+ * S_utf8_mg_pos() is used to query and update mg_ptr field of
+ * a PERL_UTF8_magic. The mg_ptr is used to store the mapping
+ * between UTF-8 and byte offsets. See also the comments of
+ * S_utf8_mg_pos_init().
+ *
+ */
+STATIC bool
+S_utf8_mg_pos(pTHX_ SV *sv, MAGIC **mgp, STRLEN **cachep, I32 i, I32 *offsetp, I32 uoff, U8 **sp, U8 *start, U8 *send)
+{
+ bool found = FALSE;
+
+ if (SvMAGICAL(sv) && !SvREADONLY(sv)) {
+ if (!*mgp)
+ *mgp = mg_find(sv, PERL_MAGIC_utf8);
+ if (*mgp && (*mgp)->mg_ptr) {
+ *cachep = (STRLEN *) (*mgp)->mg_ptr;
+ if ((*cachep)[i] == (STRLEN)uoff) /* An exact match. */
+ found = TRUE;
+ else { /* We will skip to the right spot. */
+ STRLEN forw = 0;
+ STRLEN backw = 0;
+ U8* p = NULL;
+
+ /* The assumption is that going backward is half
+ * the speed of going forward (that's where the
+ * 2 * backw in the below comes from). (The real
+ * figure of course depends on the UTF-8 data.) */
+
+ if ((*cachep)[i] > (STRLEN)uoff) {
+ forw = uoff;
+ backw = (*cachep)[i] - (STRLEN)uoff;
+
+ if (forw < 2 * backw)
+ p = start;
+ else
+ p = start + (*cachep)[i+1];
+ }
+ /* Try this only for the substr offset (i == 0),
+ * not for the substr length (i == 2). */
+ else if (i == 0) { /* (*cachep)[i] < uoff */
+ STRLEN ulen = sv_len_utf8(sv);
+
+ if ((STRLEN)uoff < ulen) {
+ forw = (STRLEN)uoff - (*cachep)[i];
+ backw = ulen - (STRLEN)uoff;
+
+ if (forw < 2 * backw)
+ p = start + (*cachep)[i+1];
+ else
+ p = send;
+ }
+
+ /* If the string is not long enough for uoff,
+ * we could extend it, but not at this low a level. */
+ }
+
+ if (p) {
+ if (forw < 2 * backw) {
+ while (forw--)
+ p += UTF8SKIP(p);
+ }
+ else {
+ while (backw--) {
+ p--;
+ while (UTF8_IS_CONTINUATION(*p))
+ p--;
+ }
+ }
+
+ /* Update the cache. */
+ (*cachep)[i] = (STRLEN)uoff;
+ (*cachep)[i+1] = p - start;
+
+ found = TRUE;
+ }
+ }
+ if (found) { /* Setup the return values. */
+ *offsetp = (*cachep)[i+1];
+ *sp = start + *offsetp;
+ if (*sp >= send) {
+ *sp = send;
+ *offsetp = send - start;
+ }
+ else if (*sp < start) {
+ *sp = start;
+ *offsetp = 0;
+ }
+ }
+ }
+ }
+ return found;
+}
+
+/*