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[emacs.git] / lisp / emacs-lisp / regexp-opt.el

;;; regexp-opt.el --- generate efficient regexps to match strings

;; Copyright (C) 1994-2015 Free Software Foundation, Inc.

;; Author: Simon Marshall <simon@gnu.org>
;; Maintainer: emacs-devel@gnu.org
;; Keywords: strings, regexps, extensions

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; The "opt" in "regexp-opt" stands for "optim\\(al\\|i[sz]e\\)".
;;
;; This package generates a regexp from a given list of strings (which matches
;; one of those strings) so that the regexp generated by:
;;
;; (regexp-opt strings)
;;
;; is equivalent to, but more efficient than, the regexp generated by:
;;
;; (mapconcat 'regexp-quote strings "\\|")
;;
;; For example:
;;
;; (let ((strings '("cond" "if" "when" "unless" "while"
;;                  "let" "let*" "progn" "prog1" "prog2"
;;                  "save-restriction" "save-excursion" "save-window-excursion"
;;                  "save-current-buffer" "save-match-data"
;;                  "catch" "throw" "unwind-protect" "condition-case")))
;;   (concat "(" (regexp-opt strings t) "\\>"))
;;  => "(\\(c\\(atch\\|ond\\(ition-case\\)?\\)\\|if\\|let\\*?\\|prog[12n]\\|save-\\(current-buffer\\|excursion\\|match-data\\|restriction\\|window-excursion\\)\\|throw\\|un\\(less\\|wind-protect\\)\\|wh\\(en\\|ile\\)\\)\\>"
;;
;; Searching using the above example `regexp-opt' regexp takes approximately
;; two-thirds of the time taken using the equivalent `mapconcat' regexp.

;; Since this package was written to produce efficient regexps, not regexps
;; efficiently, it is probably not a good idea to in-line too many calls in
;; your code, unless you use the following trick with `eval-when-compile':
;;
;; (defvar definition-regexp
;;   (eval-when-compile
;;     (concat "^("
;;             (regexp-opt '("defun" "defsubst" "defmacro" "defalias"
;;                           "defvar" "defconst") t)
;;             "\\>")))
;;
;; The `byte-compile' code will be as if you had defined the variable thus:
;;
;; (defvar definition-regexp
;;   "^(\\(def\\(alias\\|const\\|macro\\|subst\\|un\\|var\\)\\)\\>")
;;
;; Note that if you use this trick for all instances of `regexp-opt' and
;; `regexp-opt-depth' in your code, regexp-opt.el would only have to be loaded
;; at compile time.  But note also that using this trick means that should
;; regexp-opt.el be changed, perhaps to fix a bug or to add a feature to
;; improve the efficiency of `regexp-opt' regexps, you would have to recompile
;; your code for such changes to have effect in your code.

;; Originally written for font-lock.el, from an idea from Stig's hl319.el, with
;; thanks for ideas also to Michael Ernst, Bob Glickstein, Dan Nicolaescu and
;; Stefan Monnier.
;; No doubt `regexp-opt' doesn't always produce optimal regexps, so code, ideas
;; or any other information to improve things are welcome.
;;
;; One possible improvement would be to compile '("aa" "ab" "ba" "bb")
;; into "[ab][ab]" rather than "a[ab]\\|b[ab]".  I'm not sure it's worth
;; it but if someone knows how to do it without going through too many
;; contortions, I'm all ears.
\f
;;; Code:

;;;###autoload
(defun regexp-opt (strings &optional paren)
  "Return a regexp to match a string in the list STRINGS.
Each string should be unique in STRINGS and should not contain any regexps,
quoted or not.  If optional PAREN is non-nil, ensure that the returned regexp
is enclosed by at least one regexp grouping construct.
The returned regexp is typically more efficient than the equivalent regexp:

