+/* Advance 'str' past any whitespace or RFC 822 comments. A comment is
+ * a (potentially nested) parenthesized sequence with '\' used to
+ * escape any character (including parentheses).
+ *
+ * If the sequence to be skipped continues to the end of the string,
+ * then 'str' will be left pointing at the final terminating '\0'
+ * character.
+ */
+static void
+skip_space_and_comments (const char **str)
+{
+ const char *s;
+
+ s = *str;
+ while (*s && (isspace (*s) || *s == '(')) {
+ while (*s && isspace (*s))
+ s++;
+ if (*s == '(') {
+ int nesting = 1;
+ s++;
+ while (*s && nesting) {
+ if (*s == '(')
+ nesting++;
+ else if (*s == ')')
+ nesting--;
+ else if (*s == '\\')
+ if (*(s+1))
+ s++;
+ s++;
+ }
+ }
+ }
+
+ *str = s;
+}
+
+/* Parse an RFC 822 message-id, discarding whitespace, any RFC 822
+ * comments, and the '<' and '>' delimeters.
+ *
+ * If not NULL, then *next will be made to point to the first character
+ * not parsed, (possibly pointing to the final '\0' terminator.
+ *
+ * Returns a newly allocated string which the caller should free()
+ * when done with it.
+ *
+ * Returns NULL if there is any error parsing the message-id. */
+static char *
+parse_message_id (const char *message_id, const char **next)
+{
+ const char *s, *end;
+ char *result;
+
+ if (message_id == NULL)
+ return NULL;
+
+ s = message_id;
+
+ skip_space_and_comments (&s);
+
+ /* Skip any unstructured text as well. */
+ while (*s && *s != '<')
+ s++;
+
+ if (*s == '<') {
+ s++;
+ } else {
+ if (next)
+ *next = s;
+ return NULL;
+ }
+
+ skip_space_and_comments (&s);
+
+ end = s;
+ while (*end && *end != '>')
+ end++;
+ if (next) {
+ if (*end)
+ *next = end + 1;
+ else
+ *next = end;
+ }
+
+ if (end > s && *end == '>')
+ end--;
+ if (end <= s)
+ return NULL;
+
+ result = strndup (s, end - s + 1);
+
+ /* Finally, collapse any whitespace that is within the message-id
+ * itself. */
+ {
+ char *r;
+ int len;
+
+ for (r = result, len = strlen (r); *r; r++, len--)
+ if (*r == ' ' || *r == '\t')
+ memmove (r, r+1, len);
+ }
+
+ return result;
+}
+
+/* Parse a References header value, putting a copy of each referenced
+ * message-id into 'array'. */