t *idx, int64_t n);
 	hts_itr_t *hts_itr_querys(const hts_idx_t *idx, const char *reg, hts_name2id_f getid, void *hdr);
 	int hts_itr_next(BGZF *fp, hts_itr_t *iter, void *r, hts_readrec_f readrec, void *hdr);
 

vcf.c

@@ -1192,3 +1192,18 @@ int bcf_append_info_ints(const bcf_hdr_t *h, bcf1_t *b, const char *key, int n,
 	return 0;
 }
 
+int bcf_seekn(BGZF *fp, const hts_idx_t *idx, int64_t r)
+{
+	int skip, ret = 0;
+	skip = hts_idx_seekn_aux(fp, idx, r);
+	if (skip > 0) {
+		bcf1_t *b;
+		b = bcf_init1();
+		do {
+			ret = bcf_read1(fp, b);
+			--skip;
+		} while (skip > 0 && ret >= 0);
+		bcf_destroy1(b);
+	}
+	return ret;
+}

vcf.h

@@ -216,6 +216,7 @@ extern "C" {
 	#define bcf_index_load(fn) hts_idx_load(fn, HTS_FMT_CSI)
 
 	int bcf_index_build(const char *fn, int min_shift);
+	int bcf_seekn(BGZF *fp, const hts_idx_t *idx, int64_t n);
 
 	/***************
 	 *** VCF I/O ***