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+;;; 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
+;; Version: 1.04.01
+
+;; 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 (that matches
+;; one of those strings) that is equivalent to but more efficient than:
+;;
+;; (mapconcat 'identity (mapcar '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 is significantly
+;; faster than searching using the equivalent `mapconcat' regexp, taking
+;; approximately two-thirds of the time.
+;;
+;; 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\\)\\)\\>")
+;;
+;; Originally written for font-lock.el, from an idea from Stig's hl319.el.
+;; 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.
+If optional PAREN non-nil, ensure that the returned regexp is enclosed by at
+least one regexp grouping construct.
+Each string in STRINGS should be unique and should not contain any regexps.
+The returned regexp is typically more efficient than the equivalent regexp:
+
+ (mapconcat 'identity (mapcar 'regexp-quote STRINGS) \"\\\\|\")
+
+but typically contains 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 '("^.$")))
+                       (all-completions "" (mapcar 'list strings)))))
+        (cond
+         ;;
+         ;; If there is a common prefix, remove it and recurse on the suffixes.
+         ((> (length prefix) 0)
+          (let* ((length (length prefix))
+                 (suffixes (mapcar (lambda (s) (substring s length)) strings)))
+            (concat open-group
+                    (regexp-quote prefix) (regexp-opt-group suffixes t t)
+                    close-group)))
+         ;;
+         ;; If there are several one-character strings, remove them and recurse
+         ;; on the rest.
+         ((> (length letters) 1)
+          (let ((rest (let ((completion-regexp-list '("^..+$")))
+                        (all-completions "" (mapcar 'list strings)))))
+            (concat open-group
+                    (regexp-opt-charset letters) "\\|" (regexp-opt-group rest)
+                    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.
+    (let (char)
+      (while chars
+        (setq char (string-to-char (pop 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.
+    (let ((elt 0) start)
+      (while (< elt charwidth)
+        (when (aref charmap elt)
+          (setq start (1+ elt))
+          (while (and (< start charwidth) (aref charmap start))
+            (incf start))
+          (if (< (- start elt) 4)
+              (setq charset (format "%s%c" charset elt))
+            (setq charset (format "%s%c-%c" charset elt (1- start))
+                  elt start)))
+        (incf elt)))
+    ;;
+    ;; 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