Add bam_aux_first()/bam_aux_next() tagged aux field iterator API

Add new API functions for iterating through a BAM record's aux fields,
inline accessor methods for field tag and type (or code can continue
to use s-2 and *s), and a variant of bam_aux_del() that returns the
(updated) iterator to the following field (for use in iterator-based
loops that delete fields).

htslib/sam.h

@@ -1428,7 +1428,6 @@ int sam_passes_filter(const sam_hdr_t *h, const bam1_t *b,
 
 /// Converts a BAM aux tag to SAM format
 /*
- * @param b    Pointer to the bam record
  * @param key  Two letter tag key
  * @param type Single letter type code: ACcSsIifHZB.
  * @param tag  Tag data pointer, in BAM format
@@ -1616,6 +1615,29 @@ static inline const uint8_t *sam_format_aux1(const uint8_t *key,
     return NULL;
 }
 
+/// Return a pointer to a BAM record's first aux field
+/** @param b   Pointer to the BAM record
+    @return    Aux field pointer, or NULL if the record has none
+
+When NULL is returned, errno will also be set to ENOENT. ("Aux field pointers"
+point to the TYPE byte within the auxiliary data for that field; but in general
+it is unnecessary for user code to be aware of this.)
+ */
+HTSLIB_EXPORT
+uint8_t *bam_aux_first(const bam1_t *b);
+
+/// Return a pointer to a BAM record's next aux field
+/** @param b   Pointer to the BAM record
+    @param s   Aux field pointer, as returned by bam_aux_first()/_next()/_get()
+    @return    Pointer to the next aux field, or NULL if already last or error
+
+Whenever NULL is returned, errno will also be set: ENOENT if @p s was the
+record's last aux field; otherwise EINVAL, indicating that the BAM record's
+aux data is corrupt.
+ */
+HTSLIB_EXPORT
+uint8_t *bam_aux_next(const bam1_t *b, const uint8_t *s);
+
 /// Return a pointer to an aux record
 /** @param b   Pointer to the bam record
     @param tag Desired aux tag
@@ -1628,6 +1650,19 @@ static inline const uint8_t *sam_format_aux1(const uint8_t *key,
 HTSLIB_EXPORT
 uint8_t *bam_aux_get(const bam1_t *b, const char tag[2]);
 
+/// Return the aux field's 2-character tag
+/** @param s   Aux field pointer, as returned by bam_aux_first()/_next()/_get()
+    @return    Pointer to the tag characters, NOT NUL-terminated
+ */
+static inline
+const char *bam_aux_tag(const uint8_t *s) { return (const char *) (s-2); }
+
+/// Return the aux field's type character
+/** @param s   Aux field pointer, as returned by bam_aux_first()/_next()/_get()
+    @return    The type character: one of cCsSiI/fd/A/Z/H/B
+ */
+static inline char bam_aux_type(const uint8_t *s) { return *s; }
+
 /// Return a SAM formatting string containing a BAM tag
 /** @param b   Pointer to the bam record
     @param tag Desired aux tag
@@ -1739,6 +1774,19 @@ int bam_aux_append(bam1_t *b, const char tag[2], char type, int len, const uint8
 HTSLIB_EXPORT
 int bam_aux_del(bam1_t *b, uint8_t *s);
 
+/// Delete tag data from a bam record
+/* @param b   The bam record to update
+   @param s   Pointer to the aux field to delete, as returned by
+              bam_aux_first()/_next()/_get()
+   @return    Pointer to the following aux field, or NULL if none or on error
+
+Whenever NULL is returned, errno will also be set: ENOENT if the aux field
+deleted was the record's last one; otherwise EINVAL, indicating that the
+BAM record's aux data is corrupt.
+ */
+HTSLIB_EXPORT
+uint8_t *bam_aux_erase(bam1_t *b, uint8_t *s);
+
 /// Update or add a string-type tag
 /* @param b    The bam record to update
    @param tag  Tag identifier

sam.c

@@ -4508,7 +4508,52 @@ static inline uint8_t *skip_aux(uint8_t *s, uint8_t *end)
     }
 }
 
+uint8_t *bam_aux_first(const bam1_t *b)
+{
+    uint8_t *s = bam_get_aux(b);
+    uint8_t *end = b->data + b->l_data;
+    if (s >= end) { errno = ENOENT; return NULL; }
+    return s+2;
+}
+
+uint8_t *bam_aux_next(const bam1_t *b, const uint8_t *s)
+{
+    uint8_t *end = b->data + b->l_data;
+    uint8_t *next = s? skip_aux((uint8_t *) s, end) : end;
+    if (next == NULL) goto bad_aux;
+    if (next >= end) { errno = ENOENT; return NULL; }
+    return next+2;
+
+bad_aux:
+    hts_log_error("Corrupted aux data for read %s", bam_get_qname(b));
+    errno = EINVAL;
+    return NULL;
+}
+
 uint8_t *bam_aux_get(const bam1_t *b, const char tag[2])
+{
+    uint8_t *s;
+    for (s = bam_aux_first(b); s; s = bam_aux_next(b, s))
+        if (s[-2] == tag[0] && s[-1] == tag[1]) {
+            // Check the tag value is valid and complete
+            uint8_t *e = skip_aux(s, b->data + b->l_data);
+            if (e == NULL) goto bad_aux;
+            if ((*s == 'Z' || *s == 'H') && *(e - 1) != '\0') goto bad_aux;
+
+            return s;
+        }
+
+    // errno now as set by bam_aux_first()/bam_aux_next()
+    return NULL;
+
+bad_aux:
+    hts_log_error("Corrupted aux data for read %s", bam_get_qname(b));
+    errno = EINVAL;
+    return NULL;
+}
+
+#if 0
+uint8_t *bam_aux_get_old(const bam1_t *b, const char tag[2])
 {
     uint8_t *s, *end, *t = (uint8_t *) tag;
     uint16_t y = (uint16_t) t[0]<<8 | t[1];
@@ -4540,24 +4585,30 @@ uint8_t *bam_aux_get(const bam1_t *b, const char tag[2])
     errno = EINVAL;
     return NULL;
 }
+#endif
 
-// s MUST BE returned by bam_aux_get()
-int bam_aux_del(bam1_t *b, uint8_t *s)
+uint8_t *bam_aux_erase(bam1_t *b, uint8_t *s)
 {
-    uint8_t *p, *aux;
-    int l_aux = bam_get_l_aux(b);
-    aux = bam_get_aux(b);
-    p = s - 2;
-    s = skip_aux(s, aux + l_aux);
-    if (s == NULL) goto bad_aux;
-    memmove(p, s, l_aux - (s - aux));
-    b->l_data -= s - p;
-    return 0;
+    uint8_t *end = b->data + b->l_data;
+    uint8_t *next = skip_aux(s, end);
+    if (next == NULL) goto bad_aux;
 
- bad_aux:
+    memmove(s-2, next, end - next);
+    b->l_data -= next - (s-2);
+
+    if (next >= end) { errno = ENOENT; return NULL; }
+    return s;
+
+bad_aux:
     hts_log_error("Corrupted aux data for read %s", bam_get_qname(b));
     errno = EINVAL;
-    return -1;
+    return NULL;
+}
+
+int bam_aux_del(bam1_t *b, uint8_t *s)
+{
+    s = bam_aux_erase(b, s);
+    return (s || errno == ENOENT)? 0 : -1;
 }
 
 int bam_aux_update_str(bam1_t *b, const char tag[2], int len, const char *data)