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

;; Copyright (C) 1994, 1995, 1996, 1997 Free Software Foundation, Inc.

;; Author: Simon Marshall <simon@gnu.ai.mit.edu>
;; Keywords: strings, regexps

;; 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 2, 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; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:

;; The "opt" in "regexp-opt" stands for "optim\\(al\\|i\\(se\\|ze\\)\\)".
;;
;; 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 and Dan Nicolaescu.
;; Please don't tell me that it doesn't produce optimal regexps; I know that
;; already.  For example, the above explanation for the meaning of "opt" would
;; be more efficient as "optim\\(al\\|i[sz]e\\)", but this requires complex
;; forward looking.  But (ideas or) code to improve things (are) is welcome.

;;; Code:

;;;###autoload
(defun regexp-opt (strings &optional paren)
  "Return a regexp to match a string in 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-paren (if PAREN \"\\\\(\" \"\")) (close-paren (if PAREN \"\\\\)\" \"\")))
   (concat open-paren (mapconcat 'regexp-quote STRINGS \"\\\\|\") close-paren))

but typically contains more regexp grouping constructs.
Use `regexp-opt-depth' to count them."
  (save-match-data
    ;; Recurse on the sorted list.
    (let ((max-lisp-eval-depth (* 1024 1024))
	  (completion-ignore-case nil))
      (regexp-opt-group (sort (copy-sequence strings) 'string-lessp) paren))))

;;;###autoload
(defun regexp-opt-depth (regexp)
  "Return the depth of REGEXP.
This means the number of regexp grouping constructs (parenthesised 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)
      (while (string-match "\\\\(" regexp start)
	(setq count (1+ count) start (match-end 0)))
      count)))

;;; Workhorse functions.

(eval-when-compile
  (require 'cl))

(unless (fboundp 'make-bool-vector)
  (defalias 'make-bool-vector 'make-vector))

(defun regexp-opt-group (strings &optional paren lax)
  ;;
  ;; Return a regexp to match a string in 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' regexp matcher.
  ;;
  ;; The basic idea is to find the shortest common prefix, 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 (if paren "\\(" ""))
	 (close-group (if paren "\\)" ""))
	 (open-charset (if lax "" open-group))
	 (close-charset (if lax "" close-group)))
    (cond
     ;;
     ;; 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 all are one-character strings, just return a character set.
     ((= (length strings) (apply '+ (mapcar 'length strings)))
      (concat open-charset
	      (regexp-opt-charset strings)
	      close-charset))
     ;;
     ;; We have a list of different length strings.
     (t
      (let ((prefix (try-completion "" (mapcar 'list strings)))
	    (letters (let ((completion-regexp-list '("^.$")))
		       (all-completions "" (mapcar 'list strings)))))
	(cond
	 ;;
	 ;; If there is a common prefix, remove it and recurse on the suffixes.
	 ((> (length prefix) 0)
	  (let* ((length (length prefix))
		 (suffixes (mapcar (lambda (s) (substring s length)) strings)))
	    (concat open-group
		    (regexp-quote prefix) (regexp-opt-group suffixes t t)
		    close-group)))
	 ;;
	 ;; If there are several one-character strings, remove them and recurse
	 ;; on the rest (first so the final regexp finds the longest match).
	 ((> (length letters) 1)
	  (let ((rest (let ((completion-regexp-list '("^..+$")))
			(all-completions "" (mapcar 'list strings)))))
	    (concat open-group
		    (regexp-opt-group rest) "\\|" (regexp-opt-charset letters)
		    close-group)))
	 ;;
	 ;; Otherwise, divide the list into those that start with a particular
	 ;; letter and those that do not, and recurse on them.
	 (t
	  (let* ((char (substring (car strings) 0 1))
		 (half1 (all-completions char (mapcar 'list 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.
  ;;
  ;; 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* ((charwidth 256)				; Yeah, right.
	 (charmap (make-bool-vector charwidth nil))
	 (charset "")
	 (bracket "") (dash "") (caret ""))
    ;;
    ;; Make a character map but extract character set meta characters.
    (dolist (char (mapcar 'string-to-char chars))
      (case char
	(?\]
	 (setq bracket "]"))
	(?^
	 (setq caret "^"))
	(?-
	 (setq dash "-"))
	(otherwise
	 (aset charmap char t))))
    ;;
    ;; Make a character set from the map using ranges where applicable.
    (dotimes (char charwidth)
      (let ((start char))
	(while (and (< char charwidth) (aref charmap char))
	  (incf char))
	(cond ((> char (+ start 3))
	       (setq charset (format "%s%c-%c" charset start (1- char))))
	      ((> char start)
	       (setq charset (format "%s%c" charset (setq char start)))))))
    ;;
    ;; Make sure a caret is not first and a dash is first or last.
    (if (and (string-equal charset "") (string-equal bracket ""))
	(concat "[" dash caret "]")
      (concat "[" bracket charset caret dash "]"))))

(provide 'regexp-opt)

;;; regexp-opt.el ends here