 (let ((open (if PAREN \"\\\\(\" \"\")) (close (if PAREN \"\\\\)\" \"\")))
   (concat open (mapconcat 'regexp-quote STRINGS \"\\\\|\") close))

If PAREN is `words', then the resulting regexp is additionally surrounded
by \\=\\< and \\>.
If PAREN is `symbols', then the resulting regexp is additionally surrounded
by \\=\\_< and \\_>."
  (save-match-data
    ;; Recurse on the sorted list.
    (let* ((max-lisp-eval-depth 10000)
           (max-specpdl-size 10000)
           (completion-ignore-case nil)
           (completion-regexp-list nil)
           (open (cond ((stringp paren) paren) (paren "\\(")))
           (sorted-strings (delete-dups
                            (sort (copy-sequence strings) 'string-lessp)))
           (re (regexp-opt-group sorted-strings (or open t) (not open))))
      (cond ((eq paren 'words)
             (concat "\\<" re "\\>"))
            ((eq paren 'symbols)
             (concat "\\_<" re "\\_>"))
            (t re)))))

;;;###autoload
(defun regexp-opt-depth (regexp)
  "Return the depth of REGEXP.
This means the number of non-shy regexp grouping constructs
\(parenthesized expressions) in REGEXP."
  (save-match-data
    ;; Hack to signal an error if REGEXP does not have balanced parentheses.
    (string-match regexp "")
    ;; Count the number of open parentheses in REGEXP.
    (let ((count 0) start last)
      (while (string-match "\\\\(\\(\\?[0-9]*:\\)?" regexp start)
        (setq start (match-end 0))            ; Start of next search.
        (when (and (not (match-beginning 1))
                   (subregexp-context-p regexp (match-beginning 0) last))
          ;; It's not a shy group and it's not inside brackets or after
          ;; a backslash: it's really a group-open marker.
          (setq last start)         ; Speed up next regexp-opt-re-context-p.
          (setq count (1+ count))))
      count)))
\f
;;; Workhorse functions.

(defun regexp-opt-group (strings &optional paren lax)
  "Return a regexp to match a string in the sorted list STRINGS.
If PAREN non-nil, output regexp parentheses around returned regexp.
If LAX non-nil, don't output parentheses if it doesn't require them.
Merges keywords to avoid backtracking in Emacs's regexp matcher."
  ;; The basic idea is to find the shortest common prefix or suffix, remove it
  ;; and recurse.  If there is no prefix, we divide the list into two so that
  ;; \(at least) one half will have at least a one-character common prefix.

  ;; Also we delay the addition of grouping parenthesis as long as possible
  ;; until we're sure we need them, and try to remove one-character sequences
  ;; so we can use character sets rather than grouping parenthesis.
  (let* ((open-group (cond ((stringp paren) paren) (paren "\\(?:") (t "")))
         (close-group (if paren "\\)" ""))
         (open-charset (if lax "" open-group))
         (close-charset (if lax "" close-group)))
    (cond
     ;;
     ;; If there are no strings, just return the empty string.
     ((= (length strings) 0)
      "")
     ;;
     ;; If there is only one string, just return it.
     ((= (length strings) 1)
      (if (= (length (car strings)) 1)
          (concat open-charset (regexp-quote (car strings)) close-charset)
        (concat open-group (regexp-quote (car strings)) close-group)))
     ;;
     ;; If there is an empty string, remove it and recurse on the rest.
     ((= (length (car strings)) 0)
      (concat open-charset
              (regexp-opt-group (cdr strings) t t) "?"
              close-charset))
     ;;
     ;; If there are several one-char strings, use charsets
     ((and (= (length (car strings)) 1)
           (let ((strs (cdr strings)))
             (while (and strs (/= (length (car strs)) 1))
               (pop strs))
             strs))
      (let (letters rest)
        ;; Collect one-char strings
        (dolist (s strings)
          (if (= (length s) 1) (push (string-to-char s) letters) (push s rest)))

