diff options
Diffstat (limited to 'src/fns.c')
| -rw-r--r-- | src/fns.c | 35 |
1 files changed, 27 insertions, 8 deletions
diff --git a/src/fns.c b/src/fns.c index 8cff6b1b6ca..9d681017c14 100644 --- a/src/fns.c +++ b/src/fns.c @@ -1402,6 +1402,8 @@ DEFUN ("nthcdr", Fnthcdr, Snthcdr, 2, 2, 0, doc: /* Take cdr N times on LIST, return the result. */) (Lisp_Object n, Lisp_Object list) { + Lisp_Object tail = list; + CHECK_INTEGER (n); /* A huge but in-range EMACS_INT that can be substituted for a @@ -1412,24 +1414,41 @@ DEFUN ("nthcdr", Fnthcdr, Snthcdr, 2, 2, 0, EMACS_INT num; if (FIXNUMP (n)) - num = XFIXNUM (n); + { + num = XFIXNUM (n); + + /* Speed up small lists by omitting circularity and quit checking. */ + if (num < 128) + { + for (; 0 < num; num--, tail = XCDR (tail)) + if (! CONSP (tail)) + { + CHECK_LIST_END (tail, list); + return Qnil; + } + return tail; + } + } else { - num = mpz_sgn (XBIGNUM (n)->value); - if (0 < num) - num = large_num; + if (mpz_sgn (XBIGNUM (n)->value) < 0) + return tail; + num = large_num; } EMACS_INT tortoise_num = num; - Lisp_Object tail = list, saved_tail = tail; + Lisp_Object saved_tail = tail; FOR_EACH_TAIL_SAFE (tail) { - if (num <= 0) - return tail; - if (tail == li.tortoise) + /* If the tortoise just jumped (which is rare), + update TORTOISE_NUM accordingly. */ + if (EQ (tail, li.tortoise)) tortoise_num = num; + saved_tail = XCDR (tail); num--; + if (num == 0) + return saved_tail; rarely_quit (num); } |