        (if rest
            ;; several one-char strings: take them and recurse
            ;; on the rest (first so as to match the longest).
            (concat open-group
                    (regexp-opt-group (nreverse rest))
                    "\\|" (regexp-opt-charset letters)
                    close-group)
          ;; all are one-char strings: just return a character set.
          (concat open-charset
                  (regexp-opt-charset letters)
                  close-charset))))
     ;;
     ;; We have a list of different length strings.
     (t
      (let ((prefix (try-completion "" strings)))
        (if (> (length prefix) 0)
            ;; common prefix: take it and recurse on the suffixes.
            (let* ((n (length prefix))
                   (suffixes (mapcar (lambda (s) (substring s n)) strings)))
              (concat open-group
                      (regexp-quote prefix)
                      (regexp-opt-group suffixes t t)
                      close-group))

          (let* ((sgnirts (mapcar #'reverse strings))
                 (xiffus (try-completion "" sgnirts)))
            (if (> (length xiffus) 0)
                ;; common suffix: take it and recurse on the prefixes.
                (let* ((n (- (length xiffus)))
                       (prefixes
                        ;; Sorting is necessary in cases such as ("ad" "d").
                        (sort (mapcar (lambda (s) (substring s 0 n)) strings)
                              'string-lessp)))
                  (concat open-group
                          (regexp-opt-group prefixes t t)
                          (regexp-quote (nreverse xiffus))
                          close-group))

              ;; Otherwise, divide the list into those that start with a
              ;; particular letter and those that do not, and recurse on them.
              (let* ((char (substring-no-properties (car strings) 0 1))
                     (half1 (all-completions char strings))
                     (half2 (nthcdr (length half1) strings)))
                (concat open-group
                        (regexp-opt-group half1)
                        "\\|" (regexp-opt-group half2)
                        close-group))))))))))


(defun regexp-opt-charset (chars)
  "Return a regexp to match a character in CHARS.
CHARS should be a list of characters."
  ;; The basic idea is to find character ranges.  Also we take care in the
  ;; position of character set meta characters in the character set regexp.
  ;;
  (let* ((charmap (make-char-table 'case-table))
         (start -1) (end -2)
         (charset "")
         (bracket "") (dash "") (caret ""))
    ;;
    ;; Make a character map but extract character set meta characters.
    (dolist (char chars)
      (cond
       ((eq char ?\])
        (setq bracket "]"))
       ((eq char ?^)
        (setq caret "^"))
       ((eq char ?-)
        (setq dash "-"))
       (t
        (aset charmap char t))))
    ;;
    ;; Make a character set from the map using ranges where applicable.
    (map-char-table
     (lambda (c v)
       (when v
         (if (consp c)
             (if (= (1- (car c)) end) (setq end (cdr c))
               (if (> end (+ start 2))
                   (setq charset (format "%s%c-%c" charset start end))
                 (while (>= end start)
                   (setq charset (format "%s%c" charset start))
                   (setq start (1+ start))))
               (setq start (car c) end (cdr c)))
           (if (= (1- c) end) (setq end c)
             (if (> end (+ start 2))
               (setq charset (format "%s%c-%c" charset start end))
             (while (>= end start)
               (setq charset (format "%s%c" charset start))
               (setq start (1+ start))))
             (setq start c end c)))))
     charmap)
    (when (>= end start)
      (if (> end (+ start 2))
          (setq charset (format "%s%c-%c" charset start end))
        (while (>= end start)
          (setq charset (format "%s%c" charset start))
          (setq start (1+ start)))))
    ;;
    ;; Make sure a caret is not first and a dash is first or last.
    (if (and (string-equal charset "") (string-equal bracket ""))
        (if (string-equal dash "")
            "\\^"                       ; [^] is not a valid regexp
          (concat "[" dash caret "]"))
      (concat "[" bracket charset caret dash "]"))))

(provide 'regexp-opt)

;;; regexp-opt.el ends here